Geochimica et Cosmochimica Acta最新文献

{"title":"Triple oxygen isotopes in eggshell carbonate as a proxy of late Cenozoic CO2 and primary productivity","authors":"Vincent J. Hare ,&nbsp;Drake A. Yarian ,&nbsp;J. Tyler Faith ,&nbsp;Chris Harris ,&nbsp;Julia A. Lee-Thorp ,&nbsp;Benjamin H. Passey ,&nbsp;Kathryn G. Sokolowski ,&nbsp;Loïc Ségalen","doi":"10.1016/j.gca.2025.04.002","DOIUrl":"10.1016/j.gca.2025.04.002","url":null,"abstract":"<div><div>The triple oxygen isotope composition of biominerals is a promising recorder of changes in global primary productivity (GPP), as well as environment and animal physiology. However, application is largely unexplored, particularly in many regions of the world where eggshell is a common component of paleontological/archeological sites of the late Cenozoic. Ratites (ostriches and their relatives), as nonobligate drinkers, gain most of their oxygen from atmospheric O₂ and leaves, meaning that their eggshell should carry a strong imprint of anomalous <span><math><mrow><msup><mrow></mrow><mrow><mn>17</mn></mrow></msup><mi>O</mi></mrow></math></span> in atmospheric O₂ - which is ultimately related to GPP and CO₂ levels. In this study, we analyzed fossil eggshells from different climate states of the late Cenozoic, notably the Middle Miocene, Late Miocene, and Last Glacial Maximum (LGM), as well as modern eggshell from the same region, southern Africa. We interpret our results using an existing animal body water model, which we modify for parameter estimation using Bayesian inverse methods. Our results show that the predominant signal in the triple isotopic composition of biomineral carbonate is that of global carbon cycling (i.e. the ratio of GPP:CO₂), meaning that with further refinement, a relatively common material can be used to reconstruct changes in planetary primary productivity for periods beyond the ice core record. During the Middle Miocene, we find that GPP was significantly lower than today, at 95% confidence. Comparison with box-model output of the atmospheric O₂ budget shows that Middle Miocene GPP levels dropped to below 60% of modern values, assuming that Middle Miocene CO₂ levels were between 441 and 472 ppm (95% confidence). The result is discussed, with potentially important implications for future climate change.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"399 ","pages":"Pages 48-63"},"PeriodicalIF":4.5,"publicationDate":"2025-04-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143877999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Iron isotope fractionation in magnetite produced by the marine magnetotactic bacterium Magnetovibrio blakemorei","authors":"Vincent Busigny ,&nbsp;François P. Mathon ,&nbsp;Matthieu Amor ,&nbsp;François Guyot ,&nbsp;Nicolas Menguy ,&nbsp;Christopher T. Lefevre","doi":"10.1016/j.gca.2025.03.028","DOIUrl":"10.1016/j.gca.2025.03.028","url":null,"abstract":"<div><div>Magnetotactic bacteria (MTB) possess the ability to precipitate intracellular nanosized magnetite. Their emergence may date back to the early Archean (e.g. 3 Ga) but evidence of such a long history has yet to be provided. MTB identification in the rock record relies on magnetofossils, the residual magnetite crystals that may survive in time to rock deformation and low-grade metamorphism. Several criteria such as size, shape and magnetic properties have been proposed to distinguish magnetofossils from other magnetite sources. Recent experimental work on the freshwater strain <em>Magnetospirillum magneticum</em> AMB-1 revealed that Fe isotope composition represents a promising additional criterion, but isotopic characterization of distinct MTB strains and of their magnetite needs to be provided to evaluate the robustness of iron isotope signatures for paleontological applications. In the present work, we investigate the marine strain <em>Magnetovibrio blakemorei</em> MV-1 under various conditions selected to examine the influence of Fe concentration (50, 100 and 150 μM) and redox state (Fe(II)-ascorbate or Fe(III)-citrate) on bacterial biomineralization and determine associated Fe isotope fractionations. Our results confirm the preferential incorporation of light iron isotopes into magnetite relative to the bacterial growth medium previously observed in AMB-1. Modeling the evolution of the growth medium and magnetite isotope compositions by Rayleigh distillation process yields iron isotope fractionations (<em>i.e.</em> Δ⁵⁶Fe_{growth medium-magnetite} = δ⁵⁶Fe_{growth medium} − δ⁵⁶Fe_magnetite) ranging between 0.2 and 0.9 ‰ with a mean value of 0.55 (±0.35) ‰. This isotope fractionation shows no clear relation with iron concentration or redox state. Importantly, the enrichment in light iron isotope of biological magnetite contrasts with the isotopic characteristics of magnetite formed by abiotic precipitation, the latter being enriched in heavy isotopes. This suggests that Fe isotopes could be used as a biosignature for magnetofossils identification in terrestrial or extraterrestrial samples. Finally, in contrast with previous results obtained on AMB-1, no specific enrichment of ⁵⁷Fe (relative to ⁵⁴Fe, ⁵⁶Fe and ⁵⁸Fe) could be detected in MV-1 magnetite, which we interpret by differences in iron budgets between the two strains.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 83-98"},"PeriodicalIF":4.5,"publicationDate":"2025-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143901830","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Multiple episodes of serpentinite alteration revealed by progressive leaching experiments","authors":"Aled D. Evans ,&nbsp;Matthew J. Cooper ,&nbsp;Dave Craw ,&nbsp;Damon A.H. Teagle","doi":"10.1016/j.gca.2025.04.001","DOIUrl":"10.1016/j.gca.2025.04.001","url":null,"abstract":"<div><div>Serpentinized mantle rocks typically experience multiple alteration events, but to date, identifying distinct serpentinization episodes remains difficult because whole rock geochemical and isotopic analyses reflect the cumulative sum of fluid-rock interactions. Here we unravel the alteration history of serpentinized mantle rocks by undertaking multi-stage progressive leaching experiments to isolate distinct geochemical and ⁸⁷Sr/⁸⁶Sr signatures hosted within serpentinized peridotites from the Troodos Mantle Sequence and Limassol Forest Complex, Cyprus that have different geological histories. Whole rock powders underwent an initial 10 % acetic acid solution leaching step before a subsequent 3 M HCl treatment. The remaining residue was then completely dissolved and analysed following a standard HF and HNO₃ digestion. For comparison an untreated sample of whole rock powder was also completely digested and analysed. In the Troodos Mantle Sequence, an initial serpentinization event with relatively primitive ⁸⁷Sr/⁸⁶Sr ratios (0.705 to 0.706) is recorded by the rock residues and is consistent with alteration by fluids sourced from the dehydrating downgoing Cyprus slab. A more radiogenic ⁸⁷Sr/⁸⁶Sr (0.7086–88) signal, consistent with Cyprus Messinian seawater and evaporites, is mobilised by the initial leaching step. These contrasting signatures in the mantle rocks are similar to the Sr-isotopic compositions of Troodos Mantle Sequence groundwaters. Although previous field observations within the Limassol Forest Complex indicate some serpentinization by Cretaceous seawater, evidence from progressive leaching experiments of pervasive serpentinization on the Cretaceous seafloor is not forthcoming. Our results show that most samples from the Limassol Forest Complex yield a radiogenic ⁸⁷Sr/⁸⁶Sr signal (0.7087), most consistent with mid-Miocene Messinian seawater and broadly coeval with significant mid-Miocene uplift of the Limassol Forest Complex. These results demonstrate that progressive leaching approaches can reveal distinct alteration episodes with contrasting ⁸⁷Sr/⁸⁶Sr ratios and geochemical signatures that contribute to the overall integrated bulk rock signal.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 188-204"},"PeriodicalIF":4.5,"publicationDate":"2025-04-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143853635","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Basin-scale production of hyperacidic brines is critical for the formation of high-grade and large-tonnage uranium deposits in sedimentary basins","authors":"Yumeng Wang ,&nbsp;Guoxiang Chi ,&nbsp;Sean A. Bosman","doi":"10.1016/j.gca.2025.03.031","DOIUrl":"10.1016/j.gca.2025.03.031","url":null,"abstract":"<div><div>Unconformity-related uranium deposits (URU deposits) in the Proterozoic Athabasca Basin (Canada) represent the largest and richest (with average grades ranging from 0.127 to 19.5 wt% U) U deposits in the world. Fluid inclusion studies and experimental data suggest that the ore-forming fluids of URU deposits are hyperacidic (pH &lt; 3.5), oxidizing basinal brines carrying high concentrations of U (0.2 to 3700 ppm), which are several orders of magnitude higher than ordinary basinal and basement formation waters. While the oxidizing conditions of these fluids are well established, the mechanism responsible for the basin-scale generation of hyperacidic brines remains unknown. This paper aims to address this problem and to explain why the Athabasca Basin is exceptionally endowed with high-grade and large-tonnage uranium deposits. Based on petrographic and infrared hyperspectral logging data indicating basin-wide coexistence of kaolinite and illite in quartz-dominated sandstones, and fluid inclusion data showing high potassium (K) concentrations in diagenetic and ore-forming fluids within the Athabasca Basin, it is proposed that the production of hyperacidic brines resulted from a pervasive diagenetic reaction between K-rich brines and kaolinite: 3kaolinite [Al₂Si₂O₅(OH)₄] + 2 K⁺ ↔ 2illite [KAl₃Si₃O₁₀(OH)₂] + 3H₂O + 2H⁺. Geochemical modeling of fluid-rock reactions demonstrates that the basin-scale production of hyperacidic brines is achievable when a combination of three specific conditions is satisfied: 1) the presence of a thick package of compositionally mature sandstones, characterized by quartz-dominated framework grains with minimal (&lt;1%) feldspar and the interstitial space filled with kaolinite; 2) the development of K-rich brines through seawater evaporation above the sandstone package; and 3) low fluid/rock ratios, which enable the reaction between kaolinite and K-rich brines to reach equilibrium and produce illite and H⁺. Recognizing the basin-scale development of hyperacidic brines and the specific conditions entailed offers insights into why the Athabasca Basin is exceptionally endowed with high-grade, large-tonnage U deposits. The conditions of hyperacidic brine production revealed in this study may be used for evaluating the U mineralization potential of other sedimentary basins.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 1-12"},"PeriodicalIF":4.5,"publicationDate":"2025-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143817441","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Rapid crystal growth promotes the precipitation of nanoscale fluid inclusions rich in halogens and metals in colloform sphalerite","authors":"Kai Luo ,&nbsp;Alexandre Cugerone ,&nbsp;Denis Fougerouse ,&nbsp;Jia-Xi Zhou ,&nbsp;Haiyang Xian ,&nbsp;Yiping Yang ,&nbsp;David W. Saxey ,&nbsp;Vincent Motto-Ros ,&nbsp;Xiao Sun ,&nbsp;William D.A. Rickard ,&nbsp;Steve M. Reddy","doi":"10.1016/j.gca.2025.04.003","DOIUrl":"10.1016/j.gca.2025.04.003","url":null,"abstract":"<div><div>Trace elements in sulfides are commonly used to determine the physicochemical conditions of ore deposit formation. The thermodynamic models underpinning these studies rely on the assumption that trace elements are incorporated into the mineral’s crystal structure, however recent atomic-scale investigations suggest that this assumption may be erroneous, especially in metamorphosed environments. Here, in primary undeformed colloform sphalerites from two Pb-Zn deposits in South-China, we study the microstructural, geochemical, and nanoscale distribution of trace elements. Our results show that colloform sphalerite hosts trace elements such as Ge (up to 5671 ppm) and Ga (up to 16307 ppm) in nanoscale polyphase inclusions (mainly 10–20 nm), comprising an aqueous solution and solid phases such as galena and pyrite. These Ge(-Ga) polyphase inclusions are rich in light elements and halogens (H, Li, Na, Cl, K) and heavier metals such as Mn and Pb, accounting for 5 %-78 % of the trace element budget in bulk sphalerite. We propose a model whereby the rapid crystallization of colloform sphalerite favors the preservation of elevated trace element concentrations in nanoscale fluid inclusions (i.e., Ga, Ge, Pb, Mn) that are in apparent thermodynamic disequilibrium with sphalerite. A nucleation mechanism is proposed involving the entrapment of dense liquid composed of an intermediate high-density disordered state under supersaturation conditions. Based on a global geochemical data compilation of colloform sphalerite, we show significant enrichment of Pb in colloform sphalerite and multiple positive correlations between Pb and Ge. This suggests that Pb-Ge-rich nanoscale dense-liquid inclusions may be a prevalent carrier for trace elements observed in colloform sphalerite textures. Similar colloform textures resulting from supersaturated solutions in minerals such as pyrite or quartz may also contain trace element-rich nanoscale inclusions. Presence of these nanoscale inclusions appears to have a minimal effect on the estimated formation conditions derived from sphalerite chemistry (temperature, <em>f</em>S₂). This study highlights the value of chemical mapping in revealing temperature variations in sphalerite.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 119-138"},"PeriodicalIF":4.5,"publicationDate":"2025-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143853639","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Strong and persistent mineral-specific lithium isotope fractionation: no discernable kinetic isotope effects during inorganic calcite and aragonite precipitation","authors":"Corinne Hite ,&nbsp;Joji Uchikawa ,&nbsp;Sambuddha Misra ,&nbsp;I.V. Satya Chanakya ,&nbsp;Pratyusha Chanda ,&nbsp;Richard E. Zeebe","doi":"10.1016/j.gca.2025.03.030","DOIUrl":"10.1016/j.gca.2025.03.030","url":null,"abstract":"<div><div>Stable lithium isotopes (δ⁷Li) of CaCO₃ minerals have increasingly been used as a tracer for changes in silicate weathering processes. However, there is limited understanding of the influence of physical and chemical conditions on δ⁷Li values of CaCO₃ minerals during their formation in aqueous solutions. Here, we examined Li isotope fractionation in inorganic calcite and aragonite precipitation experiments with systematic manipulations of solution pH and concentrations of total dissolved inorganic carbon species ([DIC] ≈ [HCO₃⁻] + [CO₃²⁻]) and calcium ion (Ca²⁺). Calcite and aragonite samples had δ⁷Li values lower than those of dissolved Li in solutions by about 3‰ and 16‰, respectively, indicating preferential uptake of the lighter ⁶Li isotopes. Aragonite consistently had δ⁷Li values lower than those of calcite by ∼13‰, likely due to differences in Li coordination and thereby the strength of bonds formed by/with Li within the respective mineral structure. We observed no statistically significant changes in aragonite nor calcite δ⁷Li values in response to changing solution pH, [DIC], [Ca²⁺], and CaCO₃ precipitation rates, indicating our solution chemistry manipulations imposed little effect on Li isotope fractionation. These findings lead us to argue that the observed Li isotope fractionations in calcite and aragonite with respect to dissolved Li in solutions are dominated by equilibrium isotope effects, and that kinetic effects for δ⁷Li values in CaCO₃ are either non-existent or too small to be expressed under our experimental conditions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 37-52"},"PeriodicalIF":4.5,"publicationDate":"2025-04-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143823313","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Marine chemical sedimentary protoliths determined for ca. 3.71 Ga calc-silicate schists and quartzites from the Isua Supracrustal Belt","authors":"Mike J. Zawaski ,&nbsp;Johanna Marin-Carbonne ,&nbsp;Nigel M. Kelly ,&nbsp;Alexey Ulianov ,&nbsp;Anne-Sophie Bouvier ,&nbsp;Stephen J. Mojzsis","doi":"10.1016/j.gca.2025.03.029","DOIUrl":"10.1016/j.gca.2025.03.029","url":null,"abstract":"<div><div>Foliated calc-silicate schists and quartzites from a ca. 3.71 Ga domain within the northeastern part of the Isua Supracrustal Belt (southern West Greenland) may be interpreted as either deformed layered “metasedimentary” rocks, or the metasomatic fluid alteration products of a deformed mafic igneous precursor (i.e., basaltic protolith). One important reason for protolith assignment in this case stems from the debated claim that these are originally clastic sedimentary rocks which host the oldest interpretable bio-mediated sedimentary structures (cf. “stromatolites”). To determine the protolith, we used correlated mineralogical, outcrop- and regional-scale structural, and in situ major-, minor- and trace-element geochemical analyses coupled with high spatial resolution Si, O, and S isotope measurements. We conclude the protoliths were sedimentary chemical precipitates with a minor detrital clay fraction. This interpretation is bolstered by the low levels of generally immobile elements (Zr, Cr, Y, Ti, and P) which rule out either a purely igneous or clastic sedimentary origin. Further evidence for a sedimentary origin includes heterogeneous Si and O isotopes and mass-independent S isotope values (Δ³³S &gt; 0, S-MIF), along with rare earth element (REE + Y) signatures that show light REE-depletion in PAAS normalized REE patterns and seawater-like Y/Ho. Pervasive deformation has masked or obliterated most if not all primary sedimentary structures, such that no chemical or physical features can be reliably used as indicators for (shallow) water depth at time of deposition. Although we conclude these rocks are of sedimentary origin we reiterate (viz. our previous work) that the nabla-shaped (∇) features found within them are not biogenic stromatolites. Instead, evidence points towards their formation as products of local and regional strain and recrystallization, long after deposition.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"398 ","pages":"Pages 99-118"},"PeriodicalIF":4.5,"publicationDate":"2025-04-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143877904","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Sulfur outgassing and in-gassing in lunar orange glass beads and implications for 33S “Anomaly” in the Moon","authors":"Xue Su ,&nbsp;Youxue Zhang ,&nbsp;Yang Liu","doi":"10.1016/j.gca.2025.03.026","DOIUrl":"10.1016/j.gca.2025.03.026","url":null,"abstract":"<div><div>Our recent investigations have discovered inward diffusion (in-gassing) of moderately volatile elements (MVEs; e.g., Na, K and Cu) from volcanic gas into volcanic beads/droplets. In this work, we examine the distribution of sulfur in lunar orange glass beads. Our analyses reveal that sulfur exhibits a non-uniform distribution across the beads, forming “U” or “W” shaped profiles typical of in-gassing. A model developed to assess sulfur contributions from different sources (original magmatic sulfur versus atmospheric in-gassed sulfur) in the orange beads indicates that atmospheric sulfur in-gassed during eruption contributes approximately 9–24 % to the total sulfur content of an orange bead, averaging around 16 %. This in-gassed sulfur is derived from the eruption plume, where atmospheric sulfur could undergo photochemical reactions induced by UV light, leading to mass independent fractionation and a distinct sulfur isotope signature.</div><div>Interestingly, a recent study discovered a small mass independent isotope fractionation of sulfur in lunar orange glass beads in drive tube 74002/1 and a lack of such mass independent isotope fractionation in black glass beads in the same lunar sample. This finding contrasts with sulfur in lunar basalts, which typically exhibit mass dependent fractionation. With our work, the observed mass independent fractionation signal in sulfur isotopes of orange beads can be attributed to the in-gassing of photolytic sulfur in the optically thin part of the eruption plume where UV light can penetrate. Using the sulfur isotope data of lunar orange beads, we estimate that the Δ³³S value of atmospheric sulfur is approximately −0.18 ‰. Our study provides new insights into the complex dynamics of volatile elements in lunar volcanic processes, highlighting the role of in-gassing in shaping sulfur isotope signatures in volcanic glass beads.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 164-175"},"PeriodicalIF":4.5,"publicationDate":"2025-03-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143824998","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Polymetallic Te-rich melts contribute to efficient enrichment and precipitation of Au in hydrothermal ore deposits","authors":"Gao-Hua Fan ,&nbsp;Jian-Wei Li ,&nbsp;Ri-Chen Zhong ,&nbsp;Sarah A. Gleeson ,&nbsp;Zhuo-Sen Yao ,&nbsp;Daniel E. Harlov ,&nbsp;Xiao-Dong Deng ,&nbsp;Hao Cui ,&nbsp;Chang Yu ,&nbsp;Wen-Sheng Gao","doi":"10.1016/j.gca.2025.03.027","DOIUrl":"10.1016/j.gca.2025.03.027","url":null,"abstract":"<div><div>Tellurides are abundant in a large variety of hydrothermal Au deposits. Previous studies have proposed that these phases crystallize from polymetallic Te-rich melts that may act as scavengers for Au to form high grade deposits. However, the conditions under which Te-rich melts form and whether such melts can act as agents to concentrate Au have not been directly assessed. Here, we present an experimental study using natural ore samples from a giant Te-rich Au deposit (Dongping, China) to show that tellurides in Au ores crystallized from polymetallic melts and these melts behaved as a critical agent for Au enrichment and precipitation. The samples investigated contain abundant polymetallic inclusions, which consist of various telluride-gold mineral assemblages as well as minor chalcopyrite. Individual inclusions typically comprise one micron-sized gold grain. Annealing experiments of these inclusions were carried out at 300, 350, 400, and 450 °C. The polymetallic inclusions had relatively subtle changes in morphology and texture at 300 and 350 °C, but changed markedly at 400 and 450 °C, accompanying with the formation of abundant droplet-like and/or vermiculate gold grains randomly dispersed in the telluride matrix. Three <em>in-situ</em> scanning electron microscopy heating–cooling experiments with different heating rates and durations revealed a eutectic temperature of 243 to 270 °C for polymetallic tellurides, at which native gold began to dissolve into molten tellurides. During the heating processes of the three experimental runs, changes in topographic contrast occurred, with the formation of transient holes in the tellurides and the appearance of tellurides on the surface of chalcopyrite. These observations are indicative of localized migration of the molten tellurides. Upon cooling in run #1 and #2, native gold recrystallized at ∼ 340 °C from the molten tellurides and subsequently regrew in a gradual manner. After these two runs, many newly-formed telluride-gold globules were observed occurring as fracture- and/or cavity-infillings within the host pyrite, further confirming molten state of the polymetallic minerals. Collectively, our new data demonstrate that polymetallic Te-rich melts with a complete melting temperature of ∼ 340 to 400 °C are responsible for the formation of telluride-gold mineral assemblages in natural Te-rich Au ores. Such melts can act as an important Au scavenger and facilitate Au precipitation. Our findings highlight the significance of polymetallic Te-rich melts as a key agent for concentrating Au to form high-grade Au ores worldwide.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 176-187"},"PeriodicalIF":4.5,"publicationDate":"2025-03-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143824999","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Untangling the effects of crustal and source processes on the Mo isotope systematics of oceanic arcs: insights from the Tonga-Kermadec subduction system","authors":"R. Pontow ,&nbsp;R. Bezard ,&nbsp;M. Willbold ,&nbsp;S. Turner ,&nbsp;T. Di Rocco","doi":"10.1016/j.gca.2025.03.025","DOIUrl":"10.1016/j.gca.2025.03.025","url":null,"abstract":"<div><div>The Mo stable isotope system is a promising tracer of crustal recycling. As subduction zones play a major role in returning crustal material back to the Earth’s mantle, understanding the Mo isotope behavior during processes operating at these settings is paramount. The Mo isotope compositions of arc lavas are highly variable and despite recent advances in this field, questions remain about Mo isotope fractionation and mobilization during slab processes and the relative effect of magmatic differentiation on the Mo isotope compositions (<em>δ</em>^98/95Mo) of arc lavas. The Tonga-Kermadec arc is well suited to assess and disentangle crustal and source processes in intra-oceanic arc settings owing to widespread felsic volcanism and along-arc source variations, characterized by increasing proportions of a metasedimentary component from the northern Tonga to the southern Kermadec arc. In the pursuit of these objectives, we analyzed a set of basaltic to dacitic tholeiitic arc lavas from volcanoes covering the whole length of the Tonga-Kermadec arc and sediments from DSDP site 204. Tonga-Kermadec arc lavas have highly variable Mo concentrations (0.21 and 2.35<!--> <!-->µg/g) and <em>δ</em>^98/95Mo values (−0.29 to +0.34 ‰). At some volcanoes, considerable <em>δ</em>^98/95Mo variations in samples as primitive as basalt are correlated with indices of differentiation and Sr-Nd radiogenic isotopes, indicating an impact of crustal assimilation on Mo isotope compositions. However, the <em>δ</em>^98/95Mo of the most primitive lavas analyzed for each volcano appear to have preserved primary melt signatures, since they form an along-arc trend mirroring those of Pb isotope averages, a proxy that is controlled by source processes in the Tonga-Kermadec arc. Using these data, a north–south decrease in <em>δ</em>^98/95Mo of the sources of arc magmas is resolved. This is interpreted to reflect the competing signals of isotopically heavy slab-fluids (<em>δ</em>^98/95Mo higher than D-MORB values) with that of an increasing contribution of a metasedimentary component that is lighter than the DMM toward the south. Our data suggest that crustal assimilation can significantly distort primitive <em>δ</em>^98/95Mo even in mafic magmas from simple intra-oceanic arcs. The induced intra-volcano <em>δ</em>^98/95Mo scatter has the potential to obscure along-arc variability as well as systematics with typical chemical and isotopic proxies used to characterize arc lava sources (e.g., Ba/Th, radiogenic isotopes). However, we also show that a careful evaluation of these effects can, at least in the Tonga-Kermadec arc, allow the assessment of arc lava source compositions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"397 ","pages":"Pages 13-36"},"PeriodicalIF":4.5,"publicationDate":"2025-03-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143817442","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}