In the past two decades, extensive investigations have been carried out on the Trans-North China Orogen (TNCO), a Himalayan-type continental collisional belt along which the Eastern Block and the Western Block amalgamated to form the basement of the North China Craton. There are now coherent outlines of the timing and tectonic processes involved in the Paleoproterozoic amalgamation of the TNCO. However, pre-collisional tectonic setting and driving mechanism of the TNCO still remain controversial. To resolve these issues, we carried out field petrological and geochemical investigations on the Tianzhen gneisses from the Huai'an Complex in the TNCO. The Tianzhen gneisses consist predominantly of tonalitic-trondhjemitic-granodioritic (TTG) series, which can be further divided into low-Yb and high-Yb types. Our results indicate that although both low-Yb and high-Yb TTG series were formed in a magmatic arc environment, their petrogenetic origins were different. The Huai'an low-Yb TTG magma was derived from the partial melting of subducted oceanic crust consisting of eclogite or 30% garnet-bearing amphibolite under 15 to 20 kbar or even higher pressure, with garnet, amphibole, and rutile as residues. In contrast, the high-Yb TTG magma was derived from the partial melting of subducted oceanic slab consisting of garnet-free or 7% garnet amphibolite under 10 to 15 kbar, leaving residual garnet and amphibole. Both magmas then interacted with the overlying mantle wedge during ascent. Thus, it can be concluded that the Tianzhen TTG magmas were derived from the partial melting of subducted oceanic crust and interaction with mantle peridotite, supporting a magmatic arc setting for the Huai'an Complex during Neoarchean to Paleoproterozoic time. Associated with other analogous gneiss complexes in the TNCO, a long-lived Neoarchean to Paleoproterozoic magmatic arc system is established in the Trans-North China Orogen.
{"title":"A Neoarchean to Paleoproterozoic magmatic arc in the Trans-North China Orogen: Petrological and geochemical constraints from the Tianzhen gneisses in the Huai'an Complex","authors":"Dingyi Zhao, M. Sun","doi":"10.2475/02.2022.03","DOIUrl":"https://doi.org/10.2475/02.2022.03","url":null,"abstract":"In the past two decades, extensive investigations have been carried out on the Trans-North China Orogen (TNCO), a Himalayan-type continental collisional belt along which the Eastern Block and the Western Block amalgamated to form the basement of the North China Craton. There are now coherent outlines of the timing and tectonic processes involved in the Paleoproterozoic amalgamation of the TNCO. However, pre-collisional tectonic setting and driving mechanism of the TNCO still remain controversial. To resolve these issues, we carried out field petrological and geochemical investigations on the Tianzhen gneisses from the Huai'an Complex in the TNCO. The Tianzhen gneisses consist predominantly of tonalitic-trondhjemitic-granodioritic (TTG) series, which can be further divided into low-Yb and high-Yb types. Our results indicate that although both low-Yb and high-Yb TTG series were formed in a magmatic arc environment, their petrogenetic origins were different. The Huai'an low-Yb TTG magma was derived from the partial melting of subducted oceanic crust consisting of eclogite or 30% garnet-bearing amphibolite under 15 to 20 kbar or even higher pressure, with garnet, amphibole, and rutile as residues. In contrast, the high-Yb TTG magma was derived from the partial melting of subducted oceanic slab consisting of garnet-free or 7% garnet amphibolite under 10 to 15 kbar, leaving residual garnet and amphibole. Both magmas then interacted with the overlying mantle wedge during ascent. Thus, it can be concluded that the Tianzhen TTG magmas were derived from the partial melting of subducted oceanic crust and interaction with mantle peridotite, supporting a magmatic arc setting for the Huai'an Complex during Neoarchean to Paleoproterozoic time. Associated with other analogous gneiss complexes in the TNCO, a long-lived Neoarchean to Paleoproterozoic magmatic arc system is established in the Trans-North China Orogen.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46466092","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Rugenstein, K. Methner, T. Kukla, A. Mulch, Tina Lüdecke, J. Fiebig, A. Meltzer, K. Wegmann, P. Zeitler, C. Chamberlain
The timing of surface uplift of the Altai Mountains in northern Central Asia—and the climatic consequences—remains controversial. Today, the Altai Mountains cast a substantial rain shadow, effectively separating the western Gobi Desert and steppe from the Siberian Taiga. We take advantage of this stark climatic gradient to trace the interaction of climate and topography in the lee of the Altai. First, we present new water stable isotope data that demonstrate that—along with this climatic gradient—the Altai modify the δ18O of precipitation via rainout on the leeward side of the range. Second, we present a new paleosol carbonate clumped isotope (Δ47) record that spans much of the Neogene from the immediate lee of the Altai in western Mongolia to address how surface temperatures may have responded to potential uplift during the Neogene. We find that Δ47-derived temperatures have, overall, declined by approximately 7 °C over the course of the Neogene, though the precise timing of this decrease remains uncertain. Third, we pair our Δ47 record with previously published stable isotope data to demonstrate that the timing of decreasing temperatures corresponds with long-term stability in paleosol carbonate δ13C values. In contrast, increases in paleosol carbonate δ13C values—linked to declining vegetation productivity—are correlated with intervals of increasing temperatures. We speculate that declines in vegetation biomass and leaf area changed the partitioning of latent and sensible heat, resulting in rising surface temperatures during Altai uplift. In contrast, long-term Neogene cooling drove the overall decline in surface temperatures. Reconstructed soil water δ18O values (based on carbonate δ18O and Δ47 values) remain surprisingly stable over our Neogene record, differing from our expectation of decreasing δ18O values due to progressive uplift of the Altai Mountains and Neogene cooling. We demonstrate that the shift in precipitation seasonality that likely accompanied Altai uplift obscured any change in lee-side precipitation δ18O that would be expected from surface elevation change alone.
{"title":"Clumped isotope constraints on warming and precipitation seasonality in Mongolia following Altai uplift","authors":"J. Rugenstein, K. Methner, T. Kukla, A. Mulch, Tina Lüdecke, J. Fiebig, A. Meltzer, K. Wegmann, P. Zeitler, C. Chamberlain","doi":"10.2475/01.2022.02","DOIUrl":"https://doi.org/10.2475/01.2022.02","url":null,"abstract":"The timing of surface uplift of the Altai Mountains in northern Central Asia—and the climatic consequences—remains controversial. Today, the Altai Mountains cast a substantial rain shadow, effectively separating the western Gobi Desert and steppe from the Siberian Taiga. We take advantage of this stark climatic gradient to trace the interaction of climate and topography in the lee of the Altai. First, we present new water stable isotope data that demonstrate that—along with this climatic gradient—the Altai modify the δ18O of precipitation via rainout on the leeward side of the range. Second, we present a new paleosol carbonate clumped isotope (Δ47) record that spans much of the Neogene from the immediate lee of the Altai in western Mongolia to address how surface temperatures may have responded to potential uplift during the Neogene. We find that Δ47-derived temperatures have, overall, declined by approximately 7 °C over the course of the Neogene, though the precise timing of this decrease remains uncertain. Third, we pair our Δ47 record with previously published stable isotope data to demonstrate that the timing of decreasing temperatures corresponds with long-term stability in paleosol carbonate δ13C values. In contrast, increases in paleosol carbonate δ13C values—linked to declining vegetation productivity—are correlated with intervals of increasing temperatures. We speculate that declines in vegetation biomass and leaf area changed the partitioning of latent and sensible heat, resulting in rising surface temperatures during Altai uplift. In contrast, long-term Neogene cooling drove the overall decline in surface temperatures. Reconstructed soil water δ18O values (based on carbonate δ18O and Δ47 values) remain surprisingly stable over our Neogene record, differing from our expectation of decreasing δ18O values due to progressive uplift of the Altai Mountains and Neogene cooling. We demonstrate that the shift in precipitation seasonality that likely accompanied Altai uplift obscured any change in lee-side precipitation δ18O that would be expected from surface elevation change alone.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47096565","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Masterson, M. Alperin, G. L. Arnold, W. Berelson, B. Jørgensen, H. Røy, D. Johnston
Measurement of the multiple sulfur isotopes (32S/33S/34S) enables the calibration of microbial biosignatures and provides a unique diagnosis of S-based metabolic processes: sulfate reduction, disproportionation, and sulfide oxidation. All three metabolisms carry distinct geochemical consequences for S cycling in modern systems, and are particularly powerful for paleoenvironmental interpretations if their respective contributions can be separated. To hone those interpretations and to further develop a quantitative context for understanding early diagenetic sulfur cycling, we constructed a multiple S isotope reactive transport model for the sediments of a geochemically well-characterized system (Aarhus Bay, Denmark). The model reconciles pore water and solid phase concentration profiles of the major species associated with Fe/S/C cycling, and uses multiple S isotope systematics to predict the isotope profiles of the major S species, including pore water sulfate, free sulfide and solid phase pyrite. We note that very large fractionations associated with sulfate reduction (34εsr = 70‰) are required to reproduce the observed pore water profiles, and we reconcile these fractionations with low temperature theoretical predictions for isotope equilibrium fractionation. The minor sulfur isotope values (noted as Δ33S) of sulfate increase at shallow depths within the Aarhus Bay core, and decrease when sulfate drops below 10 mM. Values (Δ33S) for sulfide decrease nearly monotonically towards seawater sulfate values near the zone of sulfate depletion. Pyrite Δ33S values are nearly uniform downcore (0.170 ± 0.010‰) despite a ∼10‰ enrichment in surface versus deep pyrite δ34S values. Sulfate reduction is the most important process controlling S isotope pore water distributions, with modest contributions from oxidative S cycling. Further, microbial sulfate reduction demonstrates large fractionations typically not expected for shallow, organic rich (TOC ∼ 4%) continental margin systems.
{"title":"Understanding the isotopic composition of sedimentary sulfide: A multiple sulfur isotope diagenetic model for Aarhus Bay","authors":"A. Masterson, M. Alperin, G. L. Arnold, W. Berelson, B. Jørgensen, H. Røy, D. Johnston","doi":"10.2475/01.2022.01","DOIUrl":"https://doi.org/10.2475/01.2022.01","url":null,"abstract":"Measurement of the multiple sulfur isotopes (32S/33S/34S) enables the calibration of microbial biosignatures and provides a unique diagnosis of S-based metabolic processes: sulfate reduction, disproportionation, and sulfide oxidation. All three metabolisms carry distinct geochemical consequences for S cycling in modern systems, and are particularly powerful for paleoenvironmental interpretations if their respective contributions can be separated. To hone those interpretations and to further develop a quantitative context for understanding early diagenetic sulfur cycling, we constructed a multiple S isotope reactive transport model for the sediments of a geochemically well-characterized system (Aarhus Bay, Denmark). The model reconciles pore water and solid phase concentration profiles of the major species associated with Fe/S/C cycling, and uses multiple S isotope systematics to predict the isotope profiles of the major S species, including pore water sulfate, free sulfide and solid phase pyrite. We note that very large fractionations associated with sulfate reduction (34εsr = 70‰) are required to reproduce the observed pore water profiles, and we reconcile these fractionations with low temperature theoretical predictions for isotope equilibrium fractionation. The minor sulfur isotope values (noted as Δ33S) of sulfate increase at shallow depths within the Aarhus Bay core, and decrease when sulfate drops below 10 mM. Values (Δ33S) for sulfide decrease nearly monotonically towards seawater sulfate values near the zone of sulfate depletion. Pyrite Δ33S values are nearly uniform downcore (0.170 ± 0.010‰) despite a ∼10‰ enrichment in surface versus deep pyrite δ34S values. Sulfate reduction is the most important process controlling S isotope pore water distributions, with modest contributions from oxidative S cycling. Further, microbial sulfate reduction demonstrates large fractionations typically not expected for shallow, organic rich (TOC ∼ 4%) continental margin systems.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41927697","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The analogy between Al8/3O4 – MgAl2O4 spinels and AgSbS2 − PbS galenas is explored. Although the spinel observed in calcium-aluminum-rich inclusions in carbonaceous chondrites (CAIs) is nearly pure MgAl2O4 and the galena found in hydrothermal ore deposits is nearly pure PbS, in both occurrences a very strong case may sometimes be made that these phases originally crystallized as solid solutions with substantially higher abundance of these minor components, Al8/3O4 in CAI spinel and AgSbS2 in hydrothermal galena. It is shown that a few retrograde net-transport reactions accompanying Al8/3O4 breakdown in Al8/3O4 − MgAl2O4 spinels can account for several important features of spinel-rich CAIs including reverse zoning of åkermanite−gehlenite melilites, the production of Al-rich diopside and the depletion of anorthite in reaction rims, and the scarcity of grossite despite its previous presence as a high temperature condensate. AgSbS2 in galena is analogous, because galenas in ore deposits today have an undetectable amount of AgSbS2 (for example, Knowles,1983), yet, for example, the galena-rich ore deposits from the Coeur d'Alene mining district (ID) were responsible for the largest recorded number of ounces of Ag produced in the world until recently. Accordingly, it is appropriate that the primary Al8/3O4 contents of spinels in CAIs are reconstructed following methods analogous to those employed by Sack and others (2002,2005) to reconstruct the primary AgSbS2 contents of galenas from the Coeur d'Alene ores, if CAIs spinels are to be used to constrain temperatures, and hence pressures, of canonical condensation. Several examples are given to demonstrate the thermal constraints which may be obtained from reconstructed primary spinel Al8/3O4 concentrations starting with the Al8/3O4 -rich CAI spinels reported by Yurimoto and others (2021) and El Goresy and others (1984). The compositions of spinels which form rims around corundum and corundum-hibonite aggregates in CAIs from the Murchison and Murray chondrites (Makide and others,2013) are then used to prove that Al8/3O4 in Al8/3O4 − MgAl2O4 spinels from CAIs are completely analogous to AgSbS2 in AgSbS2 − PbS galenas from hydrothermal ore deposits.
{"title":"Temperatures of canonical condensation: Constraints from CAIs","authors":"R. Sack","doi":"10.2475/01.2022.03","DOIUrl":"https://doi.org/10.2475/01.2022.03","url":null,"abstract":"The analogy between Al8/3O4 – MgAl2O4 spinels and AgSbS2 − PbS galenas is explored. Although the spinel observed in calcium-aluminum-rich inclusions in carbonaceous chondrites (CAIs) is nearly pure MgAl2O4 and the galena found in hydrothermal ore deposits is nearly pure PbS, in both occurrences a very strong case may sometimes be made that these phases originally crystallized as solid solutions with substantially higher abundance of these minor components, Al8/3O4 in CAI spinel and AgSbS2 in hydrothermal galena. It is shown that a few retrograde net-transport reactions accompanying Al8/3O4 breakdown in Al8/3O4 − MgAl2O4 spinels can account for several important features of spinel-rich CAIs including reverse zoning of åkermanite−gehlenite melilites, the production of Al-rich diopside and the depletion of anorthite in reaction rims, and the scarcity of grossite despite its previous presence as a high temperature condensate. AgSbS2 in galena is analogous, because galenas in ore deposits today have an undetectable amount of AgSbS2 (for example, Knowles,1983), yet, for example, the galena-rich ore deposits from the Coeur d'Alene mining district (ID) were responsible for the largest recorded number of ounces of Ag produced in the world until recently. Accordingly, it is appropriate that the primary Al8/3O4 contents of spinels in CAIs are reconstructed following methods analogous to those employed by Sack and others (2002,2005) to reconstruct the primary AgSbS2 contents of galenas from the Coeur d'Alene ores, if CAIs spinels are to be used to constrain temperatures, and hence pressures, of canonical condensation. Several examples are given to demonstrate the thermal constraints which may be obtained from reconstructed primary spinel Al8/3O4 concentrations starting with the Al8/3O4 -rich CAI spinels reported by Yurimoto and others (2021) and El Goresy and others (1984). The compositions of spinels which form rims around corundum and corundum-hibonite aggregates in CAIs from the Murchison and Murray chondrites (Makide and others,2013) are then used to prove that Al8/3O4 in Al8/3O4 − MgAl2O4 spinels from CAIs are completely analogous to AgSbS2 in AgSbS2 − PbS galenas from hydrothermal ore deposits.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2022-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42811580","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ayman E. Maurice, M. Azer, P. Asimow, Fawzy F. Basta, H. Helmy, T. Shibata
Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr2O3 and Al2O3 contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO2 (0.77 wt.%) and Al2O3 (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated fO2 values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al2O3/SiO2 ratios (0.007–0.040) of harzburgite samples and the low TiO2 contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.
西奈半岛东南部Wadi Kid地区东北部的两个小型孤立超镁铁质岩体由可变蛇纹石化的方辉橄榄岩和二辉橄榄岩以及少量滑石花千枚岩组成。原生相主要为橄榄石、斜方辉石和铬尖晶石；斜辉石、角闪石和金云母也存在于二辉橄榄岩样品中。方辉橄榄岩样品的全岩Mg#（89–91）高于二辉橄榄岩（平均82）。方辉橄榄岩样品含有Mg和Ni含量较高的橄榄石、Mg#含量较高的斜方辉石和Cr含量较高的Cr尖晶石。二辉橄榄岩中单斜辉石和角闪石的REE模式与堆积成因最为一致。尽管方辉橄榄岩的几个组成特征与残余地幔的组成特征相似，但详细而言，方辉橄榄石中新鲜铬尖晶石的Cr2O3和Al2O3含量与整个阿拉伯-努比亚地盾的地幔样品或任何新元古代蛇绿橄榄岩中发现的含量不同。因此，Kabr El Bonaya的所有超镁铁质岩石最好被解释为超镁铁质堆积体，方辉橄榄岩由早期形成的堆积相组成，二辉橄榄岩包含后期形成的具有较高REE丰度的堆积相、原生含水矿物、演化的原生硅酸盐和高TiO2（0.77 wt.%）和Al2O3（18 wt.%）含量。岩石的微量元素特征表明其母岩浆与俯冲有关：全岩球粒陨石归一化REE模式为LREE富集；计算出的fO2值升高（高于辉长岩-磁铁矿-石英缓冲液+2.47至+3.39 log单位）；与斜辉石和角闪石平衡的熔体的计算的N-MORB归一化微量元素模式具有负Nb-Ta异常和大离子亲石元素富集。方辉橄榄岩样品的低Al2O3/SiO2比（0.007–0.040）及其铬尖晶石的低TiO2含量和高Cr#表明其来源于经历了高度部分熔融的地幔源。根据这些特征，我们推断出方辉橄榄岩的玻碳母熔体。我们提供了一个说明性的定量分馏模型，可以解释方辉橄榄岩和二辉橄榄岩沿着单一平衡、多气压冷却路径堆积的连续推导。我们得出的结论是，Kabr El Bonaya超镁铁质堆积体代表了在东西冈瓦纳大陆碰撞中被捕获的新元古代岛弧的裸露根部。
{"title":"The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates","authors":"Ayman E. Maurice, M. Azer, P. Asimow, Fawzy F. Basta, H. Helmy, T. Shibata","doi":"10.2475/10.2021.02","DOIUrl":"https://doi.org/10.2475/10.2021.02","url":null,"abstract":"Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr2O3 and Al2O3 contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO2 (0.77 wt.%) and Al2O3 (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated fO2 values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al2O3/SiO2 ratios (0.007–0.040) of harzburgite samples and the low TiO2 contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49512191","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xing Tian, Yuanhong Gao, T. Kukla, O. Lenz, He Huang, D. Ibarra, Shouliang Sun, Chengshan Wang
Solar cycles are important moderators of the Earth’s global climate system. Although modern-day solar cycles are well known, they have been less studied over geological time. High-resolution records such as varves have been previously used for reconstructing solar cycles from the Paleoproterozoic through Quaternary. In this paper, very fine (<1 mm) sedimentary laminations of the Early Cretaceous Yixian Formation in Xiushui Basin were studied in Northern Liaoning Province, North China. Two different microfacies of the striped shale in the Third Member of the Yixian Formation were identified. These include the light-gray to gray siltstone (Mf 1) and the gray to black organic-rich mudstone (Mf 2). Laminations of Mf 2 are mainly made of biofilms. Sub-millimeter scaled couplets of biofilm and siliciclastic-rich sublamina record seasonal growth and withering of microbial mats during the warm season (summer) and cold season (winter), respectively. Evolutionary spectral analyses of three binary rank series (the binary boxcar series, triangle series, and midpoint-triangle series), varve couplet thickness and gray scale image data (gray data) show multiple periodicities consistent with solar cycles, including the robust Schwabe sunspot cycle (10.0–10.6 year) and solar Bruckner cycle (31.0–40.6 year), and relatively weaker signals for the solar Hale cycle (21.9 year) and 16.5-year solar cycles that have been linked to solar magnetic activity. Solar cycles recognized in this paper indicate the total solar irradiance (TSI) influenced microbial mat growth in the Early Cretaceous in North China. Further, we extend our new record with a compilation of varve-recorded sunspot cycles throughout geological time to show that the 11-year Schwabe sunspot cycle and the 22-year Hale cycle have persisted since the Paleoproterozoic.
{"title":"Early Cretaceous solar cycles recorded in lacustrine laminations in North China","authors":"Xing Tian, Yuanhong Gao, T. Kukla, O. Lenz, He Huang, D. Ibarra, Shouliang Sun, Chengshan Wang","doi":"10.2475/09.2021.01","DOIUrl":"https://doi.org/10.2475/09.2021.01","url":null,"abstract":"Solar cycles are important moderators of the Earth’s global climate system. Although modern-day solar cycles are well known, they have been less studied over geological time. High-resolution records such as varves have been previously used for reconstructing solar cycles from the Paleoproterozoic through Quaternary. In this paper, very fine (<1 mm) sedimentary laminations of the Early Cretaceous Yixian Formation in Xiushui Basin were studied in Northern Liaoning Province, North China. Two different microfacies of the striped shale in the Third Member of the Yixian Formation were identified. These include the light-gray to gray siltstone (Mf 1) and the gray to black organic-rich mudstone (Mf 2). Laminations of Mf 2 are mainly made of biofilms. Sub-millimeter scaled couplets of biofilm and siliciclastic-rich sublamina record seasonal growth and withering of microbial mats during the warm season (summer) and cold season (winter), respectively. Evolutionary spectral analyses of three binary rank series (the binary boxcar series, triangle series, and midpoint-triangle series), varve couplet thickness and gray scale image data (gray data) show multiple periodicities consistent with solar cycles, including the robust Schwabe sunspot cycle (10.0–10.6 year) and solar Bruckner cycle (31.0–40.6 year), and relatively weaker signals for the solar Hale cycle (21.9 year) and 16.5-year solar cycles that have been linked to solar magnetic activity. Solar cycles recognized in this paper indicate the total solar irradiance (TSI) influenced microbial mat growth in the Early Cretaceous in North China. Further, we extend our new record with a compilation of varve-recorded sunspot cycles throughout geological time to show that the 11-year Schwabe sunspot cycle and the 22-year Hale cycle have persisted since the Paleoproterozoic.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47157312","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Archean basement in the Central Asian Orogenic Belt (CAOB) is relatively rare, but it has the potential to provide additional information on the processes of lithospheric mantle enrichment and crust extraction processes during the early history of the Earth. We identified Neoarchean amphibolite (2537−2565 Ma) and metadiorite (2481−2539 Ma) in the Biliya area of the Erguna Terrane in the southeast CAOB. The amphibolite is geochemically MORB-like and has a weakly left-leaning REE pattern, and low zircon εHf(t) (−0.7−+6.2), and whole-rock εNd(t) (−1.7–+4.5) and εHf(t) (−1.9) values. Our petrogenetic modeling reveals that the amphibolite is derived from ∼20 % partial melting of the lithospheric mantle in the spinel stability field (∼65 km depth). The metadiorite shows near-zero εNd(t) (−0.5–+3.6) and εHf(t) (+0.5–+1.4) values and is likely derived from partial melting of mafic lower crust. The metadiorite and amphibolite likely formed in an extensional continental arc/back-arc setting and represent the Archean crystalline basement of the microcontinents within the CAOB. Three-staged mantle segregation and crust extraction processes have been proposed: (a) 20 % melt extraction from primitive mantle-like lithospheric mantle, leaving behind a depleted mantle; (b) subduction-related fluid/melt metasomatism of the lithospheric mantle and its partial melting, generating the arc-type enriched mantle and mafic lower crust; and (c) partial remelting of the mafic lower crust produced the Tonalite-trondhjemite-granodiorite (TTG) crust.
{"title":"Neoarchean basement, mantle enrichment and crustal extraction in central Asia: petrogenesis of 2.5 Ga amphibolite and metadiorite in NE China","authors":"Huichuan Liu, Jun Shao, G. Zhu, Yinglei Li","doi":"10.2475/09.2021.03","DOIUrl":"https://doi.org/10.2475/09.2021.03","url":null,"abstract":"Archean basement in the Central Asian Orogenic Belt (CAOB) is relatively rare, but it has the potential to provide additional information on the processes of lithospheric mantle enrichment and crust extraction processes during the early history of the Earth. We identified Neoarchean amphibolite (2537−2565 Ma) and metadiorite (2481−2539 Ma) in the Biliya area of the Erguna Terrane in the southeast CAOB. The amphibolite is geochemically MORB-like and has a weakly left-leaning REE pattern, and low zircon εHf(t) (−0.7−+6.2), and whole-rock εNd(t) (−1.7–+4.5) and εHf(t) (−1.9) values. Our petrogenetic modeling reveals that the amphibolite is derived from ∼20 % partial melting of the lithospheric mantle in the spinel stability field (∼65 km depth). The metadiorite shows near-zero εNd(t) (−0.5–+3.6) and εHf(t) (+0.5–+1.4) values and is likely derived from partial melting of mafic lower crust. The metadiorite and amphibolite likely formed in an extensional continental arc/back-arc setting and represent the Archean crystalline basement of the microcontinents within the CAOB. Three-staged mantle segregation and crust extraction processes have been proposed: (a) 20 % melt extraction from primitive mantle-like lithospheric mantle, leaving behind a depleted mantle; (b) subduction-related fluid/melt metasomatism of the lithospheric mantle and its partial melting, generating the arc-type enriched mantle and mafic lower crust; and (c) partial remelting of the mafic lower crust produced the Tonalite-trondhjemite-granodiorite (TTG) crust.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49143291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. K. Deb, D. Saha, S. Patranabis‐Deb, A. Banerjee
Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in t
{"title":"Coexisting arc and MORB signatures in the Sonakhan greenstone belt, India: late Neoarchean – early Proterozoic subduction rollback and back-arc formation","authors":"G. K. Deb, D. Saha, S. Patranabis‐Deb, A. Banerjee","doi":"10.2475/09.2021.02","DOIUrl":"https://doi.org/10.2475/09.2021.02","url":null,"abstract":"Differentiation of rock suites related to mid-ocean ridge and subduction zone in Archean greenstone belts is important in tracing back tectonic processes related to evolution of these belts. The late Neoarchean – early Paleoproterozoic Sonakhan greenstone belt (SGB) lying between Mesoarchean gneisses of the Bastar craton and the Mesoproterozoic Chattisgarh Supergroup in central India was earlier interpreted to have arc-like affinity. New data from the SGB is presented to reinterpret the Archean tectonic setting. NNW-SSE trending SGB is constituted of three domains. The Baghmara domain in the west is dominantly a mafic metavolcanic rock succession (BGMV group), with repeated cycles of massive to pillowed basalts, pillow breccia and thin chert-BIF-shale and greywacke interlayers, representing an oceanic back-arc system. The Bilari domain in the east, with mixed mafic and felsic metavolcanic rocks (BLMV group) and minor clastic metasediments, presents an ancient magmatic arc. Overlapping these, a polymictic conglomerate-sandstone (greywacke) intercalation of the Arjuni Formation occurs in the central part of steep fold-fault belt of the SGB. Basic to intermediate intrusives (SMI group) and syn- to late-tectonic granitoids occur in all three domains. The BGMV group samples are low-K tholeiites and characterized by modern MORB like major element composition and near-flat REE patterns, reminiscent of some basalts of back-arc spreading centres, such as Parece Vela off Mariana arc. These features together with plots in Sm/Yb versus La/Sm diagram suggest derivation of their parental magmas from primitive spinel lherzolite mantle source with an N-MORB affinity that subsequently fractionated under low-pressure conditions. The BLMV and SMI samples with calc-alkaline major element composition are characterized by E-MORB type REE profiles, with enriched LREE and fractionated HREE patterns, and enrichment in trace elements more incompatible than Ti, relative to N-MORB. In addition, plots in Sm/Yb versus La/Sm diagram indicate derivation of parental magmas from partial melting of enriched garnet lherzolite mantle source at different depths, less and more deep for the BLMV and SMI groups, respectively. The BLMV magmas evolved via crystal fractionation under high water pressure conditions. The intermediate to acidic intrusives of the SGB are calc-alkaline and metaluminous, similar to I-type granites. Although in Th/Yb versus Nb/Yb diagram all the SGB mafic rocks plot above MORB array, restriction of the BGMV samples near N-MORB – PM field and distribution of the BLMV and SMI samples along AFC curve above the MORB array confirm juxtaposition of two contrasting suites, with oceanic back-arc and arc affinities, in the SGB. The Arjuni Formation apparently represents an accretionary wedge lodged in between the Baghmara and Bilari domains. Based on geological and geochemical characteristics, we suggest influence of subduction rollback and oceanic back-arc spreading in t","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47843093","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Frings, Franziska Schubring, M. Oelze, F. von Blanckenburg
Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.
{"title":"Quantifying biotic and abiotic Si fluxes in the Critical Zone with Ge/Si ratios along a gradient of erosion rates","authors":"P. Frings, Franziska Schubring, M. Oelze, F. von Blanckenburg","doi":"10.7185/gold2021.4653","DOIUrl":"https://doi.org/10.7185/gold2021.4653","url":null,"abstract":"Silicon (Si) is an important nutrient for many plant and algae species, and the ultimate source of Si is silicate mineral weathering reactions. These topics have inspired the application of Si isotope geochemistry to quantifying Si cycling in the Critical Zone, though the interpretations are often equivocal. Because germanium (Ge) geochemistry is similar to that of Si, the Ge/Si ratio is considered a tracer that provides additional constraints on Si cycling. Here, we provide Ge/Si ratios for three sites that span a gradient of erosion rates and thus time that material spends in the weathering zone before being removed. We present Ge/Si ratios in bulk rock, soil and saprolite, clay-size fractions, plant biomass, and river water from the Central Swiss Alps, the southern Californian Sierra Nevada, and the highlands of Sri Lanka. Our data perform two functions. First, they provide insight into the Ge/Si system. In particular, we document the presence of a substantial pool of Ge in plant biomass that is not associated with phytoliths, suggesting that overall plants do not discriminate against Ge relative to Si during uptake. We also quantify the preferential incorporation of Ge into clay minerals. We show that Ge/Si ratios in secondary clays may be a better proxy for weathering intensity (the fraction of denudation achieved chemically) than the Ge/Si ratio of river solutes. Ge/Si ratios in secondary clay minerals also perform as well as or even better than silicon isotopes as weathering intensity proxies. Second, the Ge/Si data are used in conjunction with silicon isotope data to develop a catchment Si mass-balance model. It suggests that the export of secondary, fractionated solids (largely clays and plant material) becomes increasingly important at shorter regolith residence times: 80−24+15% of total solubilized Si in the rapidly eroding Alps site, vs. 32−20+22% in the slowly eroding Sri Lanka site. The results also suggest that plant material is a surprisingly large contributor to Si export from these catchments, likely equivalent to 25 to110 % of dissolved Si export.","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42413775","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Metal isotopes as markers of biogeochemical processes and fluxes in the eroding Critical Zone","authors":"C. Chamberlain","doi":"10.2475/08.2021.05","DOIUrl":"https://doi.org/10.2475/08.2021.05","url":null,"abstract":"","PeriodicalId":7660,"journal":{"name":"American Journal of Science","volume":null,"pages":null},"PeriodicalIF":2.9,"publicationDate":"2021-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47408367","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"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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