加利福尼亚海岸山脉蛇绿岩橄榄岩和铬铁矿的 Re-Os 同位素和 PGE 丰度系统学:洞察前弧岩浆过程

IF 1.8 4区 地球科学 Q3 GEOCHEMISTRY & GEOPHYSICS
Sung Hi Choi, Samuel B. Mukasa, John W. Shervais, Igor S. Puchtel
{"title":"加利福尼亚海岸山脉蛇绿岩橄榄岩和铬铁矿的 Re-Os 同位素和 PGE 丰度系统学:洞察前弧岩浆过程","authors":"Sung Hi Choi, Samuel B. Mukasa, John W. Shervais, Igor S. Puchtel","doi":"10.2113/2024/lithosphere_2024_154","DOIUrl":null,"url":null,"abstract":"We report platinum-group element (PGE) and Re concentrations, and Re−Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)N ≈ 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)−Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE−Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = −8.3 to 1.4) and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite−magnetite−quartz buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after ~5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has 187Os/188Os value of 0.1250 (γOs = −3.5) and PGE−Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.Ophiolites are sections of the Earth’s oceanic crust and the underlying upper mantle that have been tectonically emplaced into continental margins, providing important insights into the processes of plate tectonics, the composition of the oceanic crust and mantle, and the dynamics of Earth’s interior. Ophiolites are also valuable as ore deposits hosting precious metals, including platinum-group elements (PGEs), ferrous metals (Cr, Mn, and Ti), and base metals (Co, Cu, and Ni). The oceanic crust preserved in ophiolites may form in any tectonic setting during the evolution of ocean basins, from the mid-ocean ridge to subduction initiation and final closure [1]. The Coast Range ophiolite (CRO) is a mid-Jurassic (~172 to 161 Ma) ophiolite terrane in central California, extending over 700 km from Elder Creek at its northernmost segment extent to Point Sal at its southernmost terminus [2-5] (Figure 1). Petrologic and geochemical data indicate its formation in a supra-subduction zone (SSZ) fore-arc setting, probably above the east-dipping proto-Franciscan subduction zone [2, 6, 7]. Initiation of the subduction is considered to have possibly started along a large-offset transform fault zone, when an exotic or fringing is collided with North America [5, 6, 8]. The arc-like geochemical characteristics of pyroclastic, volcaniclastic, and hypabyssal materials of ophiolitic crust provide important evidence for the SSZ setting [4, 9, 10]. The possibility of the CRO having formed in a distal mid-ocean ridge or back-arc settings has also been proposed [11]. However, a paleo-equatorial mid-ocean spreading origin for the CRO peridotites is ruled out due to the SSZ-like highly depleted geochemical characteristics observed in several suites of CRO peridotites [2, 7, 12] and paleomagnetic evidence for limited north–south movement [13, 14]. A back-arc setting has remained plausible because back-arc extension and magmatism would be contemporaneous with arc volcanism, and it is difficult to geochemically distinguish arc-related magmas produced in fore-arc, intra-arc, or back-arc settings [15]. Since peridotites from back-arc basin are generally similar to abyssal peridotites [16], however, such a possibility has also been excluded [2]. Meanwhile, in the SSZ setting, the melt extraction process (e.g. fore-arc basalt or boninitic) to produce the current CRO peridotites has remained unidentified.The Re−Os isotope system and PGE abundances can play a pivotal role in understanding petrogenetic processes in peridotites, such as mantle melting and fluid/melt-rock interaction [17]. In upper mantle peridotites, PGEs are dominated by base-metal sulfides (BMSs) due to their highly chalcophile characteristics [18, 19]. Based on melting temperature (Tm), the PGEs are classified into two groups: Ir-group (IPGE: Os, Ir, and Ru, Tm > 2000°C) and Pd-group (PPGE: Pt, Pd, and Rh, Tm < 2000°C) [20]. During partial melting, the IPGE group resides in residual phases (e.g. sulfides, alloys, or oxides) and thus behaves compatibly relative to the PPGE group [21]. Re behaves as a mildly incompatible element during mantle melting, which results in residual peridotites with much lower Re/Os ratios than the fertile precursor [22]. The Re−Os isotopic system thus has great potential for dating melt depletion events in peridotites [23]. The high concentration of Os in the melting residue renders the Os isotopic compositions resistant to subsequent metasomatic processes. Os and Ir are not significantly fractionated during anhydrous partial melting [20, 21]. However, Os can be partitioned more than Ir into Cl-rich oxidizing slab-derived hydrous fluids or melts [24-26].Understanding the origin of the CRO is essential for unraveling the Mesozoic evolution of Cordilleran continental margin, and its development has implications for the origin of other continental margin ophiolites and provides a point of comparison with modern intra-oceanic fore-arcs (e.g. the Izu−Bonin system; [27, 28]). PGE compositions of fore-arc mantle have been rarely reported so far, yet we have indicated their potential for providing some profound insights into melting processes. This study presents PGE abundance and Re−Os isotope data for CRO peridotites and a podiform chromitite. Our aim is to constrain the formation and evolution of the CRO in a fore-arc regime and to evaluate its significance in the broader realm of ophiolite formation.The CRO is a sequence of dismembered oceanic rocks in fault contact with the underlying late Mesozoic−early Paleogene Franciscan Complex and is overlain by Upper Jurassic strata of the Great Valley sequence [3, 4, 29]. Crustal rocks (gabbro, diorite, basalt, and andesite) are the most abundant lithologies in the ophiolite though serpentinized peridotite tectonite is also widely distributed, and in some areas, it is the predominant lithology [30-32]. Igneous rocks of the ophiolite are dominantly tholeiitic basalts and basaltic andesites with arc affinities, with less common boninite, andesite, and primitive olivine–clinopyroxene–phyric basalts [3, 4]. CRO localities with extensive crustal sections in the northern Sacramento Valley (Elder Creek, Stonyford) and Diablo Range (Del Puerto Canyon, Llanada) are characterized by common felsic volcanic rocks (andesite, “keratophyre”) and shallow plutonic rocks comprising diorite, tonalite, and trondhjemite with calc-alkaline affinities [9, 33-35]. The felsic calc-alkaline series rocks overlie or cross-cut the older arc tholeiite series rocks and are in turn overlain or cross-cut by late MORB-like lavas and dikes [4, 35]. The latter magmatism is considered to have been caused by ridge collision, resulting in termination of ophiolite formation due to the change in relative plate motion and shallow subduction of young, buoyant crust [4].The CRO ranges in age from ~172 to ~161 Ma, based on U−Pb zircon ages from plagiogranite and quartz diorites associated with the ophiolite and on Ar−Ar ages of basaltic glass at Stonyford [3, 5, 36]. High-resolution U−Pb zircon chemical abrasion ages define a tighter age range of 161.2 to 167.9 Ma [3, 36]. The youngest age (~161.2 ± 0.1 Ma) is for felsic dikes in the Del Puerto ophiolite; the oldest age is from Elder Creek (~167.9 ± 0.3 Ma). Ages for most CRO locales cluster tightly at ~165 Ma, including the Ar−Ar ages on volcanic glass (164.2 ± 0.7 Ma), suggesting rapid onset of subduction and a short formation interval of circa 7 million years.Eight peridotites and one chromitite were obtained for this study, purposely sampling outcrops previously shown to exhibit the different crustal-rock associations described earlier. Chromitite occurs as irregular pods and lenses in dunite. Sample locations are shown in Figure 1, which include Grey Eagle Mine near Chrome (Red Mountain), Black Diamond Ridge (north of the Stonyford volcanic complex), Little Stony Creek (south of the Stonyford volcanic complex), Del Puerto Canyon, and the Burro Mountain.Based on mineral chemistry, previous studies [2, 12] divided the CRO peridotites into two groups: abyssal and SSZ peridotites. Abyssal peridotites are spinel lherzolites characterized by high-Al spinels (Cr# = ~15) and relatively high Al, Ti, Na, and rare earth element (REE) abundances in pyroxene, whereas the SSZ peridotites are refractory spinel harzburgites represented by high-Cr spinels (Cr# = ~40−73) and extremely low Al, Ti, Na, and REE abundances in pyroxene; dunite and orthopyroxenites from the SSZ locales have the highest Cr#s (74−77). The abyssal-type peridotites are considered to represent remnant oceanic lithosphere trapped in a SSZ setting but before being modified by subduction processes [6]. The SSZ-type peridotites are interpreted to represent partial melting in the mantle wedge above a subduction zone to form basalt (peridotites with Cr#s = ~40−55) or boninite (harzburgites and dunites with Cr# = 70−76); the latter group includes chromitites (Cr#s = 76−79) and orthopyroxenites (Cr# = 74) that may represent boninite cumulates.Three abyssal peridotite samples from Black Diamond Ridge and five SSZ peridotite samples from Little Stony Creek, Del Puerto Canyon, and Burro Mountain were obtained for this study. Also included is one chromitite from the Grey Eagle Mine, and in this sample, SSZ-type high-Cr spinel is the main constituent mineral [2]. Our samples represent five distinct locales over a geographically wide area, and previous studies [2, 6, 12] have shown their internal similarity (except for the Black Diamond Ridge lherzolites) suggesting that they are related petrologically and tectonically to one another. The abyssal peridotites represent a remnant of large-offset transform oceanic lithosphere, and we have proposed elsewhere that proto-Franciscan subduction was initiated along this fracture zone [6]. Modeling of REE shows that the abyssal peridotites are residues after ~3% dry melting of depleted MORB mantle (DMM) source in the garnet stability field (followed by conversion to spinel lherzolite at shallower depth), and the SSZ peridotites are formed by ~15%−20% further melting in the spinel stability field, possibly under hydrous conditions [12, 37].The procedures for sample preparation closely followed those described in detail by Puchtel et al. [38]. Hand specimens ~200−300 g in weight collected from surface outcrops were cut into 1−2 cm thick slabs using a diamond saw to remove any signs of alteration. The slabs were abraded on all sides using SiC sandpaper to remove saw marks, rinsed in deionized water, dried, and crushed in an alumina-faced jaw crusher. A 50 g aliquot of crushed sample was preground in a shatter box armed with alumina grinding container and puck and then reground to a flour-grade powder in an alumina-faced disk mill; the resultant powder aliquots were used for the geochemical analyses.Whole-rock major and trace element concentrations were determined using a lithium metaborate/tetraborate fusion and an inductively coupled plasma (FUS-ICP) instrument and an ICP mass spectrometer (ICP-MS), respectively, at Actlabs, Ontario, Canada. The U.S. Geological Suervey (USGS) standards (DNC-1, SY-4, and BIR-1a) were analyzed together with the unknown samples. The precision was within 5% for major elements and within 10% for most trace elements. The results are given in online supplementary Table S1.To obtain the Re−Os isotopic and highly siderophile element (HSE) abundance data, the analytical protocols detailed in Puchtel et al. [38] were followed. Approximately 1.5 g of whole-rock peridotite and 300 mg of chromitite powder, 6 mL of triple-distilled concentrated HNO3, 3 mL of triple-distilled concentrated HCl, and appropriate amounts of mixed 185Re−190Os and PGE (99Ru, 105Pd, 191Ir, and 194Pt) spikes were sealed in double internally cleaned, chilled 25 mL Pyrex™ borosilicate Carius tubes and heated to 270°C for 96 hours. Osmium was extracted from the acid solution by CCl4 solvent extraction [39], back-extracted into concentrated HBr, and purified via microdistillation [40]. Ru, Pd, Re, Ir, and Pt were separated and purified using anion-exchange chromatography following a protocol modified after [41].Osmium isotopic measurements were done via negative thermal ionization mass spectrometry [42]. All samples were analyzed using a secondary electron multiplier detector of a ThermoFisher Triton mass spectrometer at the Isotope Geochemistry Laboratory, University of Maryland, College Park. The in-run precision of measured 187Os/188Os ratios for all samples was between 0.03% and 0.06% relative. The 187Os/188Os ratio of 500 pg loads of the in-house Johnson–Matthey Os standard measured during the 2-year period leading up to the current analytical sessions averaged 0.11377 ± 10 (2 SD, N = 64). This average 187Os/188Os value is within the uncertainty of the average 187Os/188Os = 0.1137950 ± 18 measured for the Johnson-Matthey Os standard on the Faraday cups of the IGL Triton [43]; as such, no instrumental mass-bias corrections were made. The 2SD uncertainty obtained characterizes the external precision of the isotopic analyses (0.09%), which was used to estimate the true uncertainty on the measured 187Os/188Os ratios for each individual sample in this study.The measurements of Ru, Pd, Re, Ir, and Pt were performed at the Plasma Laboratory, University of Maryland, College Park, on Faraday cups of a ThermoFisher Neptune Plus ICP-MS in static mode using 1013 Ω amplifiers for all masses of interest. Instrumental isotopic mass fractionation was monitored and corrected for by interspersing samples and standards. The external precision of the analyses was estimated, based on standard measurements performed during the period of the analytical campaign, to be 185Re/187Re, 99Ru/101Ru = 0.3%, 191Ir/193Ir = 0.2%, and 194Pt/196Pt, 105Pd/106Pd = 0.10% relative (2SD). The accuracy of the data was assessed by comparing the results for the reference materials IAG MUH-1 (Austrian harzburgite), IAG OKUM (ultramafic komatiite), and NRC TDB-1 (Diabase PGE Rock Material) obtained at the IGL with the reference values. Concentrations of all HSE and Os isotopic compositions obtained at the IGL are within the uncertainties of the certified reference values [43].The average total analytical blank (TAB) measured during the present analytical campaign was (in pg): Ru 2.1, Pd 34, Re 1.1, Os 0.74, Ir 0.14, and Pt 127 (N = 9). For the peridotite samples, the average TAB constituted less than 0.1% for Os, Ir, and Ru; less than 1.0% for Pd; less than 4% for Re; and less than 6% for Pt of the total amount of element analyzed. The average TAB for the chromitite sample constituted less than 0.1% for Os, Ir, and Ru and 3.5%, 5.5%, and 16% for Re, Pt, and Pd, respectively, of the total element analyzed. We therefore cite the uncertainties on the concentrations of each element as 1/2 of the uncertainty from the TAB contribution, assuming 50% of variation in the TAB. The calculated uncertainty on the Re/Os ratio was propagated for each sample by combining the estimated uncertainties on the Re and Os abundances for each sample. The results are given in Table 1.The whole-rock major and trace element concentrations are provided in online supplementary Table S1. The samples are slightly to moderately serpentinized (18−61%; [2]) and have loss on ignition (LOI) values of 1.8% to 12.2%. The Mg number of CRO peridotites varies from 89.8 to 92.4. The CaO and Al2O3 range from 0.2 to 2.8 wt% and 0.3 to 2.7 wt%, respectively. The Ni, Co, and Cr contents range from 1820 to 2560 µg/g, 102 to 117 µg/g, and 1920 to 2900 µg/g, respectively. The V contents range from 13 to 64 µg/g. All samples show relative depletion in Cs, Rb, U, Th, and Sr. The heavy REEs have a wide range (Yb = 0.01−0.26 µg/g), but the light REEs have a relatively limited range of values (La = 0.06−0.14 µg/g), with (La/Yb)N ratios of 0.2 to 5.0 (online supplementary Table S1).The whole-rock PGE−Re concentrations of the CRO peridotites are 1.95−7.12 ng/g for Os, 2.27−4.16 ng/g for Ir, 4.52−11.59 ng/g for Ru, 1.20−10.89 ng/g for Pt, 1.21−10.15 ng/g for Pd, and 0.02−0.34 ng/g for Re (Table 1). These concentrations are within the ranges reported for mantle peridotites [17, 22]. The chromitite from Grey Eagle Mine has 96.9 ng/g for Os, 73.7 ng/g for Ir, 241.8 ng/g for Ru, 7.8 ng/g for Pt, 0.73 ng/g for Pd, and 0.10 ng/g for Re (Table 1). None of the PGE−Re concentrations exhibit any correlation with LOI values (not shown), supporting the notion that there are no secondary alteration effects. Chondrite-normalized PGE−Re abundances are shown in Figure 2. The CRO peridotites have IPGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances. Meanwhile, PPGE and Re concentrations span several orders of magnitude and are depleted overall relative to the primitive mantle. Abyssal-type CRO peridotites have slightly subchondritic PPGE−Re abundances and flat PPGE−Re patterns, whereas the SSZ-type CRO peridotites have highly fractionated PPGE−Re patterns. DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)). 75BM-3 has higher Pt concentration and Pt/Pd ratio compared to the primitive mantle (Figure 2(a)). The chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)).The CRO peridotites yield 187Os/188Os values between 0.1188 and 0.1315 (γOs = −8.3 to +1.4) and 187Re/188Os ranging from 0.022 to 0.413 (Table 1; Figure 3). The Grey Eagle Mine chromitite has 187Os/188Os of 0.1250 (γOs = −3.5) and 187Re/188Os of 0.005 (Table 1; Figure 3(a)). The Re−Os isotopic compositions do not show a correlation with LOI value (not shown), also underscoring the insignificance of secondary alteration effects. All CRO peridotites except DP-18 have 187Os/188Os ratios less radiogenic than the estimate of the primitive upper mantle (PUM = 0.1296; [44]). All samples including DP-18, however, have lower 187Re/188Os than the PUM (Figure 3(a)). Conventional Re−Os model ages (TMA; [23]) range from 7.9 Ga to the future, and Re-depletion model ages (TRD) range from 1.5 Ga to future age (Table 1). The possibility that suprachondritic Os isotopic composition of the DP-18 was induced by Os exchange with extremely radiogenic seawater of 187Os/188Os ≈ 1 [45] is excluded due to the fact that the sample has the highest 187Re/188Os ratio among the samples studied, and it has much higher Os concentration (4 ng/g) than seawater (8−10 fg/g; [46]).Representative major oxide concentrations recalculated on an anhydrous basis are shown in Figure 4. Also shown in the plots are fields for abyssal and SSZ peridotites [47, 48]. The Black Diamond Ridge peridotites plot within the field for abyssal peridotites, but the others are comparable to SSZ peridotites. In the MgO/SiO2−Al2O3/SiO2 space (Figure 5), the samples are within the range of terrestrial mantle array, indicating nearly isochemical serpentinization for the major elements. Only sample 75BM-2 had slightly higher MgO and lower SiO2, which could be due to excess olivine in this sample, as is evident in the modal proportion of olivine of up to 93.5% [2].Figure 6 shows chondrite-normalized REE patterns. Also shown in the plot are simple-model batch melting residues of DMM. The abyssal-type peridotites are depleted in light REE (LREE), which is consistent with their being melting residues [47]. In contrast, SSZ-type peridotites have U-shaped REE patterns with much lower heavy and middle REE (HREE and MREE) abundances than the Black Diamond Ridge peridotites. Strong depletion in the HREE and MREE indicates that the rocks are residues after high degree of partial melting. On the other hand, the observed enrichment in LREE requires secondary processes to have followed melt extraction. Postmelting refertilization by trace element–enriched melt or fluid of a refractory mantle in a SSZ setting can explain the elevated abundances of LREE and ultra-depletion in HREE and MREE of the samples [37, 47-50].The PGE in mantle rocks are mostly concentrated in Fe–Ni–Cu sulfides (BMS) and, in particular, monosulfide solid solution (Mss) [18, 51]. Re is not a true PGE but is often considered in conjunction with this group due to its chemical similarity and 187Os being the radioactive decay product of 187Re. The element Re exhibits siderophile behavior in metal-silicate systems and typically resides in sulfide phases in the mantle, though with some control by silicate phases [52, 53]. Depletion in PPGE−Re (Figure 2(a)) reconciles with the origin of the CRO peridotites as residues after partial melting and melt extraction. This fractionated PGE−Re patterns can be attributed to incongruent melting of BMS during mantle melting which produces Rh–Pt–Pd-enriched sulfide melt and leaves Os–Ir–Ru-enriched Mss in the residue [21].Chondrite-normalized modeled Pt/Ir and Pd/Ir ratios in residual peridotite after incongruent melting of the BMS are shown in Figure 7 for comparison. Assuming that Ir, Pd, and Pt are 100% concentrated in the BMS, their abundances were calculated using mass balance equations for nonmodal fractional melting of the BMS. The PUM is assumed to contain 150 µg/g for S, 3.5 ng/g for Ir, 7.1 ng/g for Pd, and 7.6 ng/g for Pt [54]. We have used in the calculations Mss/silicate melt partition coefficients after [20]: DIr = 3500, DPd = 370, and DPt = 360. The modeling shows that the (Pt/Ir)N and (Pd/Ir)N ratios decrease with increasing degree of partial melting (Figure 7). Except for the two samples grossly off the defined trend (DP-18 and 75BM-3), the CRO peridotites define a trend in remarkably good agreement with the modeled curve, despite uncertainties in the S contents of the primitive mantle and S solubility in partial melts. The abyssal-type CRO peridotites are residues after less than ~5% melting, and the SSZ-type CRO peridotites are residues after ~5% to 15% melting. This is consistent with the results modeled with REE (Figure 6), considering the fact that DMM represents 2%−3% melt removal from the primitive mantle [55]. Sample DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)), probably indicating secondary refertilization processes resulting in precipitation of metasomatic Cu–Ni-rich sulfides from infiltrated silicate melts [18, 51]. Sulfidization reactions between S-rich fluids and olivine, or metals dissolved in volatile-rich alkaline melt may also result in precipitation of BMS [17, 18]. In that case, however, suprachondritic Os/Ir ratios are expected in the host peridotite due to enrichment in volatile–chalcophile element such as Os [17]. Since DP-18 has a chondritic Os/Ir ratio (Figures 2(a) and 8(a)), this possibility is excluded for this sample. Sample 75BM-3, on the other hand, has suprachondritic (Pt/Ir)N but subchondritic (Pd/Ir)N ratios (Figure 7). Also, 75BM-3 has higher Pt concentration than that in the primitive mantle (Figure 2(a)), which cannot be explained by melt removal, as Pt has slightly lower partition coefficients in sulfides than that of Pd [21]. The Pt excess observed in sample 75BM-3 may be due to nugget effects of discrete Pt-rich microphases, possibly produced during secondary melt percolation or serpentinization [51, 56-58].The 187Os/188Os ratios for the CRO peridotites display generally positive correlation with 187Re/188Os but do not define meaningful Re−Os isochrons (Figure 3(a)), likely indicating Re mobility in the samples. The data point distributions result in unrealistic TMA model ages with some sample sets yielding future ages while others produce ages older than the age of the Earth (Table 1). The open-system behavior of Re is widely observed in mantle peridotites [23, 46, 59, 60]. The TRD model age, which corresponds to the minimum estimates of the age of melt extraction and assumes no change in 187Os/188Os after time of melt extraction [23], can be an alternative dating method. However, it can provide a good approximation for the time of melting in highly refractory peridotites (e.g. SSZ type) but significantly underestimate for relatively fertile samples (e.g. abyssal type). While the Re−Os isotope data do not define an isochron, all samples possess 187Re/188Os ratios lower than the putative PUM (Figure 2(a)), suggesting that metasomatic addition of Re has not been significant. This is also substantiated by the samples’ suprachondritic (Pd/Re)N ratios (Figure 2(a)). For comparison, the reference isochron forced through PUM of 165 Ma, the CRO formation age, is shown in Figure 3(a). It should be pointed out that the CRO peridotites yield much steeper array compared with the reference line in the plot, indicating its origination from a protolith that had undergone ancient melt depletion episode(s).Al2O3, CaO, or Yb are usually considered to exhibit degrees of incompatibility similar to that of Re during mantle melting but are less easily perturbed than Re during metasomatism [60, 61]. Therefore, Al2O3 content is often used as a proxy for Re/Os to estimate the mantle separation ages of peridotites. In the alumina 187Os/188Os peudo-isochron (aluminachron; Figure 3(b)), the abyssal-type and SSZ-type CRO peridotites show two independent correlations, implying temporally distinct melting episodes. The trend of abyssal-type CRO peridotites (solid circles) passes through the PUM in the aluminachron, suggesting that these rocks might have originated from a PUM-like source. Assuming that there has been a single or multiple melting events closely spaced in time, the 187Os/188Os ratio at ~0.7 wt% Al2O3 as representative of Re/Os = 0 in anhydrous melting [61, 62] on the aluminachron can be considered as the initial ratio, which corresponds to TRD of 1.50 ± 0.13 Ga for the abyssal-type CRO peridotites (Figure 3(b)). This implies that the CRO mantle had experienced episode(s) of melt extraction before the abyssal-type CRO crust was formed. While this time of lithospheric stabilization seems too old to be expected in abyssal peridotite, the age decoupling between abyssal or fore-arc peridotites and their overlying crust, and long-term preservation of refractory domains, including a parcel of buoyant arc mantle, in the asthenosphere have been reported in several cases [46, 49, 63-67]. The aluminachron age of ~1.5 Ga for the abyssal-type lherzolites corresponds roughly to the age of rifting in the Columbia supercontinent, separating Laurentia/Baltica/Siberia from Antarctica/North Australia/South America to form an ocean basin that would evolve into the proto-Pacific Ocean basin in the Phanerozoic [68]. The Os isotopic evidence for ancient melting can be explained by unradiogenic sulfides or platinum-group metals encapsulated in silicate or oxide hosts and protected from subsequent melting or diffusion [52, 59, 65].The SSZ-type CRO peridotites show a positive correlation with a much steeper slope in the aluminachron (open symbols on the diagram in Figure 3b). However, the most refractory SSZ-type sample 75BM-2 (represented by the lowest 187Os/188Os ratio and Al2O3 content) falls on the lower limit of the correlation for abyssal-type peridotites, indicating that the abyssal-type CRO peridotites might have been the source for the second melt extraction event. It is estimated that the second event(s) occurred at ~165 Ma, but accurate dating is difficult with currently available data. We have shown a regression line for Burro Mountain peridotites for comparison in the plot (Figure 3(b)). We observe that samples with and without metasomatic overprinting in the Burro Mountain and Del Puerto peridotites are collinear in the plot (Figure 3(b)), which suggests a possibility of coeval second-stage melting episode and melt percolation events.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites [28, 69-71]. For example, in the Izu−Bonin−Marianas (IBM) fore-arc, the former stratigraphically underlie the latter [28, 69-73]. Fore-arc basalts are tholeiitic with major element compositions at the depleted end of the MORB array [70, 73-75]. They are characterized by being lower in TiO2, P2O5, Zr, and LREEs than MORB, implying their derivation from mantle that is more refractory than MORB-source mantle [70, 76, 77]. The fore-arc magmas are considered to be generated by MORB-like mantle decompression melting during near-trench spreading with little or no mass transfer from the subducting plate [28, 69-72].Compared with fore-arc basalts, boninites have higher MgO and SiO2, lower TiO2, and relatively higher concentrations of fluid-mobile trace elements (e.g. K and Sr) [28, 71, 74, 78]. Boninites were generated later when the residual, highly depleted mantle (refractory harzburgite) after extraction of the fore-arc basalts melted at relatively shallow levels after being fluxed by fluids and melts from the subducting plate [27, 28, 69, 71, 79]. Boninitic lavas are located stratigraphically above arc tholeiite basalts in the CRO [4, 35].Vanadium is a moderately incompatible element and exists in multiple oxidation states (V3+, V4+, and V5+) in terrestrial magmas. Partition coefficient for V in mantle phases therefore show a strong dependence on oxygen fugacity, which decreases with increasing fO2 [80-82]. Vanadium concentration can thus provide information on oxygen fugacity. Figure 8 is a plot of V against Yb for all CRO peridotite samples, with modeled trends for fractional melting in spinel lherzolite facies at different oxygen fugacities. The V contents of the CRO peridotites show a positive correlation with Yb, indicating a dominant control by igneous processes rather than hydrothermal alteration. It is worth noting that most CRO peridotites fall along the fayalite−magnetite−quartz (FMQ) buffer, regardless of whether they are abyssal or SSZ types. The Black Diamond Ridge peridotites fall within the range of abyssal peridotites, and SSZ-type CRO peridotites overlap with the abyssal peridotite field, but they also extend to lower concentrations beyond that field; one sample (75BM-2) appears to somewhat more oxidized than the others. The oxygen fugacity of the MORB source is at or just above the FMQ buffer, whereas subarc mantle is generally more oxidized than ambient mantle, including a substantial proportion of rocks with fO2 > FMQ+1 [83-85]. Oxidation of the mantle is attributed to the infiltration of oxidized fluids or silicic melts derived from the subduction slab, where sulfate could be the most plausible oxidant [85, 86]. This observation indicates that the CRO peridotites had limited interaction with oxidizing fluids in the fore-arc setting due to the relatively short residence time above a dehydrating slab, and fore-arc basalts are generated mainly by decompression melting [83, 87, 88]. This finding supports the claim of Birner et al. [49] that the fore-arc mantle is not pervasively oxidized relative to mid-ocean ridge mantle.Supra-chondritic Os isotope ratios (up to 187Os/188Os ~0.157) are commonly found in arc peridotite xenoliths [24, 25, 89], which is attributed to inputs of radiogenic Os from a subducted oceanic crust and sediments. Osmium can be mobile during slab dehydration or melting processes, and its mobility in a fluid or a melt increases with increasing oxygen fugacity and salinity [26, 90]. The CRO peridotites studied, however, are characterized by subchondritic Os isotopic compositions (Figure 3). In a fore-arc region, slab dehydration and fluid salinity are relatively low in cold subduction and high in hot subduction [91]. Oxygen fugacity equivalent to the FMQ buffer in a fore-arc setting (Figure 8) and possibly cold subduction as evidenced by slight to moderate serpentinization (18−61%; [2]) observed in the CRO peridotites would have resulted in Os migration being limited in this setting. Unradiogenic 187Os/188Os ratios are also observed in IBM fore-arc peridotites [66]. This is independently supported by predominantly chondritic Os/Ir ratios observed in the CRO peridotites studied (Figure 9(a)). Also note that the Os isotopic ratios do not show correlation with Os content of the peridotites (Figure 9(b)), ruling out any mixing processes. Sample 75BM-2 has slightly suprachondritic Os/Ir ratio (Figure 9(a)) and Os and Ru enrichment relative to Ir (Figure 2(a)). Meanwhile, 75BM-2 has the lowest 187Os/188Os ratio among the CRO peridotites (Figure 9(b)), indicating that the elevated Os–Ru abundances could be nugget effect (i.e. heterogeneous distribution of laurite or Ru–Os alloy) rather than secondary fluid infiltration.This finding, however, is not in line with the results of a previous study of serpentinized peridotites from Point Sal, the southernmost CRO remnant, carried out by Snortum and Day [7]. The Point Sal peridotites record significant melt depletion (>20%), which resulted in exhaustion of sulfides in the source during melting, leading to loss of Os (open diamonds in Figure 9). Meanwhile, the Point Sal peridotites are characterized by radiogenic 187Os/188Os (Figure 9(b)) and fractionated Os/Ir ratios (Figure 9(a)), as well as suprachondritic (Pd/Ir)N (~3−25) or (Pt/Ir)N (~2−6) ratios, and enrichments in Ba, U, and Sr [7]. Also note that the major element compositions of the Point Sal peridotites are less systematic than ours, and some samples (PS1703) are highly enriched in Al2O3 and CaO (Figures 4 and 5, and online supplementary Figures S1 and S2) but depleted in Sc (not shown). Taken together with the high LOI values (14−18 wt%) of the Point Sal samples, this is probably an effect of hydrothermal alteration and secondary gabbroic melt(s) infiltration. That is, the Point Sal peridotites could be a mantle section associated with Cl-rich oxidizing fluid-driven melt depletion and refertilization processes in a fore-arc setting [7].Grey Eagle Mine chromitites have a chemical composition with boninitic affinity [2]. Podiform chromitite is considered to be the combined product of crystal fractionation in the early stages and melt-harzburgite reaction in an open system as primitive hydrous melts migrate through the upper mantle in a SSZ setting [92]. Dissolution of orthopyroxene in harzburgite by reaction with hydrous melts can produce dunite coupled with crystallization of high-Cr# chromite. The chromite in boninites could thus be the products of primitive melt crystallization in the crust or of incongruent melting of orthopyroxene in the uppermost mantle, which is carried to the surface by entrapment in the melts [93]. The studied chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)). Ru shows more compatible behavior than Os and Ir in chromite as observed in the previous studies [94, 95]. The IPGE-chromite association could be due to the presence of IPGE minerals (IPGM: laurite, erlichmanite, Os–Ir ± Ru alloys, and so on) in chromite [95] or IPGE incorporation in solid solution into the chromite structure [94, 96]. The IPGM could be one of the first phases to crystallize from cooling magma, or refractory residual phase during partial melting in the mantle, which were then entrapped during growth of chromite [97-99]. Available PGE−Re abundances of boninites are shown in Figure 2(b) for comparison. The boninites show PGE−Re patterns complementary to that of chromitite. This observation suggests that the boninitic melts might be generated during concomitant fractional crystallization of olivine and chromite [93, 97]. However, the number of samples analyzed is insufficient to generalize and needs to be validated with more data in the future. The Os isotope composition of the chromitite is within the range of the initial 187Os/188Os ratios of the CRO peridotites (Figure 3(a)), suggesting indistinguishable flux of radiogenic Os from the slab into the mantle source of the boninite.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.The data are provided in the article and the supplementary materials.The authors declare that they have no conflicts of interest.This study was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (NRF-2022R1A2C1003508).We thank associate editor Chuan-Lin Zhang and two anonymous reviewers for their constructive comments on this article.The supplementary materials available include an Excel table and two figures. Table S1 provides major and trace element concentrations for spinel peridotites and chromitite from Coast Range ophiolite, California. Figure S1 shows the major element compositions for Point Sal peridotites, the southernmost Coast Range ophiolite remnant. Figure S2 shows the MgO/SiO2 versus Al2O3/SiO2 ratios for Point Sal peridotites.","PeriodicalId":18147,"journal":{"name":"Lithosphere","volume":null,"pages":null},"PeriodicalIF":1.8000,"publicationDate":"2024-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Re−Os Isotope and PGE Abundance Systematics of Coast Range Ophiolite Peridotites and Chromitite, California: Insights into Fore-Arc Magmatic Processes\",\"authors\":\"Sung Hi Choi, Samuel B. Mukasa, John W. Shervais, Igor S. Puchtel\",\"doi\":\"10.2113/2024/lithosphere_2024_154\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"We report platinum-group element (PGE) and Re concentrations, and Re−Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)N ≈ 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)−Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE−Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = −8.3 to 1.4) and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite−magnetite−quartz buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after ~5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has 187Os/188Os value of 0.1250 (γOs = −3.5) and PGE−Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.Ophiolites are sections of the Earth’s oceanic crust and the underlying upper mantle that have been tectonically emplaced into continental margins, providing important insights into the processes of plate tectonics, the composition of the oceanic crust and mantle, and the dynamics of Earth’s interior. Ophiolites are also valuable as ore deposits hosting precious metals, including platinum-group elements (PGEs), ferrous metals (Cr, Mn, and Ti), and base metals (Co, Cu, and Ni). The oceanic crust preserved in ophiolites may form in any tectonic setting during the evolution of ocean basins, from the mid-ocean ridge to subduction initiation and final closure [1]. The Coast Range ophiolite (CRO) is a mid-Jurassic (~172 to 161 Ma) ophiolite terrane in central California, extending over 700 km from Elder Creek at its northernmost segment extent to Point Sal at its southernmost terminus [2-5] (Figure 1). Petrologic and geochemical data indicate its formation in a supra-subduction zone (SSZ) fore-arc setting, probably above the east-dipping proto-Franciscan subduction zone [2, 6, 7]. Initiation of the subduction is considered to have possibly started along a large-offset transform fault zone, when an exotic or fringing is collided with North America [5, 6, 8]. The arc-like geochemical characteristics of pyroclastic, volcaniclastic, and hypabyssal materials of ophiolitic crust provide important evidence for the SSZ setting [4, 9, 10]. The possibility of the CRO having formed in a distal mid-ocean ridge or back-arc settings has also been proposed [11]. However, a paleo-equatorial mid-ocean spreading origin for the CRO peridotites is ruled out due to the SSZ-like highly depleted geochemical characteristics observed in several suites of CRO peridotites [2, 7, 12] and paleomagnetic evidence for limited north–south movement [13, 14]. A back-arc setting has remained plausible because back-arc extension and magmatism would be contemporaneous with arc volcanism, and it is difficult to geochemically distinguish arc-related magmas produced in fore-arc, intra-arc, or back-arc settings [15]. Since peridotites from back-arc basin are generally similar to abyssal peridotites [16], however, such a possibility has also been excluded [2]. Meanwhile, in the SSZ setting, the melt extraction process (e.g. fore-arc basalt or boninitic) to produce the current CRO peridotites has remained unidentified.The Re−Os isotope system and PGE abundances can play a pivotal role in understanding petrogenetic processes in peridotites, such as mantle melting and fluid/melt-rock interaction [17]. In upper mantle peridotites, PGEs are dominated by base-metal sulfides (BMSs) due to their highly chalcophile characteristics [18, 19]. Based on melting temperature (Tm), the PGEs are classified into two groups: Ir-group (IPGE: Os, Ir, and Ru, Tm > 2000°C) and Pd-group (PPGE: Pt, Pd, and Rh, Tm < 2000°C) [20]. During partial melting, the IPGE group resides in residual phases (e.g. sulfides, alloys, or oxides) and thus behaves compatibly relative to the PPGE group [21]. Re behaves as a mildly incompatible element during mantle melting, which results in residual peridotites with much lower Re/Os ratios than the fertile precursor [22]. The Re−Os isotopic system thus has great potential for dating melt depletion events in peridotites [23]. The high concentration of Os in the melting residue renders the Os isotopic compositions resistant to subsequent metasomatic processes. Os and Ir are not significantly fractionated during anhydrous partial melting [20, 21]. However, Os can be partitioned more than Ir into Cl-rich oxidizing slab-derived hydrous fluids or melts [24-26].Understanding the origin of the CRO is essential for unraveling the Mesozoic evolution of Cordilleran continental margin, and its development has implications for the origin of other continental margin ophiolites and provides a point of comparison with modern intra-oceanic fore-arcs (e.g. the Izu−Bonin system; [27, 28]). PGE compositions of fore-arc mantle have been rarely reported so far, yet we have indicated their potential for providing some profound insights into melting processes. This study presents PGE abundance and Re−Os isotope data for CRO peridotites and a podiform chromitite. Our aim is to constrain the formation and evolution of the CRO in a fore-arc regime and to evaluate its significance in the broader realm of ophiolite formation.The CRO is a sequence of dismembered oceanic rocks in fault contact with the underlying late Mesozoic−early Paleogene Franciscan Complex and is overlain by Upper Jurassic strata of the Great Valley sequence [3, 4, 29]. Crustal rocks (gabbro, diorite, basalt, and andesite) are the most abundant lithologies in the ophiolite though serpentinized peridotite tectonite is also widely distributed, and in some areas, it is the predominant lithology [30-32]. Igneous rocks of the ophiolite are dominantly tholeiitic basalts and basaltic andesites with arc affinities, with less common boninite, andesite, and primitive olivine–clinopyroxene–phyric basalts [3, 4]. CRO localities with extensive crustal sections in the northern Sacramento Valley (Elder Creek, Stonyford) and Diablo Range (Del Puerto Canyon, Llanada) are characterized by common felsic volcanic rocks (andesite, “keratophyre”) and shallow plutonic rocks comprising diorite, tonalite, and trondhjemite with calc-alkaline affinities [9, 33-35]. The felsic calc-alkaline series rocks overlie or cross-cut the older arc tholeiite series rocks and are in turn overlain or cross-cut by late MORB-like lavas and dikes [4, 35]. The latter magmatism is considered to have been caused by ridge collision, resulting in termination of ophiolite formation due to the change in relative plate motion and shallow subduction of young, buoyant crust [4].The CRO ranges in age from ~172 to ~161 Ma, based on U−Pb zircon ages from plagiogranite and quartz diorites associated with the ophiolite and on Ar−Ar ages of basaltic glass at Stonyford [3, 5, 36]. High-resolution U−Pb zircon chemical abrasion ages define a tighter age range of 161.2 to 167.9 Ma [3, 36]. The youngest age (~161.2 ± 0.1 Ma) is for felsic dikes in the Del Puerto ophiolite; the oldest age is from Elder Creek (~167.9 ± 0.3 Ma). Ages for most CRO locales cluster tightly at ~165 Ma, including the Ar−Ar ages on volcanic glass (164.2 ± 0.7 Ma), suggesting rapid onset of subduction and a short formation interval of circa 7 million years.Eight peridotites and one chromitite were obtained for this study, purposely sampling outcrops previously shown to exhibit the different crustal-rock associations described earlier. Chromitite occurs as irregular pods and lenses in dunite. Sample locations are shown in Figure 1, which include Grey Eagle Mine near Chrome (Red Mountain), Black Diamond Ridge (north of the Stonyford volcanic complex), Little Stony Creek (south of the Stonyford volcanic complex), Del Puerto Canyon, and the Burro Mountain.Based on mineral chemistry, previous studies [2, 12] divided the CRO peridotites into two groups: abyssal and SSZ peridotites. Abyssal peridotites are spinel lherzolites characterized by high-Al spinels (Cr# = ~15) and relatively high Al, Ti, Na, and rare earth element (REE) abundances in pyroxene, whereas the SSZ peridotites are refractory spinel harzburgites represented by high-Cr spinels (Cr# = ~40−73) and extremely low Al, Ti, Na, and REE abundances in pyroxene; dunite and orthopyroxenites from the SSZ locales have the highest Cr#s (74−77). The abyssal-type peridotites are considered to represent remnant oceanic lithosphere trapped in a SSZ setting but before being modified by subduction processes [6]. The SSZ-type peridotites are interpreted to represent partial melting in the mantle wedge above a subduction zone to form basalt (peridotites with Cr#s = ~40−55) or boninite (harzburgites and dunites with Cr# = 70−76); the latter group includes chromitites (Cr#s = 76−79) and orthopyroxenites (Cr# = 74) that may represent boninite cumulates.Three abyssal peridotite samples from Black Diamond Ridge and five SSZ peridotite samples from Little Stony Creek, Del Puerto Canyon, and Burro Mountain were obtained for this study. Also included is one chromitite from the Grey Eagle Mine, and in this sample, SSZ-type high-Cr spinel is the main constituent mineral [2]. Our samples represent five distinct locales over a geographically wide area, and previous studies [2, 6, 12] have shown their internal similarity (except for the Black Diamond Ridge lherzolites) suggesting that they are related petrologically and tectonically to one another. The abyssal peridotites represent a remnant of large-offset transform oceanic lithosphere, and we have proposed elsewhere that proto-Franciscan subduction was initiated along this fracture zone [6]. Modeling of REE shows that the abyssal peridotites are residues after ~3% dry melting of depleted MORB mantle (DMM) source in the garnet stability field (followed by conversion to spinel lherzolite at shallower depth), and the SSZ peridotites are formed by ~15%−20% further melting in the spinel stability field, possibly under hydrous conditions [12, 37].The procedures for sample preparation closely followed those described in detail by Puchtel et al. [38]. Hand specimens ~200−300 g in weight collected from surface outcrops were cut into 1−2 cm thick slabs using a diamond saw to remove any signs of alteration. The slabs were abraded on all sides using SiC sandpaper to remove saw marks, rinsed in deionized water, dried, and crushed in an alumina-faced jaw crusher. A 50 g aliquot of crushed sample was preground in a shatter box armed with alumina grinding container and puck and then reground to a flour-grade powder in an alumina-faced disk mill; the resultant powder aliquots were used for the geochemical analyses.Whole-rock major and trace element concentrations were determined using a lithium metaborate/tetraborate fusion and an inductively coupled plasma (FUS-ICP) instrument and an ICP mass spectrometer (ICP-MS), respectively, at Actlabs, Ontario, Canada. The U.S. Geological Suervey (USGS) standards (DNC-1, SY-4, and BIR-1a) were analyzed together with the unknown samples. The precision was within 5% for major elements and within 10% for most trace elements. The results are given in online supplementary Table S1.To obtain the Re−Os isotopic and highly siderophile element (HSE) abundance data, the analytical protocols detailed in Puchtel et al. [38] were followed. Approximately 1.5 g of whole-rock peridotite and 300 mg of chromitite powder, 6 mL of triple-distilled concentrated HNO3, 3 mL of triple-distilled concentrated HCl, and appropriate amounts of mixed 185Re−190Os and PGE (99Ru, 105Pd, 191Ir, and 194Pt) spikes were sealed in double internally cleaned, chilled 25 mL Pyrex™ borosilicate Carius tubes and heated to 270°C for 96 hours. Osmium was extracted from the acid solution by CCl4 solvent extraction [39], back-extracted into concentrated HBr, and purified via microdistillation [40]. Ru, Pd, Re, Ir, and Pt were separated and purified using anion-exchange chromatography following a protocol modified after [41].Osmium isotopic measurements were done via negative thermal ionization mass spectrometry [42]. All samples were analyzed using a secondary electron multiplier detector of a ThermoFisher Triton mass spectrometer at the Isotope Geochemistry Laboratory, University of Maryland, College Park. The in-run precision of measured 187Os/188Os ratios for all samples was between 0.03% and 0.06% relative. The 187Os/188Os ratio of 500 pg loads of the in-house Johnson–Matthey Os standard measured during the 2-year period leading up to the current analytical sessions averaged 0.11377 ± 10 (2 SD, N = 64). This average 187Os/188Os value is within the uncertainty of the average 187Os/188Os = 0.1137950 ± 18 measured for the Johnson-Matthey Os standard on the Faraday cups of the IGL Triton [43]; as such, no instrumental mass-bias corrections were made. The 2SD uncertainty obtained characterizes the external precision of the isotopic analyses (0.09%), which was used to estimate the true uncertainty on the measured 187Os/188Os ratios for each individual sample in this study.The measurements of Ru, Pd, Re, Ir, and Pt were performed at the Plasma Laboratory, University of Maryland, College Park, on Faraday cups of a ThermoFisher Neptune Plus ICP-MS in static mode using 1013 Ω amplifiers for all masses of interest. Instrumental isotopic mass fractionation was monitored and corrected for by interspersing samples and standards. The external precision of the analyses was estimated, based on standard measurements performed during the period of the analytical campaign, to be 185Re/187Re, 99Ru/101Ru = 0.3%, 191Ir/193Ir = 0.2%, and 194Pt/196Pt, 105Pd/106Pd = 0.10% relative (2SD). The accuracy of the data was assessed by comparing the results for the reference materials IAG MUH-1 (Austrian harzburgite), IAG OKUM (ultramafic komatiite), and NRC TDB-1 (Diabase PGE Rock Material) obtained at the IGL with the reference values. Concentrations of all HSE and Os isotopic compositions obtained at the IGL are within the uncertainties of the certified reference values [43].The average total analytical blank (TAB) measured during the present analytical campaign was (in pg): Ru 2.1, Pd 34, Re 1.1, Os 0.74, Ir 0.14, and Pt 127 (N = 9). For the peridotite samples, the average TAB constituted less than 0.1% for Os, Ir, and Ru; less than 1.0% for Pd; less than 4% for Re; and less than 6% for Pt of the total amount of element analyzed. The average TAB for the chromitite sample constituted less than 0.1% for Os, Ir, and Ru and 3.5%, 5.5%, and 16% for Re, Pt, and Pd, respectively, of the total element analyzed. We therefore cite the uncertainties on the concentrations of each element as 1/2 of the uncertainty from the TAB contribution, assuming 50% of variation in the TAB. The calculated uncertainty on the Re/Os ratio was propagated for each sample by combining the estimated uncertainties on the Re and Os abundances for each sample. The results are given in Table 1.The whole-rock major and trace element concentrations are provided in online supplementary Table S1. The samples are slightly to moderately serpentinized (18−61%; [2]) and have loss on ignition (LOI) values of 1.8% to 12.2%. The Mg number of CRO peridotites varies from 89.8 to 92.4. The CaO and Al2O3 range from 0.2 to 2.8 wt% and 0.3 to 2.7 wt%, respectively. The Ni, Co, and Cr contents range from 1820 to 2560 µg/g, 102 to 117 µg/g, and 1920 to 2900 µg/g, respectively. The V contents range from 13 to 64 µg/g. All samples show relative depletion in Cs, Rb, U, Th, and Sr. The heavy REEs have a wide range (Yb = 0.01−0.26 µg/g), but the light REEs have a relatively limited range of values (La = 0.06−0.14 µg/g), with (La/Yb)N ratios of 0.2 to 5.0 (online supplementary Table S1).The whole-rock PGE−Re concentrations of the CRO peridotites are 1.95−7.12 ng/g for Os, 2.27−4.16 ng/g for Ir, 4.52−11.59 ng/g for Ru, 1.20−10.89 ng/g for Pt, 1.21−10.15 ng/g for Pd, and 0.02−0.34 ng/g for Re (Table 1). These concentrations are within the ranges reported for mantle peridotites [17, 22]. The chromitite from Grey Eagle Mine has 96.9 ng/g for Os, 73.7 ng/g for Ir, 241.8 ng/g for Ru, 7.8 ng/g for Pt, 0.73 ng/g for Pd, and 0.10 ng/g for Re (Table 1). None of the PGE−Re concentrations exhibit any correlation with LOI values (not shown), supporting the notion that there are no secondary alteration effects. Chondrite-normalized PGE−Re abundances are shown in Figure 2. The CRO peridotites have IPGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances. Meanwhile, PPGE and Re concentrations span several orders of magnitude and are depleted overall relative to the primitive mantle. Abyssal-type CRO peridotites have slightly subchondritic PPGE−Re abundances and flat PPGE−Re patterns, whereas the SSZ-type CRO peridotites have highly fractionated PPGE−Re patterns. DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)). 75BM-3 has higher Pt concentration and Pt/Pd ratio compared to the primitive mantle (Figure 2(a)). The chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)).The CRO peridotites yield 187Os/188Os values between 0.1188 and 0.1315 (γOs = −8.3 to +1.4) and 187Re/188Os ranging from 0.022 to 0.413 (Table 1; Figure 3). The Grey Eagle Mine chromitite has 187Os/188Os of 0.1250 (γOs = −3.5) and 187Re/188Os of 0.005 (Table 1; Figure 3(a)). The Re−Os isotopic compositions do not show a correlation with LOI value (not shown), also underscoring the insignificance of secondary alteration effects. All CRO peridotites except DP-18 have 187Os/188Os ratios less radiogenic than the estimate of the primitive upper mantle (PUM = 0.1296; [44]). All samples including DP-18, however, have lower 187Re/188Os than the PUM (Figure 3(a)). Conventional Re−Os model ages (TMA; [23]) range from 7.9 Ga to the future, and Re-depletion model ages (TRD) range from 1.5 Ga to future age (Table 1). The possibility that suprachondritic Os isotopic composition of the DP-18 was induced by Os exchange with extremely radiogenic seawater of 187Os/188Os ≈ 1 [45] is excluded due to the fact that the sample has the highest 187Re/188Os ratio among the samples studied, and it has much higher Os concentration (4 ng/g) than seawater (8−10 fg/g; [46]).Representative major oxide concentrations recalculated on an anhydrous basis are shown in Figure 4. Also shown in the plots are fields for abyssal and SSZ peridotites [47, 48]. The Black Diamond Ridge peridotites plot within the field for abyssal peridotites, but the others are comparable to SSZ peridotites. In the MgO/SiO2−Al2O3/SiO2 space (Figure 5), the samples are within the range of terrestrial mantle array, indicating nearly isochemical serpentinization for the major elements. Only sample 75BM-2 had slightly higher MgO and lower SiO2, which could be due to excess olivine in this sample, as is evident in the modal proportion of olivine of up to 93.5% [2].Figure 6 shows chondrite-normalized REE patterns. Also shown in the plot are simple-model batch melting residues of DMM. The abyssal-type peridotites are depleted in light REE (LREE), which is consistent with their being melting residues [47]. In contrast, SSZ-type peridotites have U-shaped REE patterns with much lower heavy and middle REE (HREE and MREE) abundances than the Black Diamond Ridge peridotites. Strong depletion in the HREE and MREE indicates that the rocks are residues after high degree of partial melting. On the other hand, the observed enrichment in LREE requires secondary processes to have followed melt extraction. Postmelting refertilization by trace element–enriched melt or fluid of a refractory mantle in a SSZ setting can explain the elevated abundances of LREE and ultra-depletion in HREE and MREE of the samples [37, 47-50].The PGE in mantle rocks are mostly concentrated in Fe–Ni–Cu sulfides (BMS) and, in particular, monosulfide solid solution (Mss) [18, 51]. Re is not a true PGE but is often considered in conjunction with this group due to its chemical similarity and 187Os being the radioactive decay product of 187Re. The element Re exhibits siderophile behavior in metal-silicate systems and typically resides in sulfide phases in the mantle, though with some control by silicate phases [52, 53]. Depletion in PPGE−Re (Figure 2(a)) reconciles with the origin of the CRO peridotites as residues after partial melting and melt extraction. This fractionated PGE−Re patterns can be attributed to incongruent melting of BMS during mantle melting which produces Rh–Pt–Pd-enriched sulfide melt and leaves Os–Ir–Ru-enriched Mss in the residue [21].Chondrite-normalized modeled Pt/Ir and Pd/Ir ratios in residual peridotite after incongruent melting of the BMS are shown in Figure 7 for comparison. Assuming that Ir, Pd, and Pt are 100% concentrated in the BMS, their abundances were calculated using mass balance equations for nonmodal fractional melting of the BMS. The PUM is assumed to contain 150 µg/g for S, 3.5 ng/g for Ir, 7.1 ng/g for Pd, and 7.6 ng/g for Pt [54]. We have used in the calculations Mss/silicate melt partition coefficients after [20]: DIr = 3500, DPd = 370, and DPt = 360. The modeling shows that the (Pt/Ir)N and (Pd/Ir)N ratios decrease with increasing degree of partial melting (Figure 7). Except for the two samples grossly off the defined trend (DP-18 and 75BM-3), the CRO peridotites define a trend in remarkably good agreement with the modeled curve, despite uncertainties in the S contents of the primitive mantle and S solubility in partial melts. The abyssal-type CRO peridotites are residues after less than ~5% melting, and the SSZ-type CRO peridotites are residues after ~5% to 15% melting. This is consistent with the results modeled with REE (Figure 6), considering the fact that DMM represents 2%−3% melt removal from the primitive mantle [55]. Sample DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)), probably indicating secondary refertilization processes resulting in precipitation of metasomatic Cu–Ni-rich sulfides from infiltrated silicate melts [18, 51]. Sulfidization reactions between S-rich fluids and olivine, or metals dissolved in volatile-rich alkaline melt may also result in precipitation of BMS [17, 18]. In that case, however, suprachondritic Os/Ir ratios are expected in the host peridotite due to enrichment in volatile–chalcophile element such as Os [17]. Since DP-18 has a chondritic Os/Ir ratio (Figures 2(a) and 8(a)), this possibility is excluded for this sample. Sample 75BM-3, on the other hand, has suprachondritic (Pt/Ir)N but subchondritic (Pd/Ir)N ratios (Figure 7). Also, 75BM-3 has higher Pt concentration than that in the primitive mantle (Figure 2(a)), which cannot be explained by melt removal, as Pt has slightly lower partition coefficients in sulfides than that of Pd [21]. The Pt excess observed in sample 75BM-3 may be due to nugget effects of discrete Pt-rich microphases, possibly produced during secondary melt percolation or serpentinization [51, 56-58].The 187Os/188Os ratios for the CRO peridotites display generally positive correlation with 187Re/188Os but do not define meaningful Re−Os isochrons (Figure 3(a)), likely indicating Re mobility in the samples. The data point distributions result in unrealistic TMA model ages with some sample sets yielding future ages while others produce ages older than the age of the Earth (Table 1). The open-system behavior of Re is widely observed in mantle peridotites [23, 46, 59, 60]. The TRD model age, which corresponds to the minimum estimates of the age of melt extraction and assumes no change in 187Os/188Os after time of melt extraction [23], can be an alternative dating method. However, it can provide a good approximation for the time of melting in highly refractory peridotites (e.g. SSZ type) but significantly underestimate for relatively fertile samples (e.g. abyssal type). While the Re−Os isotope data do not define an isochron, all samples possess 187Re/188Os ratios lower than the putative PUM (Figure 2(a)), suggesting that metasomatic addition of Re has not been significant. This is also substantiated by the samples’ suprachondritic (Pd/Re)N ratios (Figure 2(a)). For comparison, the reference isochron forced through PUM of 165 Ma, the CRO formation age, is shown in Figure 3(a). It should be pointed out that the CRO peridotites yield much steeper array compared with the reference line in the plot, indicating its origination from a protolith that had undergone ancient melt depletion episode(s).Al2O3, CaO, or Yb are usually considered to exhibit degrees of incompatibility similar to that of Re during mantle melting but are less easily perturbed than Re during metasomatism [60, 61]. Therefore, Al2O3 content is often used as a proxy for Re/Os to estimate the mantle separation ages of peridotites. In the alumina 187Os/188Os peudo-isochron (aluminachron; Figure 3(b)), the abyssal-type and SSZ-type CRO peridotites show two independent correlations, implying temporally distinct melting episodes. The trend of abyssal-type CRO peridotites (solid circles) passes through the PUM in the aluminachron, suggesting that these rocks might have originated from a PUM-like source. Assuming that there has been a single or multiple melting events closely spaced in time, the 187Os/188Os ratio at ~0.7 wt% Al2O3 as representative of Re/Os = 0 in anhydrous melting [61, 62] on the aluminachron can be considered as the initial ratio, which corresponds to TRD of 1.50 ± 0.13 Ga for the abyssal-type CRO peridotites (Figure 3(b)). This implies that the CRO mantle had experienced episode(s) of melt extraction before the abyssal-type CRO crust was formed. While this time of lithospheric stabilization seems too old to be expected in abyssal peridotite, the age decoupling between abyssal or fore-arc peridotites and their overlying crust, and long-term preservation of refractory domains, including a parcel of buoyant arc mantle, in the asthenosphere have been reported in several cases [46, 49, 63-67]. The aluminachron age of ~1.5 Ga for the abyssal-type lherzolites corresponds roughly to the age of rifting in the Columbia supercontinent, separating Laurentia/Baltica/Siberia from Antarctica/North Australia/South America to form an ocean basin that would evolve into the proto-Pacific Ocean basin in the Phanerozoic [68]. The Os isotopic evidence for ancient melting can be explained by unradiogenic sulfides or platinum-group metals encapsulated in silicate or oxide hosts and protected from subsequent melting or diffusion [52, 59, 65].The SSZ-type CRO peridotites show a positive correlation with a much steeper slope in the aluminachron (open symbols on the diagram in Figure 3b). However, the most refractory SSZ-type sample 75BM-2 (represented by the lowest 187Os/188Os ratio and Al2O3 content) falls on the lower limit of the correlation for abyssal-type peridotites, indicating that the abyssal-type CRO peridotites might have been the source for the second melt extraction event. It is estimated that the second event(s) occurred at ~165 Ma, but accurate dating is difficult with currently available data. We have shown a regression line for Burro Mountain peridotites for comparison in the plot (Figure 3(b)). We observe that samples with and without metasomatic overprinting in the Burro Mountain and Del Puerto peridotites are collinear in the plot (Figure 3(b)), which suggests a possibility of coeval second-stage melting episode and melt percolation events.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites [28, 69-71]. For example, in the Izu−Bonin−Marianas (IBM) fore-arc, the former stratigraphically underlie the latter [28, 69-73]. Fore-arc basalts are tholeiitic with major element compositions at the depleted end of the MORB array [70, 73-75]. They are characterized by being lower in TiO2, P2O5, Zr, and LREEs than MORB, implying their derivation from mantle that is more refractory than MORB-source mantle [70, 76, 77]. The fore-arc magmas are considered to be generated by MORB-like mantle decompression melting during near-trench spreading with little or no mass transfer from the subducting plate [28, 69-72].Compared with fore-arc basalts, boninites have higher MgO and SiO2, lower TiO2, and relatively higher concentrations of fluid-mobile trace elements (e.g. K and Sr) [28, 71, 74, 78]. Boninites were generated later when the residual, highly depleted mantle (refractory harzburgite) after extraction of the fore-arc basalts melted at relatively shallow levels after being fluxed by fluids and melts from the subducting plate [27, 28, 69, 71, 79]. Boninitic lavas are located stratigraphically above arc tholeiite basalts in the CRO [4, 35].Vanadium is a moderately incompatible element and exists in multiple oxidation states (V3+, V4+, and V5+) in terrestrial magmas. Partition coefficient for V in mantle phases therefore show a strong dependence on oxygen fugacity, which decreases with increasing fO2 [80-82]. Vanadium concentration can thus provide information on oxygen fugacity. Figure 8 is a plot of V against Yb for all CRO peridotite samples, with modeled trends for fractional melting in spinel lherzolite facies at different oxygen fugacities. The V contents of the CRO peridotites show a positive correlation with Yb, indicating a dominant control by igneous processes rather than hydrothermal alteration. It is worth noting that most CRO peridotites fall along the fayalite−magnetite−quartz (FMQ) buffer, regardless of whether they are abyssal or SSZ types. The Black Diamond Ridge peridotites fall within the range of abyssal peridotites, and SSZ-type CRO peridotites overlap with the abyssal peridotite field, but they also extend to lower concentrations beyond that field; one sample (75BM-2) appears to somewhat more oxidized than the others. The oxygen fugacity of the MORB source is at or just above the FMQ buffer, whereas subarc mantle is generally more oxidized than ambient mantle, including a substantial proportion of rocks with fO2 > FMQ+1 [83-85]. Oxidation of the mantle is attributed to the infiltration of oxidized fluids or silicic melts derived from the subduction slab, where sulfate could be the most plausible oxidant [85, 86]. This observation indicates that the CRO peridotites had limited interaction with oxidizing fluids in the fore-arc setting due to the relatively short residence time above a dehydrating slab, and fore-arc basalts are generated mainly by decompression melting [83, 87, 88]. This finding supports the claim of Birner et al. [49] that the fore-arc mantle is not pervasively oxidized relative to mid-ocean ridge mantle.Supra-chondritic Os isotope ratios (up to 187Os/188Os ~0.157) are commonly found in arc peridotite xenoliths [24, 25, 89], which is attributed to inputs of radiogenic Os from a subducted oceanic crust and sediments. Osmium can be mobile during slab dehydration or melting processes, and its mobility in a fluid or a melt increases with increasing oxygen fugacity and salinity [26, 90]. The CRO peridotites studied, however, are characterized by subchondritic Os isotopic compositions (Figure 3). In a fore-arc region, slab dehydration and fluid salinity are relatively low in cold subduction and high in hot subduction [91]. Oxygen fugacity equivalent to the FMQ buffer in a fore-arc setting (Figure 8) and possibly cold subduction as evidenced by slight to moderate serpentinization (18−61%; [2]) observed in the CRO peridotites would have resulted in Os migration being limited in this setting. Unradiogenic 187Os/188Os ratios are also observed in IBM fore-arc peridotites [66]. This is independently supported by predominantly chondritic Os/Ir ratios observed in the CRO peridotites studied (Figure 9(a)). Also note that the Os isotopic ratios do not show correlation with Os content of the peridotites (Figure 9(b)), ruling out any mixing processes. Sample 75BM-2 has slightly suprachondritic Os/Ir ratio (Figure 9(a)) and Os and Ru enrichment relative to Ir (Figure 2(a)). Meanwhile, 75BM-2 has the lowest 187Os/188Os ratio among the CRO peridotites (Figure 9(b)), indicating that the elevated Os–Ru abundances could be nugget effect (i.e. heterogeneous distribution of laurite or Ru–Os alloy) rather than secondary fluid infiltration.This finding, however, is not in line with the results of a previous study of serpentinized peridotites from Point Sal, the southernmost CRO remnant, carried out by Snortum and Day [7]. The Point Sal peridotites record significant melt depletion (>20%), which resulted in exhaustion of sulfides in the source during melting, leading to loss of Os (open diamonds in Figure 9). Meanwhile, the Point Sal peridotites are characterized by radiogenic 187Os/188Os (Figure 9(b)) and fractionated Os/Ir ratios (Figure 9(a)), as well as suprachondritic (Pd/Ir)N (~3−25) or (Pt/Ir)N (~2−6) ratios, and enrichments in Ba, U, and Sr [7]. Also note that the major element compositions of the Point Sal peridotites are less systematic than ours, and some samples (PS1703) are highly enriched in Al2O3 and CaO (Figures 4 and 5, and online supplementary Figures S1 and S2) but depleted in Sc (not shown). Taken together with the high LOI values (14−18 wt%) of the Point Sal samples, this is probably an effect of hydrothermal alteration and secondary gabbroic melt(s) infiltration. That is, the Point Sal peridotites could be a mantle section associated with Cl-rich oxidizing fluid-driven melt depletion and refertilization processes in a fore-arc setting [7].Grey Eagle Mine chromitites have a chemical composition with boninitic affinity [2]. Podiform chromitite is considered to be the combined product of crystal fractionation in the early stages and melt-harzburgite reaction in an open system as primitive hydrous melts migrate through the upper mantle in a SSZ setting [92]. Dissolution of orthopyroxene in harzburgite by reaction with hydrous melts can produce dunite coupled with crystallization of high-Cr# chromite. The chromite in boninites could thus be the products of primitive melt crystallization in the crust or of incongruent melting of orthopyroxene in the uppermost mantle, which is carried to the surface by entrapment in the melts [93]. The studied chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)). Ru shows more compatible behavior than Os and Ir in chromite as observed in the previous studies [94, 95]. The IPGE-chromite association could be due to the presence of IPGE minerals (IPGM: laurite, erlichmanite, Os–Ir ± Ru alloys, and so on) in chromite [95] or IPGE incorporation in solid solution into the chromite structure [94, 96]. The IPGM could be one of the first phases to crystallize from cooling magma, or refractory residual phase during partial melting in the mantle, which were then entrapped during growth of chromite [97-99]. Available PGE−Re abundances of boninites are shown in Figure 2(b) for comparison. The boninites show PGE−Re patterns complementary to that of chromitite. This observation suggests that the boninitic melts might be generated during concomitant fractional crystallization of olivine and chromite [93, 97]. However, the number of samples analyzed is insufficient to generalize and needs to be validated with more data in the future. The Os isotope composition of the chromitite is within the range of the initial 187Os/188Os ratios of the CRO peridotites (Figure 3(a)), suggesting indistinguishable flux of radiogenic Os from the slab into the mantle source of the boninite.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.The data are provided in the article and the supplementary materials.The authors declare that they have no conflicts of interest.This study was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (NRF-2022R1A2C1003508).We thank associate editor Chuan-Lin Zhang and two anonymous reviewers for their constructive comments on this article.The supplementary materials available include an Excel table and two figures. Table S1 provides major and trace element concentrations for spinel peridotites and chromitite from Coast Range ophiolite, California. Figure S1 shows the major element compositions for Point Sal peridotites, the southernmost Coast Range ophiolite remnant. Figure S2 shows the MgO/SiO2 versus Al2O3/SiO2 ratios for Point Sal peridotites.\",\"PeriodicalId\":18147,\"journal\":{\"name\":\"Lithosphere\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":1.8000,\"publicationDate\":\"2024-07-05\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Lithosphere\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://doi.org/10.2113/2024/lithosphere_2024_154\",\"RegionNum\":4,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q3\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Lithosphere","FirstCategoryId":"89","ListUrlMain":"https://doi.org/10.2113/2024/lithosphere_2024_154","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
引用次数: 0

摘要

因此,Re-Os同位素系统在确定橄榄岩中熔体耗竭事件的年代方面具有巨大的潜力[23]。熔融残余物中的高浓度 Os 使 Os 同位素组成不受后续变质过程的影响。在无水部分熔融过程中,Os 和 Ir 的分馏作用并不明显[20, 21]。了解 CRO 的起源对于揭示科迪勒拉大陆边缘的中生代演化至关重要,其发展对其他大陆边缘蛇绿岩的起源也有影响,并提供了与现代洋内前弧(如伊豆-波宁系统;[27, 28])的比较点。前弧地幔的 PGE 成分迄今鲜有报道,但我们已经指出,它们有可能为熔融过程提供一些深刻的见解。本研究提供了CRO橄榄岩和一个荚状色锂辉石的PGE丰度和Re-Os同位素数据。CRO是与下伏中生代晚期-古新世早期弗朗西斯科岩群断层接触的肢解洋岩序列,上覆侏罗纪上侏罗统大山谷序列地层[3, 4, 29]。蛇绿混杂岩(辉长岩、闪长岩、玄武岩和安山岩)是蛇绿混杂岩中最丰富的岩性,但蛇纹石化橄榄岩构造岩也广泛分布,在某些地区还是最主要的岩性[30-32]。蛇绿岩的火成岩主要是透辉玄武岩和玄武安山岩,具有弧状亲缘关系,较少见的有倭黑质、安山岩和原始橄榄石-闪长玢岩[3, 4]。萨克拉门托山谷北部(Elder Creek、Stonyford)和迪亚布洛山脉(Del Puerto Canyon、Llanada)具有广泛地壳剖面的 CRO 地点的特征是常见的长英质火山岩(安山岩、"角闪岩")和浅成岩,包括闪长岩、辉长岩和具有钙碱性亲缘关系的 trondhjemite [9,33-35]。长英质钙碱性系列岩石覆盖或横切较古老的弧状透辉石系列岩石,并被晚期 MORB 类熔岩和尖晶岩覆盖或横切 [4,35]。根据与蛇绿岩相关的长花岗岩和石英闪长岩的 U-Pb 锆石年龄,以及位于 Stonyford 的玄武岩玻璃的 Ar-Ar 年龄,CRO 的年龄在 ~172 Ma 到 ~161 Ma 之间 [3、5、36]。高分辨率的 U-Pb 锆石化学磨蚀年龄确定了 161.2 至 167.9 Ma 的较小年龄范围[3, 36]。最年轻的年龄(〜161.2 ± 0.1 Ma)是德尔波托蛇绿岩中的长英质尖晶石;最古老的年龄来自 Elder Creek(〜167.9 ± 0.3 Ma)。大多数 CRO 地点的年龄紧紧集中在 ~165 Ma,包括火山玻璃上的 Ar-Ar 年龄(164.2 ± 0.7 Ma),这表明俯冲开始得很快,形成间隔很短,约为 700 万年。铬铁矿以不规则的荚状和透镜状出现在云英岩中。取样地点如图 1 所示,包括 Chrome(红山)附近的灰鹰矿、黑钻石岭(石福火山群以北)、小石溪(石福火山群以南)、德尔波托峡谷和布尔罗山。根据矿物化学成分,之前的研究[2, 12]将 CRO橄榄岩分为两类:深海橄榄岩和 SSZ橄榄岩。深渊橄榄岩是尖晶石黑云母,其特征是高铝尖晶石(Cr# = ~15)以及辉石中相对较高的 Al、Ti、Na 和稀土元素(REE)丰度;而 SSZ 橄榄岩是难熔尖晶石哈兹堡垒岩,其特征是高铬尖晶石(Cr# = ~40-73)以及辉石中极低的 Al、Ti、Na 和 REE 丰度;来自 SSZ 地区的白云石和正长石具有最高的 Cr#s (74-77)。深海型橄榄岩被认为是被困在 SSZ 环境中但尚未被俯冲过程改变的残余大洋岩石圈[6]。SSZ型橄榄岩被解释为代表俯冲带上方地幔楔的部分熔融,形成玄武岩(Cr#s = ~40-55的橄榄岩)或盂兰石(Cr# = 70-76的哈兹堡岩和云英岩);后一组橄榄岩包括铬铁矿(Cr#s = 76-79)和正长石(Cr# = 74),可能代表盂兰石累积。本研究获得了来自黑钻石岭的三个深海橄榄岩样本和来自小石溪、德尔波托峡谷和布尔罗山的五个 SSZ 橄榄岩样本。 此外,还有一个来自灰鹰矿的铬铁矿,在这个样本中,SSZ 型高铬尖晶石是主要的组成矿物[2]。我们的样本代表了地理范围广泛的五个不同地区,先前的研究[2, 6, 12]显示了它们内部的相似性(黑钻岭蛭石除外),表明它们在岩石学和构造上相互关联。深海橄榄岩代表了大偏移转换大洋岩石圈的残余,我们在其他地方提出,原法兰西俯冲是沿着这条断裂带开始的[6]。REE模型显示,深海橄榄岩是贫化MORB地幔(DMM)源在石榴石稳定场中干熔化约3%后的残留物(随后在较浅的深度转化为尖晶石蛭石),而SSZ橄榄岩是在尖晶石稳定场中进一步熔化约15%-20%后形成的,可能是在含水条件下形成的[12,37]。用金刚石锯将从地表露头采集的约 200-300 克重的手工标本切割成 1-2 厘米厚的石板,以去除任何蚀变痕迹。用碳化硅砂纸打磨石板各面以去除锯痕,在去离子水中冲洗、干燥,然后用氧化铝面颚式破碎机破碎。破碎样品的 50 克等分样品在装有氧化铝研磨容器和研磨棒的破碎箱中进行预研磨,然后在氧化铝面盘磨机中重新研磨成面粉级粉末;所得粉末等分样品用于地球化学分析。加拿大安大略省 Actlabs 公司使用偏硼酸锂/四硼酸盐熔融和电感耦合等离子体 (FUS-ICP) 仪器以及 ICP 质谱仪 (ICP-MS) 分别测定了整块岩石的主要元素和痕量元素浓度。美国地质调查局(USGS)的标准样品(DNC-1、SY-4 和 BIR-1a)与未知样品一起进行了分析。主要元素的精确度在 5%以内,大多数微量元素的精确度在 10%以内。为了获得 Re-Os 同位素和高嗜硒元素(HSE)丰度数据,我们采用了 Puchtel 等人[38]的详细分析方案。将大约 1.5 克全岩橄榄岩和 300 毫克铬铁矿粉末、6 毫升三蒸馏浓 HNO3、3 毫升三蒸馏浓 HCl 以及适量的 185Re-190Os 和 PGE(99Ru、105Pd、191Ir 和 194Pt)混合尖晶石密封在经内部双重清洁、冷却的 25 毫升 Pyrex™ 硼硅酸盐 Carius 管中,加热至 270°C 96 小时。用 CCl4 溶剂萃取法[39]从酸溶液中萃取锇,再反萃取到浓 HBr 中,然后通过微蒸馏法[40]纯化。Ru、Pd、Re、Ir 和 Pt 采用阴离子交换色谱法进行分离和纯化,该方法是根据 [41] 的方案修改的。所有样品都是在马里兰大学学院帕克分校同位素地球化学实验室使用 ThermoFisher Triton 质谱仪的二次电子倍增器检测器进行分析的。所有样品测量的 187Os/188Os 比率的运行精度在 0.03% 至 0.06% 之间。在本次分析之前的两年时间里,对内部约翰逊-马特希 Os 标准的 500 pg 样品所测得的 187Os/188Os 比率平均为 0.11377 ± 10(2 SD,N = 64)。这个 187Os/188Os 平均值在 IGL Triton [43] 法拉第杯上测量的约翰逊-马修 Os 标准平均 187Os/188Os = 0.1137950 ± 18 的不确定性范围内;因此,没有进行仪器质量偏差修正。获得的 2SD 不确定度表征了同位素分析的外部精确度(0.09%),用于估算本研究中每个样品所测得的 187Os/188Os 比值的真实不确定度。Ru、Pd、Re、Ir 和 Pt 的测量是在马里兰大学学院帕克分校等离子体实验室的 ThermoFisher Neptune Plus ICP-MS 的法拉第杯上进行的,采用静态模式,使用 1013 Ω 放大器测量所有相关质量。对仪器的同位素质量分馏进行了监测,并通过穿插样品和标准品进行了校正。根据分析活动期间进行的标准测量,估计分析的外部精确度为:185Re/187Re、99Ru/101Ru = 0.3%、191Ir/193Ir = 0.2%、194Pt/196Pt、105Pd/106Pd = 0.10% 相对值(2SD)。通过比较在 IGL 获得的参考材料 IAG MUH-1(奥地利哈兹堡垒岩)、IAG OKUM(超基性光卤石)和 NRC TDB-1(辉绿岩 PGE 岩石材料)的结果与参考值,评估了数据的准确性。 只有样品 75BM-2 的 MgO 略高,SiO2 略低,这可能是由于该样品中橄榄石过多,橄榄石的模态比例高达 93.5%[2]。图中还显示了 DMM 的简单模型批量熔化残留物。深海型橄榄岩的轻REE(LREE)含量很低,这与它们是熔融残留物的情况相符[47]。相比之下,SSZ 型橄榄岩具有 U 型 REE 模式,其重型和中型 REE(HREE 和 MREE)丰度远远低于黑钻岭橄榄岩。HREE和MREE的强烈贫化表明这些岩石是高度部分熔融后的残留物。另一方面,观测到的 LREE 丰度要求在熔融萃取之后进行二次加工。地幔岩石中的PGE主要集中在Fe-Ni-Cu硫化物(BMS),特别是单硫化物固溶体(Mss)中[18,51]。Re 并非真正的 PGE,但由于其化学性质相似,且 187Os 是 187Re 的放射性衰变产物,因此经常被认为与 PGE 同属一类。Re 元素在金属-硅酸盐系统中表现出亲硒性,通常存在于地幔中的硫化物相中,但也受到硅酸盐相的一些控制[52, 53]。PPGE-Re(图 2(a))的贫化与 CRO橄榄岩作为部分熔化和熔体提取后的残留物的起源相吻合。这种分馏的PGE-Re模式可归因于地幔熔融过程中BMS的不协调熔融,这种不协调熔融产生了富含Rh-Pt-Pd的硫化物熔体,并在残留物中留下了富含Os-Ir-Ru的Mss[21]。图7显示了BMS不协调熔融后残留橄榄岩中的软玉归一化模型Pt/Ir和Pd/Ir比值,以供比较。假设Ir、Pd和Pt在BMS中的浓度为100%,那么它们的丰度是利用BMS非模态分段熔化的质量平衡方程计算得出的。假设 PUM 中 S 的含量为 150 µg/g,Ir 为 3.5 ng/g,Pd 为 7.1 ng/g,Pt 为 7.6 ng/g[54]。在计算中,我们使用了 Mss/硅酸盐熔体分配系数[20]:DIr = 3500,DPd = 370,DPt = 360。建模结果表明,(Pt/Ir)N 和 (Pd/Ir)N 比率随着部分熔化程度的增加而降低(图 7)。除了两个严重偏离定义趋势的样品(DP-18 和 75BM-3)之外,尽管原始地幔的 S 含量和 S 在部分熔体中的溶解度存在不确定性,但 CRO橄榄岩定义的趋势与建模曲线非常吻合。深渊型CRO橄榄岩是熔融小于~5%后的残留物,而SSZ型CRO橄榄岩是熔融~5%至15%后的残留物。考虑到 DMM 代表原始地幔 2%-3% 的熔融去除,这与用 REE 模拟的结果一致(图 6)[55]。样品DP-18的Ru-Pt-Pd浓度高于在原始地幔中观测到的浓度(图2(a)),这可能表明二次再富集过程导致了硅酸盐熔体渗入后析出的元古代富Cu-Ni硫化物[18,51]。富S流体与橄榄石之间的硫化反应或溶解在富挥发性碱性熔体中的金属也可能导致BMS的沉淀[17, 18]。然而,在这种情况下,由于亲挥发性钙元素(如 Os)的富集,预计主橄榄岩中的 Os/Ir 比值会超软玉[17]。由于 DP-18 的 Os/Ir 比值属于软玉性质(图 2(a) 和图 8(a)),因此排除了这一可能性。另一方面,样品 75BM-3 具有超软玉(Pt/Ir)N 比值,但具有亚软玉(Pd/Ir)N 比值(图 7)。此外,75BM-3 的铂浓度高于原始地幔(图 2(a)),这不能用熔融去除来解释,因为铂在硫化物中的分配系数略低于钯[21]。在样品 75BM-3 中观察到的铂过量可能是由于离散富铂微相的金块效应造成的,可能是在二次熔体渗流或蛇绿岩化过程中产生的[51, 56-58]。CRO橄榄岩的 187Os/188Os 比值与 187Re/188Os 大致呈正相关,但没有定义有意义的 Re-Os 等距线(图 3(a)),这可能表明 Re 在样品中的流动性。数据点的分布导致了不现实的 TMA 模型年龄,一些样品集产生了未来年龄,而另一些则产生了比地球年龄更老的年龄(表 1)。在地幔橄榄岩中广泛观察到 Re 的开放系统行为 [23、46、59、60]。TRD模型年龄与熔融萃取年龄的最小估计值一致,并假定熔融萃取时间之后187Os/188Os没有变化[23],可以作为一种替代的年代测定方法。 Podiform 铬铁矿被认为是在 SSZ 环境中原始含水熔体通过上地幔迁移时,晶体早期分馏和熔体-哈兹堡垒岩反应的综合产物[92]。哈氏辉石中的正长石与含水熔体反应而溶解,可产生榴辉岩,同时结晶出高铬锰铬铁矿。因此,鲣石中的铬铁矿可能是地壳中原始熔体结晶的产物,也可能是最上层地幔中正辉石不协调熔化的产物,正辉石被熔体夹带至地表[93]。所研究的铬铁矿在 IPGE 中高度富集,在 PPGE 中严重贫化(图 2(b))。正如之前的研究[94, 95]所观察到的,铬铁矿中的 Ru 比 Os 和 Ir 更相容。IPGE 与铬铁矿的关联可能是由于铬铁矿中存在 IPGE 矿物(IPGM:月桂岩、糜棱岩、Os-Ir ± Ru 合金等)[95] 或 IPGE 在固溶体中融入铬铁矿结构 [94,96]。IPGM 可能是冷却岩浆中最先结晶的相之一,也可能是地幔部分熔融过程中的难熔残留相,然后在铬铁矿的生长过程中被夹带[97-99]。图 2(b)显示了榍石的 PGE-Re 丰度,以供比较。黝帘石显示出与铬铁矿互补的 PGE-Re 模式。这一观察结果表明,黝帘石熔体可能是在橄榄石和铬铁矿同时发生碎裂结晶过程中产生的[93,97]。不过,由于分析的样品数量不足,不能一概而论,需要今后用更多的数据来验证。铬铁矿的 Os 同位素组成在 CRO橄榄岩的初始 187Os/188Os 比率范围内(图 3(a)),表明从板块进入地幔的放射源 Os 通量与盂锡矿石的通量没有区别。深海类型代表了侏罗纪中期沿转换断层(原弗朗西斯坎俯冲带)开始俯冲过程中被困的海洋岩石圈的残余,该断层产生了SSZ型橄榄岩。软玉规范化模型的HREE丰度以及Pt/Ir和Pd/Ir比值表明,深海型CRO橄榄岩是在PUM熔化不到~5%之后形成的,而SSZ型橄榄岩是在PUM熔化~5%至15%之后形成的。深海型橄榄岩的铝超前模型年龄约为 1.5 Ga,这意味着 CRO 地幔经历的熔融萃取事件比 CRO 岩石圈的形成年龄要早得多,这可能反映了哥伦比亚超大陆的断裂。未经分馏的(Os/Ir)N比率约为1.1,fO2值接近FMQ缓冲区,这表明SSZ型橄榄岩主要是前弧玄武岩提取后的残留物,很少或根本没有来自俯冲板块的质量转移。深海型橄榄岩被认为是前弧机制下第二次熔融事件的来源。与此同时,荚状铬铁矿的 PGE-Re 模式与倭黑云母的 PGE-Re 模式互补,表明二者之间存在遗传联系,即在前弧玄武岩岩浆作用的结果下,在先前贫化的地幔中进行通量熔融和结晶。深海类型代表了侏罗纪中期沿转换断层(原弗朗西斯坎俯冲带)开始俯冲过程中被困的海洋岩石圈的残余,该断层产生了SSZ型橄榄岩。软玉规范化模型的HREE丰度以及Pt/Ir和Pd/Ir比值表明,深海型CRO橄榄岩是在PUM熔融不到~5%之后形成的,而SSZ型橄榄岩是在PUM熔融~5%至15%之后形成的。深海型橄榄岩的铝超前模型年龄约为 1.5 Ga,这意味着 CRO 地幔经历的熔融萃取事件比 CRO 岩石圈的形成年龄要早得多,这可能反映了哥伦比亚超大陆的断裂。未经分馏的(Os/Ir)N比率约为1.1,fO2值接近FMQ缓冲区,这表明SSZ型橄榄岩主要是前弧玄武岩提取后的残留物,很少或根本没有来自俯冲板块的质量转移。深海型橄榄岩被认为是前弧机制中第二次熔融事件的来源。 同时,荚状铬铁矿的PGE-Re模式与倭黑钨矿的PGE-Re模式互补,表明二者之间存在遗传联系,是前弧玄武岩岩浆作用导致的先前贫化地幔中的通量熔融和结晶。该研究得到了韩国政府资助的韩国国家研究基金会(NRF)(MSIT)(NRF-2022R1A2C1003508)的支持。我们感谢副主编张传林和两位匿名审稿人对本文提出的建设性意见。表 S1 提供了加利福尼亚海岸山脉蛇绿岩中尖晶石橄榄岩和铬铁矿的主要元素和微量元素浓度。图 S1 显示了最南端海岸山脉蛇绿岩残余物 Point Sal橄榄岩的主要元素组成。图 S2 显示了 Point Sal橄榄岩的 MgO/SiO2 与 Al2O3/SiO2 之比。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Re−Os Isotope and PGE Abundance Systematics of Coast Range Ophiolite Peridotites and Chromitite, California: Insights into Fore-Arc Magmatic Processes
We report platinum-group element (PGE) and Re concentrations, and Re−Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)N ≈ 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)−Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE−Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = −8.3 to 1.4) and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite−magnetite−quartz buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after ~5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has 187Os/188Os value of 0.1250 (γOs = −3.5) and PGE−Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.Ophiolites are sections of the Earth’s oceanic crust and the underlying upper mantle that have been tectonically emplaced into continental margins, providing important insights into the processes of plate tectonics, the composition of the oceanic crust and mantle, and the dynamics of Earth’s interior. Ophiolites are also valuable as ore deposits hosting precious metals, including platinum-group elements (PGEs), ferrous metals (Cr, Mn, and Ti), and base metals (Co, Cu, and Ni). The oceanic crust preserved in ophiolites may form in any tectonic setting during the evolution of ocean basins, from the mid-ocean ridge to subduction initiation and final closure [1]. The Coast Range ophiolite (CRO) is a mid-Jurassic (~172 to 161 Ma) ophiolite terrane in central California, extending over 700 km from Elder Creek at its northernmost segment extent to Point Sal at its southernmost terminus [2-5] (Figure 1). Petrologic and geochemical data indicate its formation in a supra-subduction zone (SSZ) fore-arc setting, probably above the east-dipping proto-Franciscan subduction zone [2, 6, 7]. Initiation of the subduction is considered to have possibly started along a large-offset transform fault zone, when an exotic or fringing is collided with North America [5, 6, 8]. The arc-like geochemical characteristics of pyroclastic, volcaniclastic, and hypabyssal materials of ophiolitic crust provide important evidence for the SSZ setting [4, 9, 10]. The possibility of the CRO having formed in a distal mid-ocean ridge or back-arc settings has also been proposed [11]. However, a paleo-equatorial mid-ocean spreading origin for the CRO peridotites is ruled out due to the SSZ-like highly depleted geochemical characteristics observed in several suites of CRO peridotites [2, 7, 12] and paleomagnetic evidence for limited north–south movement [13, 14]. A back-arc setting has remained plausible because back-arc extension and magmatism would be contemporaneous with arc volcanism, and it is difficult to geochemically distinguish arc-related magmas produced in fore-arc, intra-arc, or back-arc settings [15]. Since peridotites from back-arc basin are generally similar to abyssal peridotites [16], however, such a possibility has also been excluded [2]. Meanwhile, in the SSZ setting, the melt extraction process (e.g. fore-arc basalt or boninitic) to produce the current CRO peridotites has remained unidentified.The Re−Os isotope system and PGE abundances can play a pivotal role in understanding petrogenetic processes in peridotites, such as mantle melting and fluid/melt-rock interaction [17]. In upper mantle peridotites, PGEs are dominated by base-metal sulfides (BMSs) due to their highly chalcophile characteristics [18, 19]. Based on melting temperature (Tm), the PGEs are classified into two groups: Ir-group (IPGE: Os, Ir, and Ru, Tm > 2000°C) and Pd-group (PPGE: Pt, Pd, and Rh, Tm < 2000°C) [20]. During partial melting, the IPGE group resides in residual phases (e.g. sulfides, alloys, or oxides) and thus behaves compatibly relative to the PPGE group [21]. Re behaves as a mildly incompatible element during mantle melting, which results in residual peridotites with much lower Re/Os ratios than the fertile precursor [22]. The Re−Os isotopic system thus has great potential for dating melt depletion events in peridotites [23]. The high concentration of Os in the melting residue renders the Os isotopic compositions resistant to subsequent metasomatic processes. Os and Ir are not significantly fractionated during anhydrous partial melting [20, 21]. However, Os can be partitioned more than Ir into Cl-rich oxidizing slab-derived hydrous fluids or melts [24-26].Understanding the origin of the CRO is essential for unraveling the Mesozoic evolution of Cordilleran continental margin, and its development has implications for the origin of other continental margin ophiolites and provides a point of comparison with modern intra-oceanic fore-arcs (e.g. the Izu−Bonin system; [27, 28]). PGE compositions of fore-arc mantle have been rarely reported so far, yet we have indicated their potential for providing some profound insights into melting processes. This study presents PGE abundance and Re−Os isotope data for CRO peridotites and a podiform chromitite. Our aim is to constrain the formation and evolution of the CRO in a fore-arc regime and to evaluate its significance in the broader realm of ophiolite formation.The CRO is a sequence of dismembered oceanic rocks in fault contact with the underlying late Mesozoic−early Paleogene Franciscan Complex and is overlain by Upper Jurassic strata of the Great Valley sequence [3, 4, 29]. Crustal rocks (gabbro, diorite, basalt, and andesite) are the most abundant lithologies in the ophiolite though serpentinized peridotite tectonite is also widely distributed, and in some areas, it is the predominant lithology [30-32]. Igneous rocks of the ophiolite are dominantly tholeiitic basalts and basaltic andesites with arc affinities, with less common boninite, andesite, and primitive olivine–clinopyroxene–phyric basalts [3, 4]. CRO localities with extensive crustal sections in the northern Sacramento Valley (Elder Creek, Stonyford) and Diablo Range (Del Puerto Canyon, Llanada) are characterized by common felsic volcanic rocks (andesite, “keratophyre”) and shallow plutonic rocks comprising diorite, tonalite, and trondhjemite with calc-alkaline affinities [9, 33-35]. The felsic calc-alkaline series rocks overlie or cross-cut the older arc tholeiite series rocks and are in turn overlain or cross-cut by late MORB-like lavas and dikes [4, 35]. The latter magmatism is considered to have been caused by ridge collision, resulting in termination of ophiolite formation due to the change in relative plate motion and shallow subduction of young, buoyant crust [4].The CRO ranges in age from ~172 to ~161 Ma, based on U−Pb zircon ages from plagiogranite and quartz diorites associated with the ophiolite and on Ar−Ar ages of basaltic glass at Stonyford [3, 5, 36]. High-resolution U−Pb zircon chemical abrasion ages define a tighter age range of 161.2 to 167.9 Ma [3, 36]. The youngest age (~161.2 ± 0.1 Ma) is for felsic dikes in the Del Puerto ophiolite; the oldest age is from Elder Creek (~167.9 ± 0.3 Ma). Ages for most CRO locales cluster tightly at ~165 Ma, including the Ar−Ar ages on volcanic glass (164.2 ± 0.7 Ma), suggesting rapid onset of subduction and a short formation interval of circa 7 million years.Eight peridotites and one chromitite were obtained for this study, purposely sampling outcrops previously shown to exhibit the different crustal-rock associations described earlier. Chromitite occurs as irregular pods and lenses in dunite. Sample locations are shown in Figure 1, which include Grey Eagle Mine near Chrome (Red Mountain), Black Diamond Ridge (north of the Stonyford volcanic complex), Little Stony Creek (south of the Stonyford volcanic complex), Del Puerto Canyon, and the Burro Mountain.Based on mineral chemistry, previous studies [2, 12] divided the CRO peridotites into two groups: abyssal and SSZ peridotites. Abyssal peridotites are spinel lherzolites characterized by high-Al spinels (Cr# = ~15) and relatively high Al, Ti, Na, and rare earth element (REE) abundances in pyroxene, whereas the SSZ peridotites are refractory spinel harzburgites represented by high-Cr spinels (Cr# = ~40−73) and extremely low Al, Ti, Na, and REE abundances in pyroxene; dunite and orthopyroxenites from the SSZ locales have the highest Cr#s (74−77). The abyssal-type peridotites are considered to represent remnant oceanic lithosphere trapped in a SSZ setting but before being modified by subduction processes [6]. The SSZ-type peridotites are interpreted to represent partial melting in the mantle wedge above a subduction zone to form basalt (peridotites with Cr#s = ~40−55) or boninite (harzburgites and dunites with Cr# = 70−76); the latter group includes chromitites (Cr#s = 76−79) and orthopyroxenites (Cr# = 74) that may represent boninite cumulates.Three abyssal peridotite samples from Black Diamond Ridge and five SSZ peridotite samples from Little Stony Creek, Del Puerto Canyon, and Burro Mountain were obtained for this study. Also included is one chromitite from the Grey Eagle Mine, and in this sample, SSZ-type high-Cr spinel is the main constituent mineral [2]. Our samples represent five distinct locales over a geographically wide area, and previous studies [2, 6, 12] have shown their internal similarity (except for the Black Diamond Ridge lherzolites) suggesting that they are related petrologically and tectonically to one another. The abyssal peridotites represent a remnant of large-offset transform oceanic lithosphere, and we have proposed elsewhere that proto-Franciscan subduction was initiated along this fracture zone [6]. Modeling of REE shows that the abyssal peridotites are residues after ~3% dry melting of depleted MORB mantle (DMM) source in the garnet stability field (followed by conversion to spinel lherzolite at shallower depth), and the SSZ peridotites are formed by ~15%−20% further melting in the spinel stability field, possibly under hydrous conditions [12, 37].The procedures for sample preparation closely followed those described in detail by Puchtel et al. [38]. Hand specimens ~200−300 g in weight collected from surface outcrops were cut into 1−2 cm thick slabs using a diamond saw to remove any signs of alteration. The slabs were abraded on all sides using SiC sandpaper to remove saw marks, rinsed in deionized water, dried, and crushed in an alumina-faced jaw crusher. A 50 g aliquot of crushed sample was preground in a shatter box armed with alumina grinding container and puck and then reground to a flour-grade powder in an alumina-faced disk mill; the resultant powder aliquots were used for the geochemical analyses.Whole-rock major and trace element concentrations were determined using a lithium metaborate/tetraborate fusion and an inductively coupled plasma (FUS-ICP) instrument and an ICP mass spectrometer (ICP-MS), respectively, at Actlabs, Ontario, Canada. The U.S. Geological Suervey (USGS) standards (DNC-1, SY-4, and BIR-1a) were analyzed together with the unknown samples. The precision was within 5% for major elements and within 10% for most trace elements. The results are given in online supplementary Table S1.To obtain the Re−Os isotopic and highly siderophile element (HSE) abundance data, the analytical protocols detailed in Puchtel et al. [38] were followed. Approximately 1.5 g of whole-rock peridotite and 300 mg of chromitite powder, 6 mL of triple-distilled concentrated HNO3, 3 mL of triple-distilled concentrated HCl, and appropriate amounts of mixed 185Re−190Os and PGE (99Ru, 105Pd, 191Ir, and 194Pt) spikes were sealed in double internally cleaned, chilled 25 mL Pyrex™ borosilicate Carius tubes and heated to 270°C for 96 hours. Osmium was extracted from the acid solution by CCl4 solvent extraction [39], back-extracted into concentrated HBr, and purified via microdistillation [40]. Ru, Pd, Re, Ir, and Pt were separated and purified using anion-exchange chromatography following a protocol modified after [41].Osmium isotopic measurements were done via negative thermal ionization mass spectrometry [42]. All samples were analyzed using a secondary electron multiplier detector of a ThermoFisher Triton mass spectrometer at the Isotope Geochemistry Laboratory, University of Maryland, College Park. The in-run precision of measured 187Os/188Os ratios for all samples was between 0.03% and 0.06% relative. The 187Os/188Os ratio of 500 pg loads of the in-house Johnson–Matthey Os standard measured during the 2-year period leading up to the current analytical sessions averaged 0.11377 ± 10 (2 SD, N = 64). This average 187Os/188Os value is within the uncertainty of the average 187Os/188Os = 0.1137950 ± 18 measured for the Johnson-Matthey Os standard on the Faraday cups of the IGL Triton [43]; as such, no instrumental mass-bias corrections were made. The 2SD uncertainty obtained characterizes the external precision of the isotopic analyses (0.09%), which was used to estimate the true uncertainty on the measured 187Os/188Os ratios for each individual sample in this study.The measurements of Ru, Pd, Re, Ir, and Pt were performed at the Plasma Laboratory, University of Maryland, College Park, on Faraday cups of a ThermoFisher Neptune Plus ICP-MS in static mode using 1013 Ω amplifiers for all masses of interest. Instrumental isotopic mass fractionation was monitored and corrected for by interspersing samples and standards. The external precision of the analyses was estimated, based on standard measurements performed during the period of the analytical campaign, to be 185Re/187Re, 99Ru/101Ru = 0.3%, 191Ir/193Ir = 0.2%, and 194Pt/196Pt, 105Pd/106Pd = 0.10% relative (2SD). The accuracy of the data was assessed by comparing the results for the reference materials IAG MUH-1 (Austrian harzburgite), IAG OKUM (ultramafic komatiite), and NRC TDB-1 (Diabase PGE Rock Material) obtained at the IGL with the reference values. Concentrations of all HSE and Os isotopic compositions obtained at the IGL are within the uncertainties of the certified reference values [43].The average total analytical blank (TAB) measured during the present analytical campaign was (in pg): Ru 2.1, Pd 34, Re 1.1, Os 0.74, Ir 0.14, and Pt 127 (N = 9). For the peridotite samples, the average TAB constituted less than 0.1% for Os, Ir, and Ru; less than 1.0% for Pd; less than 4% for Re; and less than 6% for Pt of the total amount of element analyzed. The average TAB for the chromitite sample constituted less than 0.1% for Os, Ir, and Ru and 3.5%, 5.5%, and 16% for Re, Pt, and Pd, respectively, of the total element analyzed. We therefore cite the uncertainties on the concentrations of each element as 1/2 of the uncertainty from the TAB contribution, assuming 50% of variation in the TAB. The calculated uncertainty on the Re/Os ratio was propagated for each sample by combining the estimated uncertainties on the Re and Os abundances for each sample. The results are given in Table 1.The whole-rock major and trace element concentrations are provided in online supplementary Table S1. The samples are slightly to moderately serpentinized (18−61%; [2]) and have loss on ignition (LOI) values of 1.8% to 12.2%. The Mg number of CRO peridotites varies from 89.8 to 92.4. The CaO and Al2O3 range from 0.2 to 2.8 wt% and 0.3 to 2.7 wt%, respectively. The Ni, Co, and Cr contents range from 1820 to 2560 µg/g, 102 to 117 µg/g, and 1920 to 2900 µg/g, respectively. The V contents range from 13 to 64 µg/g. All samples show relative depletion in Cs, Rb, U, Th, and Sr. The heavy REEs have a wide range (Yb = 0.01−0.26 µg/g), but the light REEs have a relatively limited range of values (La = 0.06−0.14 µg/g), with (La/Yb)N ratios of 0.2 to 5.0 (online supplementary Table S1).The whole-rock PGE−Re concentrations of the CRO peridotites are 1.95−7.12 ng/g for Os, 2.27−4.16 ng/g for Ir, 4.52−11.59 ng/g for Ru, 1.20−10.89 ng/g for Pt, 1.21−10.15 ng/g for Pd, and 0.02−0.34 ng/g for Re (Table 1). These concentrations are within the ranges reported for mantle peridotites [17, 22]. The chromitite from Grey Eagle Mine has 96.9 ng/g for Os, 73.7 ng/g for Ir, 241.8 ng/g for Ru, 7.8 ng/g for Pt, 0.73 ng/g for Pd, and 0.10 ng/g for Re (Table 1). None of the PGE−Re concentrations exhibit any correlation with LOI values (not shown), supporting the notion that there are no secondary alteration effects. Chondrite-normalized PGE−Re abundances are shown in Figure 2. The CRO peridotites have IPGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances. Meanwhile, PPGE and Re concentrations span several orders of magnitude and are depleted overall relative to the primitive mantle. Abyssal-type CRO peridotites have slightly subchondritic PPGE−Re abundances and flat PPGE−Re patterns, whereas the SSZ-type CRO peridotites have highly fractionated PPGE−Re patterns. DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)). 75BM-3 has higher Pt concentration and Pt/Pd ratio compared to the primitive mantle (Figure 2(a)). The chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)).The CRO peridotites yield 187Os/188Os values between 0.1188 and 0.1315 (γOs = −8.3 to +1.4) and 187Re/188Os ranging from 0.022 to 0.413 (Table 1; Figure 3). The Grey Eagle Mine chromitite has 187Os/188Os of 0.1250 (γOs = −3.5) and 187Re/188Os of 0.005 (Table 1; Figure 3(a)). The Re−Os isotopic compositions do not show a correlation with LOI value (not shown), also underscoring the insignificance of secondary alteration effects. All CRO peridotites except DP-18 have 187Os/188Os ratios less radiogenic than the estimate of the primitive upper mantle (PUM = 0.1296; [44]). All samples including DP-18, however, have lower 187Re/188Os than the PUM (Figure 3(a)). Conventional Re−Os model ages (TMA; [23]) range from 7.9 Ga to the future, and Re-depletion model ages (TRD) range from 1.5 Ga to future age (Table 1). The possibility that suprachondritic Os isotopic composition of the DP-18 was induced by Os exchange with extremely radiogenic seawater of 187Os/188Os ≈ 1 [45] is excluded due to the fact that the sample has the highest 187Re/188Os ratio among the samples studied, and it has much higher Os concentration (4 ng/g) than seawater (8−10 fg/g; [46]).Representative major oxide concentrations recalculated on an anhydrous basis are shown in Figure 4. Also shown in the plots are fields for abyssal and SSZ peridotites [47, 48]. The Black Diamond Ridge peridotites plot within the field for abyssal peridotites, but the others are comparable to SSZ peridotites. In the MgO/SiO2−Al2O3/SiO2 space (Figure 5), the samples are within the range of terrestrial mantle array, indicating nearly isochemical serpentinization for the major elements. Only sample 75BM-2 had slightly higher MgO and lower SiO2, which could be due to excess olivine in this sample, as is evident in the modal proportion of olivine of up to 93.5% [2].Figure 6 shows chondrite-normalized REE patterns. Also shown in the plot are simple-model batch melting residues of DMM. The abyssal-type peridotites are depleted in light REE (LREE), which is consistent with their being melting residues [47]. In contrast, SSZ-type peridotites have U-shaped REE patterns with much lower heavy and middle REE (HREE and MREE) abundances than the Black Diamond Ridge peridotites. Strong depletion in the HREE and MREE indicates that the rocks are residues after high degree of partial melting. On the other hand, the observed enrichment in LREE requires secondary processes to have followed melt extraction. Postmelting refertilization by trace element–enriched melt or fluid of a refractory mantle in a SSZ setting can explain the elevated abundances of LREE and ultra-depletion in HREE and MREE of the samples [37, 47-50].The PGE in mantle rocks are mostly concentrated in Fe–Ni–Cu sulfides (BMS) and, in particular, monosulfide solid solution (Mss) [18, 51]. Re is not a true PGE but is often considered in conjunction with this group due to its chemical similarity and 187Os being the radioactive decay product of 187Re. The element Re exhibits siderophile behavior in metal-silicate systems and typically resides in sulfide phases in the mantle, though with some control by silicate phases [52, 53]. Depletion in PPGE−Re (Figure 2(a)) reconciles with the origin of the CRO peridotites as residues after partial melting and melt extraction. This fractionated PGE−Re patterns can be attributed to incongruent melting of BMS during mantle melting which produces Rh–Pt–Pd-enriched sulfide melt and leaves Os–Ir–Ru-enriched Mss in the residue [21].Chondrite-normalized modeled Pt/Ir and Pd/Ir ratios in residual peridotite after incongruent melting of the BMS are shown in Figure 7 for comparison. Assuming that Ir, Pd, and Pt are 100% concentrated in the BMS, their abundances were calculated using mass balance equations for nonmodal fractional melting of the BMS. The PUM is assumed to contain 150 µg/g for S, 3.5 ng/g for Ir, 7.1 ng/g for Pd, and 7.6 ng/g for Pt [54]. We have used in the calculations Mss/silicate melt partition coefficients after [20]: DIr = 3500, DPd = 370, and DPt = 360. The modeling shows that the (Pt/Ir)N and (Pd/Ir)N ratios decrease with increasing degree of partial melting (Figure 7). Except for the two samples grossly off the defined trend (DP-18 and 75BM-3), the CRO peridotites define a trend in remarkably good agreement with the modeled curve, despite uncertainties in the S contents of the primitive mantle and S solubility in partial melts. The abyssal-type CRO peridotites are residues after less than ~5% melting, and the SSZ-type CRO peridotites are residues after ~5% to 15% melting. This is consistent with the results modeled with REE (Figure 6), considering the fact that DMM represents 2%−3% melt removal from the primitive mantle [55]. Sample DP-18 has Ru–Pt–Pd concentrations higher than those observed in the primitive mantle (Figure 2(a)), probably indicating secondary refertilization processes resulting in precipitation of metasomatic Cu–Ni-rich sulfides from infiltrated silicate melts [18, 51]. Sulfidization reactions between S-rich fluids and olivine, or metals dissolved in volatile-rich alkaline melt may also result in precipitation of BMS [17, 18]. In that case, however, suprachondritic Os/Ir ratios are expected in the host peridotite due to enrichment in volatile–chalcophile element such as Os [17]. Since DP-18 has a chondritic Os/Ir ratio (Figures 2(a) and 8(a)), this possibility is excluded for this sample. Sample 75BM-3, on the other hand, has suprachondritic (Pt/Ir)N but subchondritic (Pd/Ir)N ratios (Figure 7). Also, 75BM-3 has higher Pt concentration than that in the primitive mantle (Figure 2(a)), which cannot be explained by melt removal, as Pt has slightly lower partition coefficients in sulfides than that of Pd [21]. The Pt excess observed in sample 75BM-3 may be due to nugget effects of discrete Pt-rich microphases, possibly produced during secondary melt percolation or serpentinization [51, 56-58].The 187Os/188Os ratios for the CRO peridotites display generally positive correlation with 187Re/188Os but do not define meaningful Re−Os isochrons (Figure 3(a)), likely indicating Re mobility in the samples. The data point distributions result in unrealistic TMA model ages with some sample sets yielding future ages while others produce ages older than the age of the Earth (Table 1). The open-system behavior of Re is widely observed in mantle peridotites [23, 46, 59, 60]. The TRD model age, which corresponds to the minimum estimates of the age of melt extraction and assumes no change in 187Os/188Os after time of melt extraction [23], can be an alternative dating method. However, it can provide a good approximation for the time of melting in highly refractory peridotites (e.g. SSZ type) but significantly underestimate for relatively fertile samples (e.g. abyssal type). While the Re−Os isotope data do not define an isochron, all samples possess 187Re/188Os ratios lower than the putative PUM (Figure 2(a)), suggesting that metasomatic addition of Re has not been significant. This is also substantiated by the samples’ suprachondritic (Pd/Re)N ratios (Figure 2(a)). For comparison, the reference isochron forced through PUM of 165 Ma, the CRO formation age, is shown in Figure 3(a). It should be pointed out that the CRO peridotites yield much steeper array compared with the reference line in the plot, indicating its origination from a protolith that had undergone ancient melt depletion episode(s).Al2O3, CaO, or Yb are usually considered to exhibit degrees of incompatibility similar to that of Re during mantle melting but are less easily perturbed than Re during metasomatism [60, 61]. Therefore, Al2O3 content is often used as a proxy for Re/Os to estimate the mantle separation ages of peridotites. In the alumina 187Os/188Os peudo-isochron (aluminachron; Figure 3(b)), the abyssal-type and SSZ-type CRO peridotites show two independent correlations, implying temporally distinct melting episodes. The trend of abyssal-type CRO peridotites (solid circles) passes through the PUM in the aluminachron, suggesting that these rocks might have originated from a PUM-like source. Assuming that there has been a single or multiple melting events closely spaced in time, the 187Os/188Os ratio at ~0.7 wt% Al2O3 as representative of Re/Os = 0 in anhydrous melting [61, 62] on the aluminachron can be considered as the initial ratio, which corresponds to TRD of 1.50 ± 0.13 Ga for the abyssal-type CRO peridotites (Figure 3(b)). This implies that the CRO mantle had experienced episode(s) of melt extraction before the abyssal-type CRO crust was formed. While this time of lithospheric stabilization seems too old to be expected in abyssal peridotite, the age decoupling between abyssal or fore-arc peridotites and their overlying crust, and long-term preservation of refractory domains, including a parcel of buoyant arc mantle, in the asthenosphere have been reported in several cases [46, 49, 63-67]. The aluminachron age of ~1.5 Ga for the abyssal-type lherzolites corresponds roughly to the age of rifting in the Columbia supercontinent, separating Laurentia/Baltica/Siberia from Antarctica/North Australia/South America to form an ocean basin that would evolve into the proto-Pacific Ocean basin in the Phanerozoic [68]. The Os isotopic evidence for ancient melting can be explained by unradiogenic sulfides or platinum-group metals encapsulated in silicate or oxide hosts and protected from subsequent melting or diffusion [52, 59, 65].The SSZ-type CRO peridotites show a positive correlation with a much steeper slope in the aluminachron (open symbols on the diagram in Figure 3b). However, the most refractory SSZ-type sample 75BM-2 (represented by the lowest 187Os/188Os ratio and Al2O3 content) falls on the lower limit of the correlation for abyssal-type peridotites, indicating that the abyssal-type CRO peridotites might have been the source for the second melt extraction event. It is estimated that the second event(s) occurred at ~165 Ma, but accurate dating is difficult with currently available data. We have shown a regression line for Burro Mountain peridotites for comparison in the plot (Figure 3(b)). We observe that samples with and without metasomatic overprinting in the Burro Mountain and Del Puerto peridotites are collinear in the plot (Figure 3(b)), which suggests a possibility of coeval second-stage melting episode and melt percolation events.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites [28, 69-71]. For example, in the Izu−Bonin−Marianas (IBM) fore-arc, the former stratigraphically underlie the latter [28, 69-73]. Fore-arc basalts are tholeiitic with major element compositions at the depleted end of the MORB array [70, 73-75]. They are characterized by being lower in TiO2, P2O5, Zr, and LREEs than MORB, implying their derivation from mantle that is more refractory than MORB-source mantle [70, 76, 77]. The fore-arc magmas are considered to be generated by MORB-like mantle decompression melting during near-trench spreading with little or no mass transfer from the subducting plate [28, 69-72].Compared with fore-arc basalts, boninites have higher MgO and SiO2, lower TiO2, and relatively higher concentrations of fluid-mobile trace elements (e.g. K and Sr) [28, 71, 74, 78]. Boninites were generated later when the residual, highly depleted mantle (refractory harzburgite) after extraction of the fore-arc basalts melted at relatively shallow levels after being fluxed by fluids and melts from the subducting plate [27, 28, 69, 71, 79]. Boninitic lavas are located stratigraphically above arc tholeiite basalts in the CRO [4, 35].Vanadium is a moderately incompatible element and exists in multiple oxidation states (V3+, V4+, and V5+) in terrestrial magmas. Partition coefficient for V in mantle phases therefore show a strong dependence on oxygen fugacity, which decreases with increasing fO2 [80-82]. Vanadium concentration can thus provide information on oxygen fugacity. Figure 8 is a plot of V against Yb for all CRO peridotite samples, with modeled trends for fractional melting in spinel lherzolite facies at different oxygen fugacities. The V contents of the CRO peridotites show a positive correlation with Yb, indicating a dominant control by igneous processes rather than hydrothermal alteration. It is worth noting that most CRO peridotites fall along the fayalite−magnetite−quartz (FMQ) buffer, regardless of whether they are abyssal or SSZ types. The Black Diamond Ridge peridotites fall within the range of abyssal peridotites, and SSZ-type CRO peridotites overlap with the abyssal peridotite field, but they also extend to lower concentrations beyond that field; one sample (75BM-2) appears to somewhat more oxidized than the others. The oxygen fugacity of the MORB source is at or just above the FMQ buffer, whereas subarc mantle is generally more oxidized than ambient mantle, including a substantial proportion of rocks with fO2 > FMQ+1 [83-85]. Oxidation of the mantle is attributed to the infiltration of oxidized fluids or silicic melts derived from the subduction slab, where sulfate could be the most plausible oxidant [85, 86]. This observation indicates that the CRO peridotites had limited interaction with oxidizing fluids in the fore-arc setting due to the relatively short residence time above a dehydrating slab, and fore-arc basalts are generated mainly by decompression melting [83, 87, 88]. This finding supports the claim of Birner et al. [49] that the fore-arc mantle is not pervasively oxidized relative to mid-ocean ridge mantle.Supra-chondritic Os isotope ratios (up to 187Os/188Os ~0.157) are commonly found in arc peridotite xenoliths [24, 25, 89], which is attributed to inputs of radiogenic Os from a subducted oceanic crust and sediments. Osmium can be mobile during slab dehydration or melting processes, and its mobility in a fluid or a melt increases with increasing oxygen fugacity and salinity [26, 90]. The CRO peridotites studied, however, are characterized by subchondritic Os isotopic compositions (Figure 3). In a fore-arc region, slab dehydration and fluid salinity are relatively low in cold subduction and high in hot subduction [91]. Oxygen fugacity equivalent to the FMQ buffer in a fore-arc setting (Figure 8) and possibly cold subduction as evidenced by slight to moderate serpentinization (18−61%; [2]) observed in the CRO peridotites would have resulted in Os migration being limited in this setting. Unradiogenic 187Os/188Os ratios are also observed in IBM fore-arc peridotites [66]. This is independently supported by predominantly chondritic Os/Ir ratios observed in the CRO peridotites studied (Figure 9(a)). Also note that the Os isotopic ratios do not show correlation with Os content of the peridotites (Figure 9(b)), ruling out any mixing processes. Sample 75BM-2 has slightly suprachondritic Os/Ir ratio (Figure 9(a)) and Os and Ru enrichment relative to Ir (Figure 2(a)). Meanwhile, 75BM-2 has the lowest 187Os/188Os ratio among the CRO peridotites (Figure 9(b)), indicating that the elevated Os–Ru abundances could be nugget effect (i.e. heterogeneous distribution of laurite or Ru–Os alloy) rather than secondary fluid infiltration.This finding, however, is not in line with the results of a previous study of serpentinized peridotites from Point Sal, the southernmost CRO remnant, carried out by Snortum and Day [7]. The Point Sal peridotites record significant melt depletion (>20%), which resulted in exhaustion of sulfides in the source during melting, leading to loss of Os (open diamonds in Figure 9). Meanwhile, the Point Sal peridotites are characterized by radiogenic 187Os/188Os (Figure 9(b)) and fractionated Os/Ir ratios (Figure 9(a)), as well as suprachondritic (Pd/Ir)N (~3−25) or (Pt/Ir)N (~2−6) ratios, and enrichments in Ba, U, and Sr [7]. Also note that the major element compositions of the Point Sal peridotites are less systematic than ours, and some samples (PS1703) are highly enriched in Al2O3 and CaO (Figures 4 and 5, and online supplementary Figures S1 and S2) but depleted in Sc (not shown). Taken together with the high LOI values (14−18 wt%) of the Point Sal samples, this is probably an effect of hydrothermal alteration and secondary gabbroic melt(s) infiltration. That is, the Point Sal peridotites could be a mantle section associated with Cl-rich oxidizing fluid-driven melt depletion and refertilization processes in a fore-arc setting [7].Grey Eagle Mine chromitites have a chemical composition with boninitic affinity [2]. Podiform chromitite is considered to be the combined product of crystal fractionation in the early stages and melt-harzburgite reaction in an open system as primitive hydrous melts migrate through the upper mantle in a SSZ setting [92]. Dissolution of orthopyroxene in harzburgite by reaction with hydrous melts can produce dunite coupled with crystallization of high-Cr# chromite. The chromite in boninites could thus be the products of primitive melt crystallization in the crust or of incongruent melting of orthopyroxene in the uppermost mantle, which is carried to the surface by entrapment in the melts [93]. The studied chromitite is highly enriched in IPGE and strongly depleted in PPGE (Figure 2(b)). Ru shows more compatible behavior than Os and Ir in chromite as observed in the previous studies [94, 95]. The IPGE-chromite association could be due to the presence of IPGE minerals (IPGM: laurite, erlichmanite, Os–Ir ± Ru alloys, and so on) in chromite [95] or IPGE incorporation in solid solution into the chromite structure [94, 96]. The IPGM could be one of the first phases to crystallize from cooling magma, or refractory residual phase during partial melting in the mantle, which were then entrapped during growth of chromite [97-99]. Available PGE−Re abundances of boninites are shown in Figure 2(b) for comparison. The boninites show PGE−Re patterns complementary to that of chromitite. This observation suggests that the boninitic melts might be generated during concomitant fractional crystallization of olivine and chromite [93, 97]. However, the number of samples analyzed is insufficient to generalize and needs to be validated with more data in the future. The Os isotope composition of the chromitite is within the range of the initial 187Os/188Os ratios of the CRO peridotites (Figure 3(a)), suggesting indistinguishable flux of radiogenic Os from the slab into the mantle source of the boninite.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.Geochemical data show that the CRO of California contains both SSZ and abyssal peridotites. The abyssal types represent a remnant of oceanic lithosphere trapped during subduction initiation along transform fault(s) (the proto-Franciscan subduction zone) in mid-Jurassic which produced the SSZ-type peridotites.Depletion in PPGE−Re and subchondritic 187Os/188Os ratios indicate that the CRO peridotites are residues after partial melting and melt extraction. Chondrite-normalized modeled HREE abundances, and Pt/Ir and Pd/Ir ratios indicate that the abyssal-type CRO peridotites are formed after less than ~5% melting and the SSZ-type ones after ~5% to 15% melting of the PUM.The aluminachron model age of ~1.5 Ga for the abyssal-type peridotites implies that the CRO mantle had experienced melt extraction event(s) much older than the formation age of the CRO lithosphere, which may reflect rifting of the Columbia supercontinent.Volcanism in a near-trench setting during subduction initiation is represented by fore-arc basalts and boninites. Unfractionated (Os/Ir)N ratios of ~1.1 and the fO2 values close to the FMQ buffer indicate that the SSZ-type peridotites are residues after mainly fore-arc basalt extraction with little or no mass transfer from the subducting plate. The abyssal-type peridotites are considered to be the source for the second melting event in a fore-arc regime. Meanwhile, the podiform chromitite has PGE−Re pattern complementary to that of boninites, suggesting a genetic link between the two by flux melting and crystallization in a previously depleted mantle as a result of fore-arc basalt magmatism.The data are provided in the article and the supplementary materials.The authors declare that they have no conflicts of interest.This study was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIT) (NRF-2022R1A2C1003508).We thank associate editor Chuan-Lin Zhang and two anonymous reviewers for their constructive comments on this article.The supplementary materials available include an Excel table and two figures. Table S1 provides major and trace element concentrations for spinel peridotites and chromitite from Coast Range ophiolite, California. Figure S1 shows the major element compositions for Point Sal peridotites, the southernmost Coast Range ophiolite remnant. Figure S2 shows the MgO/SiO2 versus Al2O3/SiO2 ratios for Point Sal peridotites.
求助全文
通过发布文献求助,成功后即可免费获取论文全文。 去求助
来源期刊
Lithosphere
Lithosphere GEOCHEMISTRY & GEOPHYSICS-GEOLOGY
CiteScore
3.80
自引率
16.70%
发文量
284
审稿时长
>12 weeks
期刊介绍: The open access journal will have an expanded scope covering research in all areas of earth, planetary, and environmental sciences, providing a unique publishing choice for authors in the geoscience community.
×
引用
GB/T 7714-2015
复制
MLA
复制
APA
复制
导出至
BibTeX EndNote RefMan NoteFirst NoteExpress
×
提示
您的信息不完整,为了账户安全,请先补充。
现在去补充
×
提示
您因"违规操作"
具体请查看互助需知
我知道了
×
提示
确定
请完成安全验证×
copy
已复制链接
快去分享给好友吧!
我知道了
右上角分享
点击右上角分享
0
联系我们:info@booksci.cn Book学术提供免费学术资源搜索服务,方便国内外学者检索中英文文献。致力于提供最便捷和优质的服务体验。 Copyright © 2023 布克学术 All rights reserved.
京ICP备2023020795号-1
ghs 京公网安备 11010802042870号
Book学术文献互助
Book学术文献互助群
群 号:481959085
Book学术官方微信