Geochimica et Cosmochimica Acta最新文献

{"title":"Stable and radiogenic strontium isotopes trace the composition and diagenetic alteration of remnant glacial seawater","authors":"Madison M. Wood ,&nbsp;Clara L. Blättler ,&nbsp;Ana Kolevica ,&nbsp;Anton Eisenhauer ,&nbsp;Adina Paytan","doi":"10.1016/j.gca.2024.10.028","DOIUrl":"10.1016/j.gca.2024.10.028","url":null,"abstract":"<div><div>A remnant of glacial seawater preserved in the pore fluids of sediment cores from the Maldives Inner Sea provided an opportunity to investigate the stable strontium isotopic composition (<span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr) of the ocean during the Last Glacial Maximum and explore the usefulness of <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr as a tracer of early marine diagenesis. We used paired measurements of <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr and radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) in pore fluids and surrounding carbonate sediments to constrain the diagenetic history of the preserved glacial water mass at IODP Sites U1466 and U1468. These pore fluid profiles document variability in <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr in a shallow marine setting, revealing distinct diagenetic processes dominating within different depth intervals. We find evidence for isotope fractionation during secondary calcite precipitation at intermediate depths and observe that in aragonite-dominated settings, fractionation during recrystallization may be obscured by the dissolution of aragonite in the uppermost sediments. Correcting for the effect of carbonate recrystallization on pore fluid Sr concentration ([Sr]) and isotopic composition, we estimate that glacial seawater [Sr] was higher (<span><math><mrow><mo>∼</mo><mn>98</mn><mspace></mspace><mi>μ</mi></mrow></math></span>M) and <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr lower (<span><math><mo>∼</mo></math></span>0.32‰) compared to the modern ocean, consistent with hypotheses attributing the present-day disequilibrium of the ocean Sr budget to glacial/interglacial changes in shelf carbonate weathering and burial. Our results provide evidence that the ocean [Sr] and <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr are sensitive to carbon cycle changes on timescales much shorter than its residence time (<span><math><mo>∼</mo></math></span>2 Myr) and demonstrate that pore fluid <span><math><msup><mrow><mi>δ</mi></mrow><mrow><mn>88</mn><mo>/</mo><mn>86</mn></mrow></msup></math></span>Sr measurements are a useful addition to multi-tracer studies of diagenesis in complex marine systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 211-223"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673701","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"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":"Influence of the Amazon River on the composition of particulate organic carbon in the western tropical Atlantic Ocean","authors":"Giovanna S.A. Utsumi ,&nbsp;Ding He ,&nbsp;William M. Berelson ,&nbsp;Renato M. Castelao ,&nbsp;Patricia L. Yager ,&nbsp;Patricia M. Medeiros","doi":"10.1016/j.gca.2024.09.011","DOIUrl":"10.1016/j.gca.2024.09.011","url":null,"abstract":"<div><div>The Amazon River exports ∼13.7 Tg yr⁻¹ of particulate organic carbon (POC) to the ocean, a portion of which is transported vertically through the water column. Here, we investigated in detail the composition of POC using a multi-tracer approach, including POC concentration, stable carbon isotopes (δ¹³C) and a wide range of molecular biomarkers to assess the contribution of POC export from the Amazon River to the western tropical Atlantic Ocean (WTAO). For this purpose, suspended and sediment trap POC were collected at multiple depths along the Amazon River plume under high and low river discharge conditions. Concentrations of POC detected in the plume were ∼2 to 3-fold higher than those below the plume. Overall, δ¹³C signatures of suspended and sediment trap POC samples were enriched, averaging −19.9 ‰ and –22.8 ‰, respectively. At the surface, POC composition was mainly characterized by low molecular weight <em>n-</em>alkanoic acids (&lt;C₂₀), mono- and disaccharides (e.g., glucose, <em>scyllo-</em>inositol and melibiose) and triglyceride derivatives (e.g., glycerol and 2-<em>O</em>-glycerol-α-D-galactopyranoside), indicating fresh inputs of organic matter (OM) presumably derived from algal blooms. These biomarkers were observed in lower concentrations in sediment traps. Trap samples were marked by zooplankton- (e.g., cholesterol, occelasterol) and bacteria-derived (e.g., C₁₅ and C₁₇ branched <em>n-</em>alkanoic acids) biomarkers, indicating metabolic alteration of OM at those depths. Solvent-extractable terrestrial biomarkers (e.g., dehydroabietic acid and levoglucosan) were only found in minor concentrations in most samples, which contrasts with the dissolved fraction which has been previously shown to have a primary signature of terrigenous inputs. Our results clarify the Amazon River plume’s impact on the biological pump of the WTAO, consistent with a river plume fueling primary production, and with increased zooplankton and bacteria contributions to POC composition at depth and in the POC that is vertically exported.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 84-99"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143095858","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":"Polycrystalline diamond aggregates and their role in Earth’s deep carbon cycle","authors":"Dorrit E. Jacob ,&nbsp;Richard A. Stern ,&nbsp;Janina Czas ,&nbsp;Magnus Reutter ,&nbsp;Sandra Piazolo ,&nbsp;Thomas Stachel","doi":"10.1016/j.gca.2024.10.021","DOIUrl":"10.1016/j.gca.2024.10.021","url":null,"abstract":"<div><div>Polycrystalline diamond aggregates (PDAs) are small rocks composed mostly of diamond grains, but often containing also peridotitic, eclogitic and/or websteritic minerals as accessory phases. PDAs are formed rapidly in Earth’s mantle, and the diamonds preserve heterogeneity not often seen in monocrystalline diamond. Here, diamond grains from forty-three PDAs from the Venetia diamond mine (RSA) with grain-sizes &lt; 1 mm are presented. They have heterogeneous and complex cathodoluminescence signatures that are best explained by multiple, separate diamond growth episodes from compositionally distinct COH fluids/melts. The diamonds show a large range of nitrogen concentrations (0.5 to 2,891 at. ppm), δ¹⁵N (−4.3 to + 16.8 ‰) and δ¹³C values (−27.8 to −7.6 ‰). The positive δ¹⁵N median of + 6.4 ‰ and negative δ¹³C median of −21.2 ‰ indicate derivation of the diamond-forming fluid from organic materials in subducted oceanic crust and lithosphere. Two PDAs have δ¹³C and δ¹⁵N values typical for Earth’s mantle. Thirty-three PDAs contain websteritic garnets, or peridotitic garnets and clinopyroxenes and/or micas. Unradiogenic εNd_i values in the garnets (−15.9 to −29.7) and clinopyroxene (−8.3) and δ¹⁸O values of 6.49 to 8.09 ‰ in websteritic garnets are consistent with an origin from subducted altered oceanic crust and support the findings from N and C systematics in the diamonds. Nitrogen aggregation data for the diamonds range from 25 % to 100 %B and vary by as much as 60 % within some individual PDAs. We explain the geochemical and isotopic heterogeneity of diamonds and silicates as well as the complex cathodoluminescence features with a model of episodic melt/fluid −rock interaction involving a reducing asthenospheric melt in the cratonic roots or the thermal boundary layer. We suggest that large volumes of PDA are formed in the cratonic roots and thermal boundary layer by this mechanism, making them an important reservoir for carbon storage, which is corroborated with their locally high abundance (ca. 20 %) in some kimberlites.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 136-156"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"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":"Climate controls on speleothem initial 234U/238U ratios in midlatitude settings over two glacial cycles","authors":"Carlos Pérez-Mejías ,&nbsp;Jian Wang ,&nbsp;Youfeng Ning ,&nbsp;Ana Moreno ,&nbsp;Antonio Delgado-Huertas ,&nbsp;R. Lawrence Edwards ,&nbsp;Hai Cheng ,&nbsp;Heather M. Stoll","doi":"10.1016/j.gca.2024.11.016","DOIUrl":"10.1016/j.gca.2024.11.016","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Despite early hydrological studies of &lt;sup&gt;234&lt;/sup&gt;U/&lt;sup&gt;238&lt;/sup&gt;U in groundwaters, their utilization as a paleoclimatic proxy in stalagmites has remained sporadic. This study explores uranium isotope ratios in 235 datings (&lt;sup&gt;230&lt;/sup&gt;Th) from six stalagmites in Ejulve cave, northeastern Iberia, covering the last 260 ka. The observed &lt;sup&gt;234&lt;/sup&gt;U enrichment is attributed to selective leaching of &lt;sup&gt;234&lt;/sup&gt;U from damaged lattice sites, linked to the number of microfractures in the drip route and wetness frequency, which under certain conditions, may result in the accumulation of &lt;sup&gt;234&lt;/sup&gt;U recoils. This selective leaching process diminishes with enhanced bedrock dissolution, leading to low δ&lt;sup&gt;234&lt;/sup&gt;U. Temperature variations significantly influence bedrock dissolution intensity. During stadial periods and glacial maxima, lower temperatures likely reduced vegetation and respiration rates, thereby decreasing soil CO&lt;sub&gt;2&lt;/sub&gt; and overall rock dissolution rates. This reduction could enhance the preferential leaching of &lt;sup&gt;234&lt;/sup&gt;U from bedrock surfaces due to lower bulk rock dissolution. Additionally, the temperature regime during cold periods may have facilitated more frequent freeze–thaw cycles, resulting in microfracturing and exposure of fresh surfaces. Conversely, warmer temperatures increased soil respiration rates and soil CO&lt;sub&gt;2&lt;/sub&gt;, accelerating rock dissolution rates during interstadials and interglacials, when low δ&lt;sup&gt;234&lt;/sup&gt;U is consistent with high bedrock dissolution rates. The contribution of a number of variables sensitive to bedrock dissolution and wetness frequency processes successfully explains 57% and 74% of the variability observed in the δ&lt;sup&gt;234&lt;/sup&gt;U in &lt;em&gt;Andromeda&lt;/em&gt; stalagmite during MIS 3–4 and MIS 5b-5e, respectively. Among these variables, the growth rate has emerged as crucial to explain δ&lt;sup&gt;234&lt;/sup&gt;U variability, highlighting the fundamental role of soil respiration and soil CO&lt;sub&gt;2&lt;/sub&gt; in δ&lt;sup&gt;234&lt;/sup&gt;U through bedrock dissolution. I-STAL simulations provides the potential for a combination of Prior Calcite Precipitation (PCP) indicators like Mg/Ca with PCP-insensitive indicators of bedrock dissolution such as δ&lt;sup&gt;234&lt;/sup&gt;U, along with growth rate data, may be useful to diagnose when PCP variations reflect predominantly changes in drip intervals and when changes in bedrock dissolution intensity contribute. The relationship between stalagmite δ&lt;sup&gt;234&lt;/sup&gt;U, bedrock dissolution, and initial dripwater oversaturation suggests two significant advancements in paleoclimate proxies. First, δ&lt;sup&gt;234&lt;/sup&gt;U could serve as a valuable complement to δ&lt;sup&gt;13&lt;/sup&gt;C since it is significantly influenced by soil respiration and soil CO&lt;sub&gt;2&lt;/sub&gt;, thereby reflecting soil and vegetation productivity sensitive to both humidity and temperature. Secondly, since PCP does not fractionate uranium isotopes, δ&lt;sup&gt;234&lt;/sup&gt;U could be used in combination with Mg/Ca or δ&lt;sup&gt;4","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 265-279"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841528","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":"Chemical characteristics of the Yamato-type (CY) carbonaceous chondrites","authors":"Mitsuru Ebihara ,&nbsp;Naoki Shirai ,&nbsp;Takahito Osawa ,&nbsp;Akira Yamaguchi","doi":"10.1016/j.gca.2024.10.026","DOIUrl":"10.1016/j.gca.2024.10.026","url":null,"abstract":"<div><div>Fifteen elements, including most of the major elements, were quantified using neutron-induced prompt gamma-ray analysis for five Antarctic carbonaceous chondrites with CI affinities and seven with CM affinities. Common among the twelve meteorites is the depletion of volatile elements H and chlorine, showing a positive correlation and being depleted compared to non-Antarctic CI levels. This depletion is not thought to have occurred after the fall on Antarctica, but to have been caused by thermal metamorphism on the parent body. Among the meteorites analyzed in this study, six meteorites (Y-86029, Y 980115 and Y-82162 (with CI affinities), and Y-86720, Y-86789 and B-7904 (with CM affinities)) have previously been proposed to constitute a new meteorite group, the Yamato-type (CY), based on their oxygen isotopic compositions and petrological features. The elemental compositional characteristics of the remaining six meteorites analyzed in this study, Y-86737 and Y 980134 (with CI affinities), and Y-86770, Y-86771, Y-86772 and Y-86773 (with CM affinities), suggest that these meteorites are all classified into the same chemical group CY. Based on the abundance of moderately volatile elements Mn and S, the twelve meteorites can be divided into two groups: one with levels similar to non-Antarctic CI and the other with intermediate levels between CI and CM. These results suggest that CY chondrites originate from two distinct parent bodies. To facilitate further discussions on CY chondrites, we propose naming the groups with compositions close to CI and CM as CYi and CYm, respectively.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 200-210"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673645","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":"Partial molar volumes of 1–1 electrolytes at high T and P: correlations and predictions","authors":"Andrey V. Plyasunov,&nbsp;Elena V. Cherkasova","doi":"10.1016/j.gca.2024.10.024","DOIUrl":"10.1016/j.gca.2024.10.024","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Knowledge of the partial molar volumes of aqueous ions allows accurate calculation of the pressure dependence of equilibrium constants, solubility of minerals, etc., thus being useful for thermodynamic modeling of hydrothermal processes. This study analyzed methods to correlate and predict the values of the partial molar volumes at infinite dilution, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, for 1–1 electrolytes and singly charged ions at elevated T and P. Since the precise experimental values of the dielectric constant of water are measured only up to 873 K, we were interested only in non-electrostatic ways to correlate &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; data. First of all, we compiled the &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; values at T &gt; 373 K for the following 1–1 electrolytes: HCl, LiCl, LiI, LiNO&lt;sub&gt;3&lt;/sub&gt;, LiOH, NaF, NaCl, NaBr, NaI, NaNO&lt;sub&gt;3&lt;/sub&gt;, NaOH, NaHCO&lt;sub&gt;3&lt;/sub&gt;, NaClO&lt;sub&gt;4&lt;/sub&gt;, NaH&lt;sub&gt;2&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;, NaTr (Tr stands for triflate), KF, KCl, KBr, KI, KNO&lt;sub&gt;3&lt;/sub&gt;, KOH, CsBr, and NH&lt;sub&gt;4&lt;/sub&gt;Cl. Relations, following from the “density” model and from the Fluctuation Solution Theory (FST) were employed to analyze data. It was concluded that at the current state of knowledge of &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msubsup&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mrow&gt;&lt;mn&gt;2&lt;/mn&gt;&lt;/mrow&gt;&lt;mi&gt;o&lt;/mi&gt;&lt;/msubsup&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; the FST-relations for electrolytes are recommended mainly to reject strongly deviating experimental outliers. However, the “density” model provides a simple and fairly accurate way to describe the compiled set of data with only two parameters for each ion, &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mtext&gt;hc&lt;/mtext&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;, values of which were evaluated for the following singly-charged ions: H&lt;sup&gt;+&lt;/sup&gt;, Li&lt;sup&gt;+&lt;/sup&gt;, Na&lt;sup&gt;+&lt;/sup&gt;, K&lt;sup&gt;+&lt;/sup&gt;, Cs&lt;sup&gt;+&lt;/sup&gt;, NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt;, F&lt;sup&gt;-&lt;/sup&gt;, Cl&lt;sup&gt;-&lt;/sup&gt;, Br&lt;sup&gt;-&lt;/sup&gt;, I&lt;sup&gt;-&lt;/sup&gt;, OH&lt;sup&gt;–&lt;/sup&gt;, NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;–&lt;/sup&gt;, H&lt;sub&gt;2&lt;/sub&gt;PO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;–&lt;/sup&gt;, ClO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, Tr&lt;sup&gt;-&lt;/sup&gt; (Tr = triflate). Following &lt;span&gt;&lt;span&gt;Mesmer et al. (1988)&lt;/span&gt;&lt;/span&gt;, we consider the fitting parameters &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mtext&gt;hc&lt;/mtext&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; to be related to the intrinsic volume of the ion and to the number of water molecules transferred from the bulk water to a hydration shell around the ion, respectively.&lt;/div&gt;&lt;div&gt;Although the values of &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;mi&gt;n&lt;/mi&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; and &lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msub&gt;&lt;mi&gt;V&lt;/mi&gt;&lt;mtext&gt;hc&lt;/mtext&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt; were fitted from data at temperatures from 373 to 673 K and water densities, &lt;span&gt;&lt;math&gt;&lt;mrow","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 168-185"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673650","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":"Petrogenesis of the unbrecciated pigeonite cumulate eucrite Northwest Africa 8326: Bridging the gap between eucrites and diogenites","authors":"Xiao-Wen Liu ,&nbsp;Ai-Cheng Zhang ,&nbsp;Li-Hui Chen ,&nbsp;Lang Zhang ,&nbsp;Xiao-Jun Wang ,&nbsp;Jia Liu ,&nbsp;Li-Ping Qin ,&nbsp;Yu Liu ,&nbsp;Qiu-Li Li ,&nbsp;Xiao-Xiao Ling","doi":"10.1016/j.gca.2024.11.004","DOIUrl":"10.1016/j.gca.2024.11.004","url":null,"abstract":"<div><div>Understanding of the diversity and petrogenesis of achondrites is critical for deciphering magmatic processes and the early evolution of planets and asteroids. Here, we report the detailed petrologic, mineralogical, geochemical, and chronological features of the unbrecciated Vestan meteorite Northwest Africa (NWA) 8326. We found that NWA 8326 is composed of coarse-grained orthopyroxene (∼74 vol%), plagioclase (∼19 vol%), fine-grained augite (∼5 vol%), and many accessory minerals such as chromite, ilmenite, Fe-sulfide, silica phases, K-feldspar, Ca-phosphate phases, zircon, baddeleyite, rutile, and primary Si,Al,K-rich glass, differing from typical howardite-eucrite-diogenite meteorites. Based on textural feature and compositional calculation of pyroxene, we suggest that the coarse-grained orthopyroxene was inverted from primary pigeonite and NWA 8326 should be classified as a pigeonite cumulate eucrite. The oxygen and chromium isotope data (Δ¹⁷O =  − 0.254 ± 0.009 ‰; ε⁵⁴Cr =  − 0.60 ± 0.06) support this classification. A few zircon aggregates are observed in NWA 8326 and the grains therein show a core-mantle zoned texture in cathodoluminescence (CL) images, with the cores being dark and Al-rich while the mantles being bright and Al-poor. We interpret that the CL-dark cores are xenocrystic zircon grains derived from eucrites, whose presence indicates that NWA 8326 should have formed through partial melting of the Vestan mantle, with assimilation of eucritic material. The presence of xenocrystic zircon and primary Si,Al,K-rich glass and the large compositional variation of plagioclase indicate that NWA 8326 is an unequilibrated cumulate eucrite and hence the zircon ²⁰⁷Pb/²⁰⁶Pb age of 4559.2 ± 5.2 (2σ) Ma represents the crystallization of NWA 8326. Reconciling the cumulative texture with the presence of the chemically evolved glass, NWA 8326 would be excavated during the late stage of its crystallization and escaped the prevalent crustal thermal metamorphism of the eucrite parent body. The Mg isotopic composition of NWA 8326 is higher than most diogenites, which suggests that the parent magma of such a pigeonite cumulate eucrite was derived from a source region with heavier magnesium isotopic composition (μ²⁵Mg: −90 to − 96 ppm).</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 224-238"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673612","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":"Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions","authors":"Ping-Ping Liu ,&nbsp;Ben Ma ,&nbsp;Fang-Zhen Teng ,&nbsp;Zhen-Chao Wang","doi":"10.1016/j.gca.2024.09.025","DOIUrl":"10.1016/j.gca.2024.09.025","url":null,"abstract":"<div><div>Stable isotopes of metals, such as magnesium (Mg) and iron (Fe), typically undergo minimal fractionation at magmatic temperatures. However, in Fe-Ti oxide-bearing intrusions, significant Mg isotope fractionation (Δ²⁶Mg values of up to 23 ‰) has been observed between silicates and Fe-Ti oxides. The mechanism responsible for this substantial fractionation remains unclear. This study presents Mg isotopic compositions for bulk rocks and mineral separates—including olivine, clinopyroxene, titanomagnetite, ilmenite—from sixteen Fe-Ti oxide ores in both the lower and the upper zones of the Baima layered mafic–ultramafic intrusion, as well as bulk rocks from three high-Ti basalts of the Emeishan large igneous province, SW China. Oxide ores from the lower zone and the lower section of the upper zone of the Baima intrusion are characterized by a low abundance of volatile-bearing minerals, such as apatite and hornblende. These ores exhibit δ²⁶Mg values ranging from −0.30 to −0.14 ‰ for olivine, −0.26 to −0.06 ‰ for clinopyroxene, 0.16 to 0.60 ‰ for titanomagnetite and −0.33 to −0.21 ‰ for ilmenite, indicative of near-equilibrium fractionation. In contrast, oxide ores from the upper section of the upper zone exhibit a high abundance of volatile-bearing minerals and significantly different δ²⁶Mg values, ranging from −0.19 to −0.05 ‰ for olivine, −0.25 to −0.10 ‰ for clinopyroxene, 0.57 to 0.60 ‰ for titanomagnetite and 0.86 to 2.34 ‰ for ilmenite, indicative of disequilibrium fractionation. A negative correlation between Fo content and δ²⁶Mg value in olivine suggests that Mg isotopes fractionate as olivine undergoes fractional crystallization. Trapped liquid fractions, calculated based on Ce contents in apatite, suggest that rocks with high volatile-bearing mineral content have undergone re-equilibration with trapped liquids during cooling. The large disequilibrium Mg isotopic fractionation between silicates and Fe-Ti oxides is likely due to sub-solidus re-equilibration between minerals and trapped liquids. We propose that the extent of sub-solidus, diffusion-driven kinetic Mg isotopic fractionation in Fe-Ti oxide-bearing intrusions likely correlates with the volume of trapped liquids, which may serve as a significant medium accelerating ion exchange between silicates and Fe-Ti oxides. Moreover, mass balance calculations indicate that data of Mg isotopes on their own are insufficient to determine the petrogenesis of Fe-Ti oxides in layered intrusions. This insufficiency is primarily due to the low MgO content in titanomagnetite and ilmenite, where their separation causes minimal Mg isotopic fractionation of the melts.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"389 ","pages":"Pages 100-109"},"PeriodicalIF":4.5,"publicationDate":"2025-01-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143095859","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":"The role of scapolite-bearing granulites in sequestering and releasing sulfur: Implications for S isotope signatures of crustal fluids during lower-crustal exhumation","authors":"Johannes Hammerli ,&nbsp;Anthony I.S. Kemp ,&nbsp;Anne-Sophie Bouvier ,&nbsp;Roberta L. Rudnick ,&nbsp;Pierre Boivin ,&nbsp;Robert M. Holder ,&nbsp;Thomas Chacko ,&nbsp;Kevin Blake","doi":"10.1016/j.gca.2025.01.010","DOIUrl":"10.1016/j.gca.2025.01.010","url":null,"abstract":"<div><div>To understand sulfur and carbon sequestration and release within the continental crust, scapolite minerals from a variety of granulite facies rocks were analyzed for their elemental composition and S isotope signatures. These high-grade scapolites host significant amounts of SO₃ and CO₂, up to approximately 5 wt% and 3 wt%, respectively, with δ³⁴S_VCDT from −3 to +10 ‰, and formed in relatively oxidizing environments characterized by low <em>a</em>H₂O in which scapolite may form as a primary igneous mineral or via metamorphic reactions involving sulfides and silicates. The range of scapolite sulfur isotope compositions mirrors those observed in mantle xenoliths, suggesting transport of S from the mantle into the lower crust via fluids and melts. Although scapolite’s contribution to the global S and C cycles may be modest, it is significant in the context of sulfur fluxing from the mantle to the lower crust, particularly in its oxidized form. We estimate that at least 10 % of lower crustal sulfur is sequestered within scapolite. The exhumation of scapolite-bearing lower crustal rocks can therefore liberate substantial quantities of sulfur species and CO₂, which may serve to both supply and compositionally buffer retrograde metamorphic fluids. These fluids may exhibit a range of S isotope compositions from mantle-like (δ³⁴S_VCDT ≈ 0 ‰) to relatively ³⁴S-enriched signatures. Consequently, retrograde fluids may have S isotope signatures indistinguishable from those of mantle fluids, even in the absence of direct mantle S input during fluid formation. Exhumation of scapolite-bearing lower crust may facilitate element mobilization through S and Cl complexing, particularly with respect to base metals, within exhumed lower crustal sections, thus providing sources of metals and fluids in mid- to high-grade metamorphic rocks. Globally, scapolite-bearing lower crust may help balance the global sulfur cycle through catch-and-release from scapolite.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 175-194"},"PeriodicalIF":4.5,"publicationDate":"2025-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143386854","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"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":"Uranium isotope systematics of a low-productivity ferruginous ocean analog: Implications for the uranium isotope record of early Earth","authors":"Geoffrey J. Gilleaudeau ,&nbsp;Xinming Chen ,&nbsp;Stephen J. Romaniello ,&nbsp;Sajjad A. Akam ,&nbsp;Chad Wittkop ,&nbsp;Sergei Katsev ,&nbsp;Ariel D. Anbar ,&nbsp;Elizabeth D. Swanner","doi":"10.1016/j.gca.2025.01.011","DOIUrl":"10.1016/j.gca.2025.01.011","url":null,"abstract":"<div><div>The uranium isotope (δ²³⁸U) paleo-redox proxy has emerged as a premier tool for understanding ocean oxygenation through Earth’s history. The fidelity of this important proxy depends, however, on our mechanistic understanding of the pathways that induce uranium isotope fractionation during U(VI) reduction to U(IV). Uranium reduction has been extensively studied in euxinic (anoxic + sulfidic) environments, yet relatively few constraints currently exist on δ²³⁸U fractionation in ferruginous (anoxic + iron-rich) environments, even though ferruginous conditions may have been a dominant feature of Earth’s oceans for much of the geologic past. Here, we present a comprehensive uranium isotope study of modern, meromictic, oligotrophic to mesotrophic, ferruginous Canyon Lake, Upper Peninsula, Michigan (USA), including investigation of a high-resolution profile of lake waters, lake inlet and outlet waters, and groundwater, as well as shallow cores through both oxic and ferruginous sediments. The key observation of this study is that the entire water column, oxic sediments, and ferruginous sediments have indistinguishable δ²³⁸U values near the composition of the upper continental crust. This implies a lack of δ²³⁸U fractionation in the low-productivity, ferruginous environments of Canyon Lake. We suggest that uranium cycling in Canyon Lake is dominated by adsorption and co-precipitation with iron oxides, with only a limited role for U(VI) reduction. These processes result partly from aqueous uranium speciation in the lake, with the dominance of UO₂-CO₃ complexes in the upper water column leading to a high partition coefficient of uranium during sorption to iron oxides. In addition, the dominance of CaUO₂(CO₃)₃^2– and Ca₂UO₂(CO₃)_3(aq) in bottom waters kinetically inhibits U(VI) reduction by Fe(II)_(aq). The lack of U(VI) reduction and hence δ²³⁸U fractionation in Canyon Lake, despite Fe(II)_(aq) concentrations &gt;1.5 mM, is potentially analogous to the lack of δ²³⁸U fractionation that occurred in the Archean and Proterozoic oceans, as indicated by the carbonate δ²³⁸U record. In contrast with predictions that U(VI) should be rapidly reduced and scavenged from the water column in the presence of Fe(II)_(aq), our data suggest a limited role for U(VI) reduction and δ²³⁸U fractionation under the low-nutrient, low-productivity, ferruginous conditions that characterized the oceans on the early Earth.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"392 ","pages":"Pages 195-206"},"PeriodicalIF":4.5,"publicationDate":"2025-01-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143055388","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}