Contributions to Mineralogy and Petrology最新文献

{"title":"Mineralogical constraints on magma storage conditions in ultramafic arc cumulates and the nature and role of cryptic fugitive melts: Tulameen Alaskan-type intrusion, North American Cordillera","authors":"Graham T. Nixon,&nbsp;Matthew J. Manor,&nbsp;James S. Scoates,&nbsp;Dylan W. Spence,&nbsp;Dejan Milidragovic","doi":"10.1007/s00410-024-02181-6","DOIUrl":"10.1007/s00410-024-02181-6","url":null,"abstract":"<div><p>Alaskan-type ultramafic–mafic intrusions in convergent-margin settings provide valuable information on melt-cumulate petrogenetic processes operating at depth in the sub-arc crust. Here, we report the compositions and textural relationships of cumulus and postcumulus minerals in a suite of clinopyroxenites and hornblendites from the peripheral zone of the Tulameen Alaskan-type intrusion in British Columbia, Canada. Mineral chemistry is used to establish magma storage conditions (P, T, <i>f</i>O₂, H₂O_melt) and to reconstruct the composition of cryptic residual liquids that equilibrated with the mineral phases and subsequently escaped the local mush system. Residual liquids in equilibrium with clinopyroxene (diopside) are metaluminous calc-alkaline basalt to andesite; melts equilibrated with amphibole (magnesio-hastingsite) are metaluminous to peraluminous calc-alkaline dacite to low-silica rhyolite. Thermobarometry yields a robust estimate of storage pressure of 400 ± 50 MPa (~ 15 km paleodepth) for the Tulameen magma reservoir and equilibration temperatures of 1130–960 °C for clinopyroxene and 950–850 °C for amphibole. The large cooling interval between the early crystallization of clinopyroxene and late appearance and continued crystallization of peritectic amphibole facilitated progressive extraction of residual liquids from clinopyroxene-rich cumulates, consistent with textural relationships, mass balance calculations and experimental petrology. Peritectic dacitic melts are hydrous (~ 6–8.3 wt% H₂O_melt), oxidized (<i>f</i>O₂ ~ NNO + 1.6 to NNO + 3.6 log units) and buoyantly mobile with low density (~ 2200 kg/m³) and viscosity (~ 10³ poise). Lower water contents likely reflect degassing of peritectic melts driven by amphibole crystallization; relatively high redox conditions are attributed to precursor fractionation of olivine and clinopyroxene preserved as cumulates in the core of the Tulameen intrusion. Peritectic amphibole crystallized in response to migration of a thermally buffered reaction front marking the stability limit of amphibole (≤ 950 °C) and driven by near-isobaric cooling. Pervasive infiltration of reactive dacitic liquids through the clinopyroxene mush formed intergranular/poikilitic amphibole and channelized flow was captured in part by cm-scale hornblendite segregations; aggregated melts formed in situ bodies of replacive hornblendite. The absence of orthopyroxene and rarity of plagioclase in the evolved ultramafic cumulates of Alaskan-type intrusions and similar arc-related rocks is attributed primarily to high H₂O_melt and oxygen fugacity in differentiated arc magmas.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142587853","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Nucleation delay controlling the formation of mafic enclaves and banded pumice","authors":"Masatoshi Ohashi,&nbsp;Ben Kennedy,&nbsp;Darren Gravley","doi":"10.1007/s00410-024-02180-7","DOIUrl":"10.1007/s00410-024-02180-7","url":null,"abstract":"<div><p>The presence of mafic enclaves and banded pumice reveals key physical processes associated with volcanic eruptions. Here, through the textural and geochemical analyses of the 3550 B.P. Waimihia deposits in Taupō, New Zealand, we demonstrate how disequilibrium crystallization controls the way magmas mix. Andesitic enclaves in pyroclastic deposits from this predominantly rhyolitic eruption consist of microlites that crystallized rapidly during mafic injection into rhyolitic host magma. The variation of microlite textures depends on enclave size, implying that mafic enclaves crystallized as discrete blobs within a host rhyolitic magma. However, gray pumice and dark bands in banded pumice are characterized by a lack of or less plagioclase microlites that should be present if equilibrium crystallization occurred. Our textural and chemical data suggest that the lack of plagioclase in gray pumice and dark bands resulted from the nucleation delay arising from the mixing with rhyolitic magma. After mafic magma broke up in a magma chamber as discrete mafic blobs, the plagioclase-free rim of the blobs was disaggregated by shear flow. The eroded mafic blobs form a hybrid magma by mixing with rhyolitic magma, which further delays the plagioclase nucleation. This hybrid magma eventually erupted as gray pumice or banded pumice, depending on the intensity of magma mingling in the conduit. We use a plagioclase nucleation delay model to calculate residence times of hours to tens of hours prior to eruption. Our mixing model with nucleation delay enables small volumes of mafic magma to mix with large amounts of silicic magma.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00410-024-02180-7.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142587851","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Hydrothermal mineral replacement of bastnäsite by rhabdophane and monazite: effects of temperature on mineralogy, REE immobilisation, and fractionation","authors":"Manuel Knorsch,&nbsp;Tobias G. Bamforth,&nbsp;Fang Xia,&nbsp;Artur P. Deditius,&nbsp;Mark A. Pearce,&nbsp;Louise Schoneveld,&nbsp;Malcolm P. Roberts","doi":"10.1007/s00410-024-02183-4","DOIUrl":"10.1007/s00410-024-02183-4","url":null,"abstract":"<div><p>The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO₃F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO₄∙xH₂O, x = 0–1) and monazite (REEPO₄) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00410-024-02183-4.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142587852","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The impact of exhumation onto fluid-mobile element budget and Rb-Sr isotope heterogeneity of the subducted eclogitic crust (Alag-Khadny, SW Mongolia)","authors":"Sergei Yu. Skuzovatov,&nbsp;Anfisa V. Skoblenko,&nbsp;Adrien Vezinet,&nbsp;Anas A. Karimov,&nbsp;Tatsuki Tsujimori","doi":"10.1007/s00410-024-02179-0","DOIUrl":"10.1007/s00410-024-02179-0","url":null,"abstract":"<div><p>Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ¹⁸O values and in-situ Rb–Sr constraints on the initial (⁸⁷Sr/⁸⁶Sr)_I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H₂O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable ⁸⁷Rb/⁸⁶Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (⁸⁷Sr/⁸⁶Sr)_I released into eclogites. Instead, unradiogenic ⁸⁷Sr/⁸⁶Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (⁸⁷Sr/⁸⁶Sr)_I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142555287","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Experimental constraints on serpentinite carbonation in the presence of a H2O–CO2–NaCl fluid","authors":"Yongsheng Huang,&nbsp;Satoshi Okumura,&nbsp;Kazuhisa Matsumoto,&nbsp;Naoko Takahashi,&nbsp;Hong Tang,&nbsp;Guoji Wu,&nbsp;Tatsuki Tsujimori,&nbsp;Michihiko Nakamura,&nbsp;Atsushi Okamoto,&nbsp;Yuan Li","doi":"10.1007/s00410-024-02175-4","DOIUrl":"10.1007/s00410-024-02175-4","url":null,"abstract":"<div><p>Serpentinite carbonation contributes to the deep carbon (C) cycle. Recently, geophysical and numerical studies have inferred considerable hydrothermal alteration in plate bending faults, opening the possibility of significant C storage in the slab mantle. However, there is a lack of quantitative determination of C uptake in serpentinized mantle rocks. Here, we experimentally constrain serpentinite carbonation in H₂O–CO₂–NaCl fluids to estimate C uptake in hydrated mantle rocks. We find that serpentinite carbonation results in the formation of talc and magnesite along the serpentinite surface. The presence of porous reaction zones (49.2% porosity) promotes the progress of carbonation reactions through a continuous supply of CO₂-bearing fluids to the reaction front. Added NaCl effectively decreases the serpentinite carbonation efficiency, particularly at low salinities (&lt; 5.0 wt%), which is likely attributed to the reduction in fluid pH and the transport rate of reactants, and the increase in magnesite solubility. Based on previous and our experiments, we fit an empirical equation for the reaction rate of serpentinite carbonation. Extrapolation of this equation to depths of plate bending fault systems suggests that serpentinite carbonation may contribute to an influx of up to 7.3–28.5 Mt C/yr in subduction zones. Our results provide new insights into serpentinite carbonation in environments with high fluid salinities and potentially contribute to the understanding of the C cycle in subduction zones.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00410-024-02175-4.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142410632","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Spatial and temporal mush heterogeneity during eruptions recorded in clinopyroxene from the 2021 paroxysms at Mt. Etna, Italy","authors":"Alice MacDonald,&nbsp;Teresa Ubide,&nbsp;Silvio Mollo,&nbsp;Jacopo Taddeucci","doi":"10.1007/s00410-024-02174-5","DOIUrl":"10.1007/s00410-024-02174-5","url":null,"abstract":"<div><p>Textural and compositional zoning of volcanic minerals archives pre-eruptive magma processes. Crystals erupted simultaneously may be sampled from different regions of the plumbing system and hence record variable histories due to complex magma dynamics. In addition, crystals erupted throughout the course of an eruption may record temporal variations in the plumbing system. To resolve mush variability on both spatial and temporal scales, we investigate clinopyroxene erupted during a series of paroxysmal episodes between February–April 2021 at Mt. Etna, Italy. Using a combination of high-resolution geochemical techniques, we observe that Cr enrichments in clinopyroxene mantle zones, grown upon eruption-triggering mafic rejuvenation, exhibit both temporal and spatial (sample-scale) variability. Temporal variability correlates with changes in glass compositions, attesting to the ability of clinopyroxene to track magma maficity throughout an eruption. Spatial variability, indicated by the scatter of Cr concentrations, is greatest for the first event and lowest for the final paroxysm. In conjunction with core textures, degree of sector enrichment and thermobarometry, our data suggest that the onset of the paroxysms was preceded by the remobilisation of a mid-crustal clinopyroxene mush (534 ± 46 MPa) by hot, mafic magma causing variable resorption of mush-derived crystal cores. Towards the end of the eruption, waning magma supply led to less efficient mush remobolisation and mixing, resulting in homogenous crystal populations. Our results highlight that clinopyroxene Cr contents and sector enrichment can be used to track mafic rejuvenation and magma evolution throughout eruptions, while also reflecting spatial heterogeneities within the plumbing system.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00410-024-02174-5.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142410643","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Stability of hydrous basaltic melts at low water fugacity: evidence for widespread melting at the lithosphere-asthenosphere boundary","authors":"Marija Putak Juriček,&nbsp;Hans Keppler","doi":"10.1007/s00410-024-02177-2","DOIUrl":"10.1007/s00410-024-02177-2","url":null,"abstract":"<div><p>The upper mantle low velocity zone is often attributed to partial melting at the lithosphere-asthenosphere boundary. This implies that basaltic melts may be stable along plausible geotherms due to the freezing point depression in the presence of water and other incompatible impurities. However, the freezing point depression (<i>ΔT</i>) as a function of water content in the near-solidus basaltic melt (<i>c</i>_<i>H2O</i>) cannot be precisely determined from peridotite melting experiments because of difficulties in recovering homogeneous basaltic glasses at high pressures. We therefore used an alternative approach to reinvestigate and accurately constrain the <i>ΔT–c</i>_<i>H2O</i> relationship for basaltic melts at the low water fugacities that are expected in the upper mantle. Internally heated pressure vessel (IHPV) experiments were performed at water-saturated conditions in the anorthite-diopside-H₂O system at confining pressures of 0.02 to 0.2 GPa and temperatures between 940 and 1450 ℃. We determined the water-saturated solidus, and obtained <i>ΔT</i> by combining our data with reports of dry melting temperatures in the anorthite-diopside system. In another series of experiments, we measured water solubility in haplobasaltic melts and extrapolated <i>c</i>_<i>H2O</i> to pressures and temperatures of the water-saturated solidus. By combining the results from these two series of experiments, we showed that the effect of water on <i>ΔT</i> was previously underestimated by at least 50 ℃. The new <i>ΔT–c</i>_<i>H2O</i> relationship was then used to revise predictions of melt distribution in the upper mantle. Hydrous melt is almost certainly stable beneath extensive regions of the oceanic lithosphere, and may be present in younger and water-enriched zones of the subcontinental mantle.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://link.springer.com/content/pdf/10.1007/s00410-024-02177-2.pdf","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142410382","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Crustal anatectic origin of the pegmatitic carbonate rocks in the Proterozoic Highland Complex, Sri Lanka","authors":"Ben-Xun Su,&nbsp;Shi-Ying Wang,&nbsp;P. L. Dharmapriya,&nbsp;Jing Wang,&nbsp;Sanjeewa P. K. Malaviarachchi,&nbsp;Kui-Feng Yang,&nbsp;Hong-Rui Fan","doi":"10.1007/s00410-024-02178-1","DOIUrl":"10.1007/s00410-024-02178-1","url":null,"abstract":"<div><p>Pegmatitic dyke-like carbonate rocks mainly composed of very coarse-grained calcite, are a rare type of carbonate rocks found in some of orogenic belts in the world. These specific carbonate rocks generally occur intimately with high-temperature granulites and marbles. In the Proterozoic Highland Complex of Sri Lanka which is a segment of the Mozambique suture, they are associated with marbles and granitic pegmatites, and intercalated with high-grade calc-silicate gneisses and highly folded ortho- and para-gneisses. These pegmatitic carbonate rocks do not show any signs of metamorphic or deformed overprint, but instead well preserve igneous textures and contain various silicate crustal xenoliths. The calcite crystals occur as euhedral to subhedral grains and are large in size from 1 to 15 cm. The diverse colors of calcite from white to yellow and blue derive from mineral inclusions and their own compositions. Non-carbonate minerals, commonly present in typical carbonatites such as phlogopite, apatite, clinopyroxene, olivine, plagioclase, iron oxides and spinel, are all found in the rocks. Meanwhile, a skarn-type assemblage of wollastonite, garnet, clinopyroxene and sulfide occurs in contact between the carbonate rocks and gneiss xenoliths, which probably resulted from antiskarn reactions. Chemical compositions of major constituent minerals (calcite, dolomite and apatite) of the carbonate rocks are intermediate between typical marbles and mantle-derived carbonatites and akin to crustal-origin carbonatites worldwide. We thus classify the studied rocks as ‘anatectic carbonatite pegmatite’, and suggest that they originated from the melting of a mixture of marbles and surrounding silicate rocks at crustal levels during high-temperature metamorphism.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142410048","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithological and compositional diversity of diamond substrates beneath the Koidu kimberlite reveals addition of subducted sediments","authors":"Mei Yan Lai,&nbsp;Thomas Stachel,&nbsp;Matthew F. Hardman,&nbsp;D. Graham Pearson,&nbsp;Jeff W. Harris,&nbsp;Richard A. Stern,&nbsp;Stephen E. Haggerty","doi":"10.1007/s00410-024-02176-3","DOIUrl":"10.1007/s00410-024-02176-3","url":null,"abstract":"<div><p>Six diamond-bearing eclogite xenoliths with oceanic crust protoliths and 370 mineral inclusions in 104 diamonds recovered from the Koidu kimberlite complex in Sierra Leone provide insight into the lithological and compositional diversity of the lithospheric mantle beneath the West African Craton. Diamond formation beneath Koidu is predominantly associated with eclogitic substrates that originated from subduction and high-pressure metamorphism of oceanic crust, as indicated by a dominance of eclogitic (78%) over peridotitic (17%) and mixed paragenesis diamonds (5%). Peridotitic diamonds contain olivine inclusions with very high Mg# (92.2–94.7; median = 94.2), indicative of derivation from dunite or harzburgite protoliths. Moreover, a peridotitic spinel with Cr# = 50.9 suggests that it equilibrated with orthopyroxene-free dunite. 44% of Koidu diamonds contain coesite, of which some coexist with omphacite, eclogitic garnet, and/or kyanite. Most analysed eclogitic garnet inclusions have extremely high δ¹⁸O values ( ≥ + 9.9‰) and occur with clinopyroxene inclusions that have very high jadeite components (~ 70 mol%). These high jadeite components are a close match to clinopyroxenes in high-pressure metapelites, which have a phase assemblage that includes coesite and kyanite. Our data suggest that the eclogitic mineral inclusions in most Koidu diamonds have oceanic basalt protoliths that were mingled with pelagic sediments, which may have increased δ¹⁸O values to levels much higher than observed for other eclogites at Koidu and shifted the originally basaltic bulk compositions closer to that of pelites. Most eclogitic mineral inclusions in Koidu diamonds have elemental compositions not observed for Koidu eclogite xenoliths, which have clear oceanic crust protolith (oceanic lavas and cumulates) signatures without significant crustal sediment contamination. These findings suggest the subduction of distinct packages of oceanic crust into the Koidu lithospheric mantle through time.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 11","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142409886","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Magma degassing of ore-metals into submarine hydrothermal systems: a case study from the Xunmei hydrothermal field, South Mid-Atlantic Ridge","authors":"Sai Wang,&nbsp;Chuanshun Li,&nbsp;Bing Li,&nbsp;Yili Guan,&nbsp;Yuan Dang,&nbsp;Jun Ye,&nbsp;Xuefa Shi,&nbsp;Yongqiang Yang","doi":"10.1007/s00410-024-02168-3","DOIUrl":"10.1007/s00410-024-02168-3","url":null,"abstract":"<div><p>The Xunmei hydrothermal field, located at 26°S along the South Mid-Atlantic Ridge, is an active submarine hydrothermal system underlain by a basaltic substrate. This field comprises two distinct types of basalts: massive basalts, characterized by aphyric to moderately porphyritic textures without large vesicles, and vesicular basalts, known for their highly vesicular nature. Olivine-hosted melt inclusions within the massive basalts exhibit a diverse range of chemical compositions. Type-A melt inclusions are distinguished by lower levels of K₂O, Rb, Ba and U, but higher concentrations of S, Co, Ni, and Cu. Conversely, Type-B melt inclusions exhibit higher levels of K₂O, Rb, Ba and U, but lower concentrations of S, Co, Ni, and Cu. Although both types of melt inclusions show similar ranges of La/Sm, La/Yb, Sr/Yb, and Zr/Nb, the significant differences in K₂O/TiO₂ and Nb/U indicate that the massive basalts likely originate from the mixing of two distinct melts derived from different source regions. Data from melt inclusions and quenched basaltic glasses, combined with theoretical calculations, indicate that Type-I melts, represented by the Type-A melt inclusions, were sulfide-saturated during the crystallization of olivine at depth, evolving into sulfide-unsaturated melts as they ascended towards the seafloor. Approximately 50% of the Cu in the Type-I melts transitioned to the gas phase and were eventually released from the magma to the overlying hydrothermal system. Conversely, Type-II melts, represented by the Type-B melt inclusions, did not reach sulfide saturation. The presence of magmatic sulfides within or attached to vesicles, occupying voids in the primocryst frameworks, and lining the walls of vapor bubbles in melt inclusions, may suggest a volatile-driven transport of magmatic sulfides from the magma system as compound drops during magma degassing. This mechanism likely plays a crucial role in the supply ore-metals during the formation of seafloor massive sulfides in the Xunmei and possibly other hydrothermal fields along mid-ocean ridges.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 10","pages":""},"PeriodicalIF":3.5,"publicationDate":"2024-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142409356","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}