Chang Yu, Richen Zhong, Andrew G. Tomkins, Hao Cui, Yanjing Chen
Orogenic gold deposits contribute the largest proportion of the world’s gold reserves, and the source of their ore-forming components has been recognized as the metamorphic devolatilization of metapelites or metabasites across the greenschist- to amphibolite-facies transition. However, hypozonal orogenic gold deposits represent an enigma in this context. Some of these apparently formed in higher-grade metamorphic rocks when temperatures were beyond the wet solidus of quartz-feldspar–bearing rocks; it is therefore puzzling how these fluids were generated in the source and migrated through the crust without causing partial melting. Here, we show that devolatilization of hydrated komatiites, a volumetrically significant lithological unit in Precambrian greenstone belts, is a viable model that can plausibly lead to gold mineralization at amphibolite-facies conditions. Our thermodynamic simulations indicate that subsolidus metamorphic devolatilization of komatiites at ~700 °C (upper amphibolite facies) can unlock significant amounts of gold via dehydration of talc and chlorite. This genetic model is supported by the geochemical characteristics of, and estimated pressure-temperature (P-T) formation conditions of, hypozonal gold deposits and the intimate spatiotemporal association between hypozonal deposits and komatiites in greenstone belts. This work expands the P-T range of the metamorphic devolatilization model and enhances its robustness in explaining gold mineralization in metamorphic terranes.Orogenic gold deposits are widely studied because they contribute over a quarter of the world’s gold supply (Goldfarb et al., 2005); however, the source(s) of their ore-forming components (fluid, sulfur, gold, and other metals) has(have) long been debated (Goldfarb and Groves, 2015; Groves et al., 2020; Kolb et al., 2015; Phillips and Powell, 2010; Selvaraja et al., 2017; Tomkins, 2010; Wang et al., 2022; Zhao et al., 2019). The widely accepted metamorphic devolatilization model (Phillips and Powell, 2010) emphasizes that gold-bearing fluids are produced by metamorphic dehydration of hydrous crustal rocks, particularly at the greenschist to amphibolite transition, largely through breakdown of chlorite (~12% H2O) to minerals like biotite (~4% H2O), hornblende (~2% H2O), and garnet (anhydrous) (Goldfarb et al., 2005; Phillips and Powell, 2010; Pitcairn et al., 2006; Tomkins, 2010; Zhong et al., 2015). At temperatures higher than the greenschist-amphibolite transition, there is minimal opportunity for fluid liberation from the metamorphosed mafic and sedimentary rocks. As a result, gold and sulfur are thought to be inaccessible in the sources under these conditions (Tomkins, 2013). Since fluids generated at the greenschist-amphibolite transition tend to migrate upward into rocks of lower metamorphic grades, this model satisfactorily explains the formation of orogenic gold in lower-amphibolite- to greenschist-facies terranes.Deposits are also found in higher-gra
{"title":"Expanding the metamorphic devolatilization model: Komatiites as a source for orogenic gold deposits in high-grade metamorphic rocks","authors":"Chang Yu, Richen Zhong, Andrew G. Tomkins, Hao Cui, Yanjing Chen","doi":"10.1130/g51446.1","DOIUrl":"https://doi.org/10.1130/g51446.1","url":null,"abstract":"Orogenic gold deposits contribute the largest proportion of the world’s gold reserves, and the source of their ore-forming components has been recognized as the metamorphic devolatilization of metapelites or metabasites across the greenschist- to amphibolite-facies transition. However, hypozonal orogenic gold deposits represent an enigma in this context. Some of these apparently formed in higher-grade metamorphic rocks when temperatures were beyond the wet solidus of quartz-feldspar–bearing rocks; it is therefore puzzling how these fluids were generated in the source and migrated through the crust without causing partial melting. Here, we show that devolatilization of hydrated komatiites, a volumetrically significant lithological unit in Precambrian greenstone belts, is a viable model that can plausibly lead to gold mineralization at amphibolite-facies conditions. Our thermodynamic simulations indicate that subsolidus metamorphic devolatilization of komatiites at ~700 °C (upper amphibolite facies) can unlock significant amounts of gold via dehydration of talc and chlorite. This genetic model is supported by the geochemical characteristics of, and estimated pressure-temperature (P-T) formation conditions of, hypozonal gold deposits and the intimate spatiotemporal association between hypozonal deposits and komatiites in greenstone belts. This work expands the P-T range of the metamorphic devolatilization model and enhances its robustness in explaining gold mineralization in metamorphic terranes.Orogenic gold deposits are widely studied because they contribute over a quarter of the world’s gold supply (Goldfarb et al., 2005); however, the source(s) of their ore-forming components (fluid, sulfur, gold, and other metals) has(have) long been debated (Goldfarb and Groves, 2015; Groves et al., 2020; Kolb et al., 2015; Phillips and Powell, 2010; Selvaraja et al., 2017; Tomkins, 2010; Wang et al., 2022; Zhao et al., 2019). The widely accepted metamorphic devolatilization model (Phillips and Powell, 2010) emphasizes that gold-bearing fluids are produced by metamorphic dehydration of hydrous crustal rocks, particularly at the greenschist to amphibolite transition, largely through breakdown of chlorite (~12% H2O) to minerals like biotite (~4% H2O), hornblende (~2% H2O), and garnet (anhydrous) (Goldfarb et al., 2005; Phillips and Powell, 2010; Pitcairn et al., 2006; Tomkins, 2010; Zhong et al., 2015). At temperatures higher than the greenschist-amphibolite transition, there is minimal opportunity for fluid liberation from the metamorphosed mafic and sedimentary rocks. As a result, gold and sulfur are thought to be inaccessible in the sources under these conditions (Tomkins, 2013). Since fluids generated at the greenschist-amphibolite transition tend to migrate upward into rocks of lower metamorphic grades, this model satisfactorily explains the formation of orogenic gold in lower-amphibolite- to greenschist-facies terranes.Deposits are also found in higher-gra","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840039","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}
In recent decades, new Ti-based thermometers have found widespread use in geosciences, providing a convenient and powerful tool for investigating the crystallization temperatures of quartz and zircons in magmatic systems. However, a commonly overlooked aspect is the constraint of TiO2 activity (aTiO2liquid–rutile). Many studies assume aTiO2 to be constant or equate the presence of Ti-rich phases, such as ilmenite, with fixed activity levels. Using solubility models and data from natural systems, we demonstrate that aTiO2 is a dynamic parameter, influenced by temperature, mineral assemblage, and TiO2 content in the melt. Focusing on examples from several volcanic fields (Bishop Tuff, Fish Canyon Tuff, Yellowstone, and Shiveluch), we discuss the impact of these factors on aTiO2 and highlight how inadequate constraint of aTiO2 can lead to erroneous interpretations of magma storage conditions.
{"title":"The dynamic nature of aTiO2: Implications for Ti-based thermometers in magmatic systems","authors":"L.M. Fonseca Teixeira, J. Troch, O. Bachmann","doi":"10.1130/g51587.1","DOIUrl":"https://doi.org/10.1130/g51587.1","url":null,"abstract":"In recent decades, new Ti-based thermometers have found widespread use in geosciences, providing a convenient and powerful tool for investigating the crystallization temperatures of quartz and zircons in magmatic systems. However, a commonly overlooked aspect is the constraint of TiO2 activity (aTiO2liquid–rutile). Many studies assume aTiO2 to be constant or equate the presence of Ti-rich phases, such as ilmenite, with fixed activity levels. Using solubility models and data from natural systems, we demonstrate that aTiO2 is a dynamic parameter, influenced by temperature, mineral assemblage, and TiO2 content in the melt. Focusing on examples from several volcanic fields (Bishop Tuff, Fish Canyon Tuff, Yellowstone, and Shiveluch), we discuss the impact of these factors on aTiO2 and highlight how inadequate constraint of aTiO2 can lead to erroneous interpretations of magma storage conditions.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840383","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}
Zheng Gong, David A.D. Evans, Zhongtian Zhang, Chi Yan
Abstract not available
无摘要
{"title":"Mid-Proterozoic geomagnetic field was more consistent with a dipole than a quadrupole: REPLY","authors":"Zheng Gong, David A.D. Evans, Zhongtian Zhang, Chi Yan","doi":"10.1130/g51903y.1","DOIUrl":"https://doi.org/10.1130/g51903y.1","url":null,"abstract":"Abstract not available","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840080","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}
Lead isotopic data imply that thorium and uranium were fractionated from one another in Earth’s early history; however, the origin of this fractionation is poorly understood. We report new in situ Pb isotope data from orthoclase in 144 granites sampled across the Archean Yilgarn craton (Western Australia) to characterize its Pb isotope variability and evolution. Granite Pb isotope compositions reveal three Pb sources, a mantle-derived Pb reservoir and two crustal Pb reservoirs, distinguished by their implied source 232Th/238U (κPb). High-κPb granites reflect sources with high 232Th/238U (~4.7) and are largely co-located with Eoarchean–Paleoarchean crust. The Pb isotope compositions of most granites, and those of volcanic-hosted massive sulfide (VHMS) and gold ores, define a mixing array between a mantle Pb source and a Th-rich Eoarchean–Paleoarchean source. Pb isotope modeling indicates that the high-κPb source rocks experienced Th/U fractionation at ca. 3.3 Ga. As Th/U fractionation in the Yilgarn craton must have occurred before Earth’s atmosphere was oxygenated, subaerial weathering cannot explain the apparent differences in their geochemical behavior. Instead, the high Th/U source reflects Eoarchean–Paleoarchean rocks that experienced prior high-temperature metamorphism, partial melting, and melt loss in the presence of a Th-sequestering mineral like monazite. Archean Pb isotope variability thus has its origins in open-system high-temperature metamorphic processes responsible for the differentiation and stabilization of Earth’s continental crust.Thorium and uranium are highly incompatible trace elements that are partitioned into Earth’s crust over geological time (Galer and O’nions, 1985; Allègre et al., 1986). Being two of the main heat-producing elements in the silicate Earth, understanding their partitioning between different geochemical reservoirs is important for tracking our planet’s thermal evolution and internal differentiation.Thorium has a single valence state (4+) whereas U exists in two valence states (4+ and 6+), with the highly water-soluble hexavalent species dominant under oxidized surface conditions (Zartman and Haines, 1988). Since the Great Oxidation Event (2.5–2.4 Ga), U has preferentially been recycled into the mantle, causing a progressive lowering of the Th/U ratio in the mantle and in rocks derived from it (McCulloch, 1993; Collerson and Kamber, 1999; Elliott et al., 1999; Zartman and Richardson, 2005). However, in the Archean, when Earth’s atmosphere was largely devoid of oxygen, neither mantle melting, fractional crystallization, nor weathering and recycling processes could have fractionated U and Th. Hence, it is thought the geochemical behavior of these elements was identical from the surface down to the upper mantle (Liu et al., 2019). Nevertheless, some lines of evidence show that Th and U were fractionated from one another early in Earth’s history. For example, variability in the 208Pb/204Pb ratios of some Arche
然而，早期地壳分异过程中形成的致密低Th/U岩浆残留物的脱层也可能产生同样的结果（图4B；Emo等人，2023年）。这些机制并不相互排斥，都可能在克拉通发展的不同时期导致高Th/U特征。无论如何，很显然，早阿基坦时期 Th 和 U 的分馏与稳定地球大陆地壳的地壳内分化过程密切相关，而附属矿物在其中发挥了关键作用。Tim Elliott 和 Balz Kamber 富有洞察力的审阅大大改进了手稿。感谢 Brian Kennett 提供 AUSREM 数据，感谢 Noreen Evans、Brad McDonald 和 Kai Rankenburg 的分析协助，以及 Marc Norman 的编辑处理。科廷大学的研究工作得到了AuScope和澳大利亚政府的支持，由澳大利亚研究理事会（ARC）提供资金（LE150100013）。SPJ和RHS经GSWA执行董事许可出版。DRM 经澳大利亚地球科学协会首席执行官许可发布。
{"title":"Origin of Archean Pb isotope variability through open-system Paleoarchean crustal anatexis","authors":"M.I.H. Hartnady, C.L. Kirkand, S.P. Johnson, R.H. Smithies, L.S. Doucet, D.R. Mole","doi":"10.1130/g51507.1","DOIUrl":"https://doi.org/10.1130/g51507.1","url":null,"abstract":"Lead isotopic data imply that thorium and uranium were fractionated from one another in Earth’s early history; however, the origin of this fractionation is poorly understood. We report new in situ Pb isotope data from orthoclase in 144 granites sampled across the Archean Yilgarn craton (Western Australia) to characterize its Pb isotope variability and evolution. Granite Pb isotope compositions reveal three Pb sources, a mantle-derived Pb reservoir and two crustal Pb reservoirs, distinguished by their implied source 232Th/238U (κPb). High-κPb granites reflect sources with high 232Th/238U (~4.7) and are largely co-located with Eoarchean–Paleoarchean crust. The Pb isotope compositions of most granites, and those of volcanic-hosted massive sulfide (VHMS) and gold ores, define a mixing array between a mantle Pb source and a Th-rich Eoarchean–Paleoarchean source. Pb isotope modeling indicates that the high-κPb source rocks experienced Th/U fractionation at ca. 3.3 Ga. As Th/U fractionation in the Yilgarn craton must have occurred before Earth’s atmosphere was oxygenated, subaerial weathering cannot explain the apparent differences in their geochemical behavior. Instead, the high Th/U source reflects Eoarchean–Paleoarchean rocks that experienced prior high-temperature metamorphism, partial melting, and melt loss in the presence of a Th-sequestering mineral like monazite. Archean Pb isotope variability thus has its origins in open-system high-temperature metamorphic processes responsible for the differentiation and stabilization of Earth’s continental crust.Thorium and uranium are highly incompatible trace elements that are partitioned into Earth’s crust over geological time (Galer and O’nions, 1985; Allègre et al., 1986). Being two of the main heat-producing elements in the silicate Earth, understanding their partitioning between different geochemical reservoirs is important for tracking our planet’s thermal evolution and internal differentiation.Thorium has a single valence state (4+) whereas U exists in two valence states (4+ and 6+), with the highly water-soluble hexavalent species dominant under oxidized surface conditions (Zartman and Haines, 1988). Since the Great Oxidation Event (2.5–2.4 Ga), U has preferentially been recycled into the mantle, causing a progressive lowering of the Th/U ratio in the mantle and in rocks derived from it (McCulloch, 1993; Collerson and Kamber, 1999; Elliott et al., 1999; Zartman and Richardson, 2005). However, in the Archean, when Earth’s atmosphere was largely devoid of oxygen, neither mantle melting, fractional crystallization, nor weathering and recycling processes could have fractionated U and Th. Hence, it is thought the geochemical behavior of these elements was identical from the surface down to the upper mantle (Liu et al., 2019). Nevertheless, some lines of evidence show that Th and U were fractionated from one another early in Earth’s history. For example, variability in the 208Pb/204Pb ratios of some Arche","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840104","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}
Sami Mikhail, Eva E. Stüeken, Toby J. Boocock, Megan Athey, Nick Mappin, Adrian J. Boyce, Janne Liebmann, Christopher J. Spencer, Claire E. Bucholz
Strongly peraluminous granites (SPGs) are generated by the partial melting of sedimentary rocks and can thus provide a novel archive to reveal secular trends in Earth’s environmental history that integrate siliciclastic sedimentary lithologies. The nitrogen (N) content of Archean, Proterozoic, and Phanerozoic SPGs reveals a systematic increase across the Precambrian–Phanerozoic boundary. This rise is supported by a coeval increase in the phosphorus (P) contents of SPGs. Collectively, these data are most parsimoniously explained by an absolute increase in biomass burial in the late Proterozoic or early Phanerozoic by a factor of ~5 and as much as 8. The Precambrian–Phanerozoic transition was a time of progressive oxygenation of surface environments paired with major biological innovations, including the rise of eukaryotic algae to ecological dominance. Because oxygenation suppresses biomass preservation in sediments, the increase in net biomass burial preserved in SPGs reveals an expansion of the biosphere and an increase in primary production across this interval.
{"title":"Strongly peraluminous granites provide independent evidence for an increase in biomass burial across the Precambrian–Phanerozoic boundary","authors":"Sami Mikhail, Eva E. Stüeken, Toby J. Boocock, Megan Athey, Nick Mappin, Adrian J. Boyce, Janne Liebmann, Christopher J. Spencer, Claire E. Bucholz","doi":"10.1130/g51800.1","DOIUrl":"https://doi.org/10.1130/g51800.1","url":null,"abstract":"Strongly peraluminous granites (SPGs) are generated by the partial melting of sedimentary rocks and can thus provide a novel archive to reveal secular trends in Earth’s environmental history that integrate siliciclastic sedimentary lithologies. The nitrogen (N) content of Archean, Proterozoic, and Phanerozoic SPGs reveals a systematic increase across the Precambrian–Phanerozoic boundary. This rise is supported by a coeval increase in the phosphorus (P) contents of SPGs. Collectively, these data are most parsimoniously explained by an absolute increase in biomass burial in the late Proterozoic or early Phanerozoic by a factor of ~5 and as much as 8. The Precambrian–Phanerozoic transition was a time of progressive oxygenation of surface environments paired with major biological innovations, including the rise of eukaryotic algae to ecological dominance. Because oxygenation suppresses biomass preservation in sediments, the increase in net biomass burial preserved in SPGs reveals an expansion of the biosphere and an increase in primary production across this interval.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840335","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":"Mid-Proterozoic geomagnetic field was more consistent with a dipole than a quadrupole: COMMENT","authors":"James W. Sears","doi":"10.1130/g51799c.1","DOIUrl":"https://doi.org/10.1130/g51799c.1","url":null,"abstract":"Abstract not available","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840362","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}
J. Gilgannon, Damien Freitas, R. E. Rizzo, John Wheeler, Ian B. Butler, Sohan Seth, Federica Marone, C. Schlepütz, Gina McGill, Ian Watt, Oliver Plümper, Lisa Eberhard, Hamed Amiri, A. Chogani, F. Fusseis
Detailing the relationship between stress and reactions in metamorphic rocks has been controversial, and much of the debate has centered on theory. Here, we add to this discussion and make a major advance by showing in time-resolved synchrotron microtomography experiments that a reacting and deforming sample experiencing an elastic differential stress produces a fabric orthogonal to the largest principal stress. This fabric forms very early in the reaction and can be shown to be unrelated to strain. The consequences of this are significant because a non-hydrostatic stress state is a very common geological occurrence. Our data provide the basis for new interpretations of the classical, and enigmatic, serpentine fabrics of Val Malenco, Italy, and Cerro del Almirez, Spain, where we relate the reported fabrics to transient, and cyclical, differential stresses from magma intrusion and the earthquake cycle.
关于变质岩中应力与反应之间关系的详细研究一直存在争议，大部分争论都集中在理论上。在这里，我们通过时间分辨同步辐射显微层析成像实验证明，正在经历弹性差应力的反应和变形样品会产生与最大主应力正交的结构，从而为这一讨论添砖加瓦，并取得重大进展。这种结构在反应初期就已形成，而且可以证明与应变无关。由于非静水压力状态是一种非常常见的地质现象，因此其后果非常重要。我们的数据为重新解释意大利 Val Malenco 和西班牙 Cerro del Almirez 的经典和神秘蛇纹石构造提供了依据，我们将报告的构造与岩浆侵入和地震周期产生的瞬时和周期性差异应力联系起来。
{"title":"Elastic stresses can form metamorphic fabrics","authors":"J. Gilgannon, Damien Freitas, R. E. Rizzo, John Wheeler, Ian B. Butler, Sohan Seth, Federica Marone, C. Schlepütz, Gina McGill, Ian Watt, Oliver Plümper, Lisa Eberhard, Hamed Amiri, A. Chogani, F. Fusseis","doi":"10.1130/g51612.1","DOIUrl":"https://doi.org/10.1130/g51612.1","url":null,"abstract":"Detailing the relationship between stress and reactions in metamorphic rocks has been controversial, and much of the debate has centered on theory. Here, we add to this discussion and make a major advance by showing in time-resolved synchrotron microtomography experiments that a reacting and deforming sample experiencing an elastic differential stress produces a fabric orthogonal to the largest principal stress. This fabric forms very early in the reaction and can be shown to be unrelated to strain. The consequences of this are significant because a non-hydrostatic stress state is a very common geological occurrence. Our data provide the basis for new interpretations of the classical, and enigmatic, serpentine fabrics of Val Malenco, Italy, and Cerro del Almirez, Spain, where we relate the reported fabrics to transient, and cyclical, differential stresses from magma intrusion and the earthquake cycle.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2023-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138959733","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}
Ping-Ping Liu, Dian-Bing Wang, Mei-Fu Zhou, Xian-Hua Li, Qiuli Li, G. Gaetani, B. Monteleone, Vadim Kamenetsky
Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8−7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ∼0.02‰. Approximately 6−9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.
弧岩浆产生于地幔楔块，可能还有来自俯冲板块中蛇纹岩和沉积物的流体和熔体。确定此类岩浆的各种来源及其相关贡献具有挑战性；特别是在大陆弧，地壳同化可能会覆盖最初的地球化学特征。本研究介绍了来自后火山口玄武岩的带状橄榄石颗粒的氧同位素组成，以及这些玄武岩和印度尼西亚多巴火山系统硅质凝灰岩的石英和挛辉石中玻璃状熔融包裹体的硼含量和同位素。玄武岩中橄榄石的高镁核（≥87 mol% Fo [sterite]）的δ18O值范围为5.12‰至6.14‰，表明多巴火山下的地幔源不同程度地富含18O。40%的流体来自蛇纹岩，其他则来自蚀变的大洋地壳和沉积物。这些流体只能使岩浆的δ18O增加0.02‰。还需要地幔楔中大约6-9%的沉积物衍生熔体杂化，才能产生δ18O值与高Fo橄榄石岩芯平衡的玄武岩熔体。另一方面，同生硅质凝灰岩似乎记录了较高比例的流体添加，除地壳同化外，主要是沉积物衍生的流体添加到地幔源。因此，我们的勘察研究证明了 B 和 O 组合同位素在区分来自俯冲板块中蛇纹岩和沉积物的熔体和流体方面的应用--这种应用可适用于世界各地的弧岩浆。
{"title":"Constraints of boron and oxygen stable isotopes on dehydration fluids, sediment-derived melts, and crustal assimilation of the Toba volcanic system (Indonesia)","authors":"Ping-Ping Liu, Dian-Bing Wang, Mei-Fu Zhou, Xian-Hua Li, Qiuli Li, G. Gaetani, B. Monteleone, Vadim Kamenetsky","doi":"10.1130/g51690.1","DOIUrl":"https://doi.org/10.1130/g51690.1","url":null,"abstract":"Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8−7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ∼0.02‰. Approximately 6−9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2023-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138997287","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}
Kai Xiang, Zongjun Yin, Wei Liu, Fangchen Zhao, Maoyan Zhu
The origin of Bryozoa has long been an intriguing mystery, largely due to the dearth of unambiguous bryozoan fossils within Cambrian strata. The earliest bryozoan thus far identified, Protomelission, an early Cambrian microfossil from Australia and China, was recently contested and proposed to be dasyclad algae. Our reanalysis of micro-computed tomography data from Protomelission specimens, however, unveils previously unrevealed structures, indicating Protomelission to be neither bryozoan nor alga, but a scleritome of Cambroclavus, an early Cambrian microfossil typically found as disarticulated sclerites. The long-sought scleritomes suggest that enigmatic cambroclaves likely represent an extinct group of epitheliozoan-grade eumetazoan, with spiny sclerites covering their epidermis for defense. The prevalence of bizarre animals like cambroclaves, lacking modern analogues, implies a greater disparity of animal body plans during the Cambrian explosion than previously recognized. Our findings underscore the absence of definitive early Cambrian bryozoan fossils, further highlighting the dilemma of the origin of Bryozoa.
{"title":"Early Cambrian Cambroclavus is a scleritomous eumetazoan unrelated to bryozoan or dasyclad algae","authors":"Kai Xiang, Zongjun Yin, Wei Liu, Fangchen Zhao, Maoyan Zhu","doi":"10.1130/g51663.1","DOIUrl":"https://doi.org/10.1130/g51663.1","url":null,"abstract":"The origin of Bryozoa has long been an intriguing mystery, largely due to the dearth of unambiguous bryozoan fossils within Cambrian strata. The earliest bryozoan thus far identified, Protomelission, an early Cambrian microfossil from Australia and China, was recently contested and proposed to be dasyclad algae. Our reanalysis of micro-computed tomography data from Protomelission specimens, however, unveils previously unrevealed structures, indicating Protomelission to be neither bryozoan nor alga, but a scleritome of Cambroclavus, an early Cambrian microfossil typically found as disarticulated sclerites. The long-sought scleritomes suggest that enigmatic cambroclaves likely represent an extinct group of epitheliozoan-grade eumetazoan, with spiny sclerites covering their epidermis for defense. The prevalence of bizarre animals like cambroclaves, lacking modern analogues, implies a greater disparity of animal body plans during the Cambrian explosion than previously recognized. Our findings underscore the absence of definitive early Cambrian bryozoan fossils, further highlighting the dilemma of the origin of Bryozoa.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2023-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139008739","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}
We present 17 new 10Be erosion rates from southern Peru sampled across an extreme orographic rainfall gradient. Using a rainfall-weighted variant of the normalized channel steepness index, ksnQ, we show that channel steepness values, and thus topography, are adjusted to spatially varying rainfall. Rocks with similar physical characteristics define distinct relationships between ksnQ and erosion rate (E), suggesting ksnQ is also resolving lithologic variations in erodibility. However, substantial uncertainty exists in parameters describing these relationships. By combining our new data with 38 published erosion rates from Peru and Bolivia, we collapse the range of compatible parameter values and resolve robust, nonlinear ksnQ−E relationships suggestive of important influences of erosional thresholds, rock properties, sediment characteristics, and temporal runoff variability. In contrast, neither climatic nor lithologic effects are clear using the traditional channel steepness metric, ksn. Our results highlight that accounting for spatial rainfall variations is essential for disentangling the multiple influences of climate, lithology, and tectonics common in mountain landscapes, which is a necessary first step toward greater understanding of how these landscapes evolve.
{"title":"Controls on topography and erosion of the north-central Andes","authors":"Joel S. Leonard, K. Whipple, Arjun M. Heimsath","doi":"10.1130/g51618.1","DOIUrl":"https://doi.org/10.1130/g51618.1","url":null,"abstract":"We present 17 new 10Be erosion rates from southern Peru sampled across an extreme orographic rainfall gradient. Using a rainfall-weighted variant of the normalized channel steepness index, ksnQ, we show that channel steepness values, and thus topography, are adjusted to spatially varying rainfall. Rocks with similar physical characteristics define distinct relationships between ksnQ and erosion rate (E), suggesting ksnQ is also resolving lithologic variations in erodibility. However, substantial uncertainty exists in parameters describing these relationships. By combining our new data with 38 published erosion rates from Peru and Bolivia, we collapse the range of compatible parameter values and resolve robust, nonlinear ksnQ−E relationships suggestive of important influences of erosional thresholds, rock properties, sediment characteristics, and temporal runoff variability. In contrast, neither climatic nor lithologic effects are clear using the traditional channel steepness metric, ksn. Our results highlight that accounting for spatial rainfall variations is essential for disentangling the multiple influences of climate, lithology, and tectonics common in mountain landscapes, which is a necessary first step toward greater understanding of how these landscapes evolve.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2023-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139009842","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}
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