Geochimica et Cosmochimica Acta最新文献

{"title":"The Role of Mafic Intrusions in Producing Alkaline Vent Fluids at the Lost City Hydrothermal Field: Evidence from Stable Potassium Isotopes","authors":"Guy N. Evans,&nbsp;Soisiri Charin,&nbsp;William E. Seyfried Jr.,&nbsp;Xin-Yuan Zheng","doi":"10.1016/j.gca.2024.08.030","DOIUrl":"10.1016/j.gca.2024.08.030","url":null,"abstract":"<div><div>The alkaline, H₂-rich vent fluids of the Lost City Hydrothermal Field (LCHF) have generally been attributed to serpentinization of tectonically uplifted mantle peridotite. However, elevated concentrations of highly incompatible and fluid mobile trace alkali metals, Rb and Cs, indicate reaction with relatively unaltered mafic components (<span><span>Seyfried et al., 2015</span></span>). Here, we present high-precision stable-K isotope analyses of LCHF vent fluids, which provide new constraints on source-rock lithology and support more quantitative estimates of fluid:rock ratio, providing a new constraint on heat and mass transfer processes. We find that δ⁴¹K values of LCHF vent fluids (0.03–0.07 ‰) are distinct from local seawater (0.13 ± 0.02 ‰). In combination with near-seawater concentrations of K (10.4 ± 0.1 mmol/kg), these data are incompatible with hydrothermal alteration of peridotite alone, which is highly depleted in K. Instead, K-isotope and concentration data are consistent with hydrothermal alteration of mafic rocks (e.g., gabbro or diabase) at a fluid:rock ratio of 6–26, a range that agrees well with Rb, Cs, and ⁸⁷Sr/⁸⁶Sr-based estimates of fluid:rock ratio, updated here to reflect derivation from mafic source rocks.</div><div>Geochemical modeling of hydrothermal fluid-rock reactions indicates that LCHF vent fluids can be effectively reproduced by a two-stage reaction in which seawater initially reacts at 200–300 °C with mafic rocks at in a fluid:rock ratio of ∼ 10 —as constrained by the above isotopic and trace-element systematics— and subsequently reacts with peridotite at a fluid:rock ratio of ∼ 30 at roughly the same temperature, pressure conditions. We also note that LCHF vent fluids are ∼ 2 % depleted in Cl compared to seawater, which may reflect admixture of a vapor component derived from ongoing phase separation at higher (&gt; 450 °C) temperatures. We thus propose that the LCHF represents waning-stage hydrothermal activity initiated by a discrete off-axis mafic intrusion that has subsequently cooled to intermediate temperatures conducive to olivine hydrolysis and production of alkaline vent fluids. In this interpretation, alkaline H₂-rich vent fields occupy a narrow thermal window in the expected evolution of hydrothermal activity associated with episodic off-axis mafic intrusions into tectonically exposed ultramafic host rocks and are thus likely to be rare on the modern seafloor.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 84-95"},"PeriodicalIF":4.5,"publicationDate":"2024-09-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142662971","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":"Re-evaluating the diffusivity of phosphorus in olivine: Implications of low diffusive mobility for thermochronology","authors":"William Nelson,&nbsp;Julia Hammer,&nbsp;Thomas Shea","doi":"10.1016/j.gca.2024.08.025","DOIUrl":"10.1016/j.gca.2024.08.025","url":null,"abstract":"<div><div>Heterogeneities in the phosphorus (P) content of olivine are relatively resistant to diffusive homogenization when compared with other compositional heterogeneities. Thus, heterogeneities in the spatial distribution of P can preserve petrological information about olivine crystals from the earliest stages of crystallization which have been otherwise eliminated. However, compared to independent determinations of protracted cooling timescales in slowly cooled rocks, P enrichments are so sharp as to suggest a rate of diffusive mobility that is many orders of magnitude slower than previously suggested. Here, we heat single natural olivine crystals with sharply defined P heterogeneities (0.02–0.15 wt% P₂O₅ over 0.5 μm spatial scale) in a 1-atmosphere gas-mixing furnace for durations of 10–20 days at 1400°C, thus exposing them to conditions that should be sufficient for complete diffusive relaxation according to published P diffusivity values. However, high-precision chemical analysis of the same interior section before and after heating reveals no discernable difference in the sharpness of phosphorus concentration patterns. Therefore, diffusion chronometry applied to skeletal P enrichments in olivine currently provides erroneously short diffusive timescales. We discuss several possible causes for these discrepancies and the implications for diffusion chronometry as applied to phosphorus in olivine.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 74-83"},"PeriodicalIF":4.5,"publicationDate":"2024-09-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245896","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":"Theoretical and experimental constraints on hydrogen isotope equilibrium in C1-C5 alkanes","authors":"Hao Xie ,&nbsp;Michael J. Formolo ,&nbsp;Alex L. Sessions ,&nbsp;John M. Eiler","doi":"10.1016/j.gca.2024.08.023","DOIUrl":"10.1016/j.gca.2024.08.023","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Stable isotope ratios of C&lt;sub&gt;1&lt;/sub&gt;–C&lt;sub&gt;5&lt;/sub&gt; alkanes, the major constituents of subsurface gaseous hydrocarbons, can provide valuable insights on their origins, transport, and fates. Equilibrium isotope effects are fundamental to interpreting stable isotope signatures, as recognition of them in natural materials indicates reversible processes and constrains the temperatures of equilibrated systems. Hydrogen isotope equilibrium of C&lt;sub&gt;1&lt;/sub&gt;–C&lt;sub&gt;5&lt;/sub&gt; alkanes is of particular interest because evidence shows that alkyl H can undergo isotopic exchange with coexisting compounds under subsurface conditions. We present the results of a combined experimental and theoretical effort to determine equilibrium hydrogen isotope distributions in mixtures of these hydrocarbon compounds. We created two mixtures: one with C&lt;sub&gt;1&lt;/sub&gt;, C&lt;sub&gt;2&lt;/sub&gt; and C&lt;sub&gt;3&lt;/sub&gt; (where C1 indicates methane, C&lt;sub&gt;2&lt;/sub&gt; ethane, etc., hereinafter called ‘C&lt;sub&gt;1&lt;/sub&gt;–C&lt;sub&gt;3&lt;/sub&gt; mixture’) and another one with C&lt;sub&gt;2&lt;/sub&gt;, C&lt;sub&gt;3&lt;/sub&gt;, &lt;em&gt;i&lt;/em&gt;C&lt;sub&gt;4&lt;/sub&gt;, &lt;em&gt;n&lt;/em&gt;C&lt;sub&gt;4&lt;/sub&gt;, &lt;em&gt;i&lt;/em&gt;C&lt;sub&gt;5&lt;/sub&gt; and &lt;em&gt;n&lt;/em&gt;C&lt;sub&gt;5&lt;/sub&gt; (hereinafter called ‘C&lt;sub&gt;2&lt;/sub&gt;–C&lt;sub&gt;5&lt;/sub&gt; mixture’); in both cases, the mixtures were created to start out of hydrogen isotope equilibrium. We tested the performance of several metal catalysts as aids to H isotopic exchange by exposing these mixtures to different metal catalysts at 100 or 200 °C and analyzing the compound-specific hydrogen isotope ratios of the product gases. The C&lt;sub&gt;1&lt;/sub&gt;–C&lt;sub&gt;3&lt;/sub&gt; mixture exchanged hydrogen isotopes among the starting gas components rapidly in the presence of Ru/Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; at 200 °C. The isotope ratios reach a steady-state (presumably equilibrium) after 72 h of heating (up to 120 h). The hydrogen isotopic ratios of alkanes in the C&lt;sub&gt;2&lt;/sub&gt;–C&lt;sub&gt;5&lt;/sub&gt; mixture shifted significantly in the presence of Rh/Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; at 100 °C and C&lt;sub&gt;3&lt;/sub&gt;-C&lt;sub&gt;5&lt;/sub&gt; compounds approached steady state after 70 h, whereas C&lt;sub&gt;2&lt;/sub&gt; had not yet reached a steady state D/H ratio after 216 h of heating. We evaluated the reaction progress of isotope exchange for each compound as a function of time with a reaction-network model informed by constraints on chemical kinetics. The model indicates that C&lt;sub&gt;3&lt;/sub&gt;, &lt;em&gt;i&lt;/em&gt;C&lt;sub&gt;4&lt;/sub&gt;, &lt;em&gt;n&lt;/em&gt;C&lt;sub&gt;4&lt;/sub&gt;, &lt;em&gt;i&lt;/em&gt;C&lt;sub&gt;5&lt;/sub&gt; and &lt;em&gt;n&lt;/em&gt;C&lt;sub&gt;5&lt;/sub&gt; in the C&lt;sub&gt;2&lt;/sub&gt;-C&lt;sub&gt;5&lt;/sub&gt; mixture reached internal isotope equilibrium after 70 h, hence we used their values to calculate experimental equilibrium results. We also calculated equilibrium isotope fractionations with the Bigeleisen-Mayer theorem using vibrational frequencies computed from the density functional theory (B3LYP/aug-cc-pVTZ). We included torsional conformers and explicit H positions in the calculations. We found that the experimental results from both the 200 ˚C C&lt;sub&gt;1&lt;/sub&gt;–C&lt;sub&gt;3&lt;/sub&gt; experiment and 100 ˚C","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 63-73"},"PeriodicalIF":4.5,"publicationDate":"2024-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142245686","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":"HIDALGO: A FUN object from the earliest epoch of the solar system’s history","authors":"Ming-Chang Liu ,&nbsp;Nozomi Matsuda ,&nbsp;Kevin D. McKeegan ,&nbsp;Emilie T. Dunham ,&nbsp;Kaitlyn A. McCain","doi":"10.1016/j.gca.2024.08.015","DOIUrl":"10.1016/j.gca.2024.08.015","url":null,"abstract":"<div><div>Chemical and isotopic measurements of HIDALGO, a stoichiometrically pure hibonite inclusion found in the matrix of the Dar al Gani 027 meteorite, were conducted by secondary ion mass spectrometry to investigate its origin and evolution. HIDALGO is characterized by large mass-dependent isotope fractionations in O, Ca, and Ti, as well as large negative anomalies in neutron-rich ⁴⁸Ca and ⁵⁰Ti, making it the newest member of the HAL-type FUN inclusions. The highly fractionated Ca and Ti isotopes but unfractionated Mg isotopes are consistent with HIDALGO being a residue from an extensive evaporation event, during which large fractions of initial Ca and Ti, and essentially all the initial Mg, in the precursor material were lost. HIDALGO appears to have incorporated live ²⁶Al at a higher level than other HAL-type inclusions, but still at a lower amount compared to the Solar System’s initial ²⁶Al abundance typically found in non-FUN CAIs. Interestingly, the inferred ¹⁰Be abundance in HIDALGO is comparable to the values observed in the majority of CV3 CAIs but ∼ 2.5 times higher than those in HAL-type samples. HIDALGO’s unusual ²⁶Al/²⁷Al and ¹⁰Be/⁹Be ratios, together with the ⁴⁸Ca-⁵⁰Ti anomalies, can be best explained by the formation of its precursor material in the isotopically heterogeneous solar nebula. Finally, large ⁷Li excesses correlating with Be/Li were found in HIDALGO, a behavior that can be interpreted as due to in-situ decay of live ⁷Be. Charged particle spallation of initially Li-free HIDALGO can simultaneously account for the inferred ⁷Be abundance and the measured Li elemental concentration. The consistency between the measurement and spallation calculation results provides support for the prior existence of ⁷Be in HIDALGO, possibly produced by irradiation close to the Sun.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 48-62"},"PeriodicalIF":4.5,"publicationDate":"2024-08-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142137961","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":"Symplectite formation in ultramafic achondrites by impact percolation of a sulfide melt","authors":"Z. Váci ,&nbsp;P.M. Kruttasch ,&nbsp;M.J. Krawczynski ,&nbsp;R.C. Ogliore ,&nbsp;K. Mezger","doi":"10.1016/j.gca.2024.08.011","DOIUrl":"10.1016/j.gca.2024.08.011","url":null,"abstract":"<div><div>The ungrouped dunitic achondrite Northwest Africa (NWA) 12217 contains symplectic spinel-pyroxene veins that are mineralogically identical to symplectites in other ultramafic planetary materials. The morphology and amount of chromite present in these features relative to the Cr in their olivine hosts suggest an exogenous origin. Petrological experiments show that a Cr laden sulfide liquid reacts with olivine to produce pyroxene by scavenging Mg and Fe from olivine to crystallize chromite. The liquid infiltrates cracks and grain boundaries within the olivine and produces a vein-like symplectic chromite-pyroxene mineralogy similar to that observed in NWA 12217. This process is likely responsible for forming the symplectites in the related ultramafic achondrites NWA 12319, 12562, and 13954, along with many other achondrites. The nucleosynthetic Cr isotopic composition of chromites appears to be in disequilibrium with that of silicates in NWA 12217, suggesting that the liquids responsible for the symplectite forming reaction are at least partially sourced from a different parent body and result from an impact.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"386 ","pages":"Pages 33-47"},"PeriodicalIF":4.5,"publicationDate":"2024-08-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142249301","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":"Trace element redistributions during metamorphism of E-chondrites: Implications for reduced bodies and the Earth","authors":"Jean-Alix Barrat ,&nbsp;Addi Bischoff ,&nbsp;Brigitte Zanda","doi":"10.1016/j.gca.2023.07.003","DOIUrl":"https://doi.org/10.1016/j.gca.2023.07.003","url":null,"abstract":"<div><p>We report on new trace element analyses of enstatite chondrites (ECs) to clarify their behavior during the metamorphism. During the transition from a type 3 to a type 5 or higher, silicates lose a large portion of their trace elements to sulfides. Our procedure allows us to obtain trace element abundances of the silicate fraction of an EC quite easily. The element patterns of these fractions (especially REE patterns) are quite different for EH and EL chondrites, and are furthermore dependent on the metamorphic grade. This procedure can be usefull to classify meteorites, in particular when the sulfides are altered. Applied to anomalous ECs, it allows direct recognition of the EH affinity of QUE 94204, and suggests that Zakłodzie, NWA 4301, and NWA 4799 derive from the same EH-like body of previously unsampled composition.</p><p>We have used the concentrations obtained on the silicate fractions of the most metamorphosed chondrites to discuss the chemical characteristics of the primitive mantles of reduced bodies of EH or EL affinity (i.e., after core segregation). Our data indicate that these mantles are very depleted in refractory lithophile elements (RLEs), particularly in rare earth elements (REEs), and notably show significant positive anomalies in Sr, Zr, Hf, and Ti. These estimates imply that the cores contain most of the REEs, U and Th of these bodies. Interestingly, the inferred primitive mantles of these reduced bodies contrast with that of the Earth. If the Earth accreted essentially from ECs, one would expect similar signatures to be preserved, which is not the case. This mismatch can be explained either by a later homogenization of the bulk silicate Earth, or alternatively, that the materials that were accreted were isotopically similar to ECs, but mineralogically different (i.e., oldhamite-free).</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"356 ","pages":"Pages 51-65"},"PeriodicalIF":5.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"1626346","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":"Reactive vanadium and iron fluxes in different modern sedimentary environments","authors":"Jonathan Garcia-Orozco ,&nbsp;Miguel Angel Huerta-Diaz ,&nbsp;Xosé L. Otero ,&nbsp;Karla Gabriela Mejia-Piña ,&nbsp;Jacob Alberto Valdivieso-Ojeda ,&nbsp;Daniel David Gregory ,&nbsp;Margarita Díaz-de-Alba ,&nbsp;D. Arenas-Islas ,&nbsp;Roxana Cervantes-Flores","doi":"10.1016/j.gca.2023.07.006","DOIUrl":"https://doi.org/10.1016/j.gca.2023.07.006","url":null,"abstract":"<div><p>This study evaluated the removal of the metals (Me) vanadium and iron associated with the operationally defined HCl-extractable (Me_HCl), pyrite (Me_pyr), and reactive (Me_reac = Me_HCl + Me_pyr) fractions in the sediments of five contrasting sedimentary environments: (1) the oxygen minimum zone (OMZ; San Lázaro and San Blas basins); (2) the anoxic/anoxic-sulfidic sediments of the California Continental Borderland basins and Todos Santos Bay Canyon; (3) the oxic sediments of the Patton Escarpment, Baja California continental shelf, and deep sediments of the Gulf of Mexico (GoM); (4) the Guerrero Negro Hypersaline System (GNHS) in Baja California Sur, Mexico; and (5) the turbiditic deposits of the GoM. The average V_HCl concentration ranged from 8 ± 15 nmol g⁻¹ in turbidites to (7.9 ± 4.3) × 10² nmol g⁻¹ in the OMZ, respectively. The V_pyr concentrations were within the narrow range of 6.6 ± 4.2 nmol g⁻¹ (oxic sediments) to 8.6 ± 5.4 nmol g⁻¹ (turbidites), which indicates that the pyrite fraction is not an important reservoir of reactive V. The relative consistency of the V_pyr concentrations (7.2 ± 5.2 nmol g⁻¹; <em>n</em> = 1098), regardless of the sedimentary environment and redox state of the system, allowed us to calculate a global burial value for oceanic sediments of 4.2 ± 3.0 Gg y^–1. The V_HCl enrichment observed in OMZ and anoxic/anoxic-sulfidic sediments may be due to its incorporation into acid volatile sulfide. The lowest average values of the degree of V pyritization corresponded to the OMZ (0.98 ± 0.68%) while the highest average values corresponded to turbidites (69 ± 32%), with the latter being due to their extremely low V_HCl values. The mass accumulation rate (MAR) calculations for reactive Fe and V show that the sediments of all OMZ regions could annually incorporate (12.1 ± 8.6) × 10⁴ Gg and (6.3 ± 3.8) × 10² Gg of reactive Fe and V, respectively. Although the GNHS exhibited the greatest MAR value of reactive Fe and V [(32 ± 28) × 10² g m⁻² ky⁻¹ and 35 ± 35 g m⁻² ky⁻¹, respectively], this environment currently does not contribute an important percentage of the MAR at the global level due to its small geographical area. However, in the geological past when its geographical area was much greater, the MAR values of reactive Fe and V may have been more important. Although deep sediments (&gt;1000 m water depth) cover a substantial area, they contribute a relatively small, yet significant proportion of the total vanadium (V) deposited in ocean sediments, ranging from 9.7% to 24.8%.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"356 ","pages":"Pages 14-37"},"PeriodicalIF":5.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3452356","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":"Determination of activities and condensation temperatures of GaO1.5 and InO1.5 in anorthite-diopside eutectic melts by Knudsen Effusion Mass Spectrometry","authors":"Lukas Bischof ,&nbsp;Paolo A. Sossi ,&nbsp;Dmitry Sergeev ,&nbsp;Michael Müller ,&nbsp;Max W. Schmidt","doi":"10.1016/j.gca.2023.08.027","DOIUrl":"https://doi.org/10.1016/j.gca.2023.08.027","url":null,"abstract":"<div><p>The group 13 elements Ga and In are overabundant in bulk silicate Earth (BSE) when compared to lithophile elements of similar 50% nebular condensation temperature <span><math><mrow><mfenced><mrow><msubsup><mi>T</mi><mrow><mi>c</mi></mrow><mn>50</mn></msubsup></mrow></mfenced></mrow></math></span>. To understand whether evaporation from silicate melts provides a more accurate description of volatility during the later stages of planetary accretion, namely, at higher temperatures and oxygen fugacities than in the solar nebula, knowledge of the activities of GaO_1.5 and InO_1.5 in silicate melts and their stable gaseous species are required. To this end, we doped anorthite-diopside (An-Di) eutectic glasses with ∼1000 and ∼10,000 ppm of Ga and In and determined their equilibrium partial pressures above the silicate liquid by Knudsen Effusion Mass Spectrometry (KEMS) using Ir cells at 1550–1740 K over the log(<em>f</em>O₂) range ΔIW+1.5 to ΔIW+2.5 (IW = iron-wüstite buffer). We detect Ga⁰ and In⁰ as the dominant vapour species and determine activity coefficients of <em>γ</em>(GaO_1.5) = 0.036(6) at 1700 K and of <em>γ</em>(InO_1.5) = 0.017(12) at 1674 K. Using these activity coefficients, we calculate partial pressures of Ga and In, together with those of similarly volatile elements, K and Zn and show that their relative volatilities from An-Di eutectic melts are in the in order Ga &gt; K ∼ In &gt; Zn, different from those predicted from their <span><math><mrow><msubsup><mi>T</mi><mrow><mi>c</mi></mrow><mn>50</mn></msubsup></mrow></math></span> under nebular conditions but in line with their relative abundances in the BSE. This substantiates the view that the abundances of volatiles in BSE, such as Ga and In, may have been set by evaporation from silicate melts under oxidising conditions at later stages of planetary accretion. Moreover, chondrules likely never underwent significant evaporation during melting and their volatile-depleted nature is likely inherited from the earliest solid condensates.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"360 ","pages":"Pages 1-15"},"PeriodicalIF":5.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"6727367","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":"Fate of arsenic during the interactions between Mn-substituted goethite and dissolved Fe(II)","authors":"Huan Liu ,&nbsp;Xiancai Lu ,&nbsp;Elaine D. Flynn ,&nbsp;Jeffrey G. Catalano","doi":"10.1016/j.gca.2023.06.028","DOIUrl":"https://doi.org/10.1016/j.gca.2023.06.028","url":null,"abstract":"<div><p>Geogenic arsenic has become a globally-distributed groundwater contaminant, liberated from the weathering of arsenic-bearing sulfide minerals and often transported to aquifer sediments adsorbed to iron oxides. Among the iron oxides, goethite (α-FeOOH) is uniquely important for the fate of arsenic because of its widespread abundance, stability, and high affinity for binding arsenic. Goethite is ubiquitous in soils and sediments and often contains substituted elements, including manganese. Structural manganese may affect the surface reactivity and redox capacity of goethite and alter the mechanisms of recrystallization catalyzed by dissolved Fe(II), potentially affecting arsenic adsorption. This study examined the fate of As(V) during the interactions between dissolved Fe(II) and Mn-substituted goethites at pH 4 and 7 as well as associated changes in arsenic speciation. At pH 7, the addition of dissolved Fe(II) initially increases the adsorption of As(V) onto Mn-bearing and Mn-free goethites. For the Mn-substituted goethites, the adsorbed As(V) slowly releases to solution at longer aging times. Fe(II) addition at pH 4 slightly increases As(V) uptake by Mn-substituted goethites, with differences in total sorption correlating with the Mn content in goethite. The addition of Fe(II) releases substantial dissolved manganese but the amount solubilized is higher at pH 4 compared to 7, suggesting that the presence of adsorbed As(V) may substantially promote the Mn release at pH 4. X-ray absorption fine structure spectroscopy shows that arsenic is stabilized as As(V) in all the samples and adsorbed on goethite via a bidentate binuclear mechanism. Fitting results show that the binding distance and coordination numbers are stable in Mn-free goethite and Mn-substituted goethite samples; the effect of substituted Mn on the surface complex structure is minor. High resolution transmission electron microscopy and X-ray diffraction confirm that no secondary ferrous arsenate minerals precipitate under both pH conditions. This study improves our understanding of the Fe(II)-As(V) interactions on iron oxides, and demonstrates that the substituted cations such as manganese may quantitatively alter the geochemical fate of arsenic during the reaction of dissolved Fe(II) with Fe(III) oxides.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"356 ","pages":"Pages 1-13"},"PeriodicalIF":5.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"2375791","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":"Sabkha and salina dolomite preserves the biogeochemical conditions of its depositional paleoenvironment","authors":"Mónica Sánchez-Román ,&nbsp;Luis Gibert ,&nbsp;Juan Diego Martín-Martín ,&nbsp;Kirsten van Zuilen ,&nbsp;Victoriano Pineda-González ,&nbsp;Pieter Vroon ,&nbsp;Sylvie Bruggmann","doi":"10.1016/j.gca.2023.06.031","DOIUrl":"https://doi.org/10.1016/j.gca.2023.06.031","url":null,"abstract":"<div><p>Precipitation of Mg-carbonate minerals, ranging from Mg-calcite to Ca-rich dolomite, is typically associated with microbial activity, and commonly takes place in saline environments along with other evaporitic minerals. The geochemistry of such evaporitic minerals can contribute to the interpretation of the environmental conditions and those (bio)geochemical processes occurring at the time of deposition. We use a multiproxy approach that combines stratigraphic, petrographic, mineralogical and geochemical data, such as Fe and Sr isotope values, to describe the environmental and biogeochemical conditions under which dolomite precipitated in a sabkha/salina environment using the Miocene Vilobí Gypsum Unit (Penedès Basin, NE Spain) as a case study.</p><p>The lithofacies of Vilobí Gypsum Unit are interpreted to indicate sedimentary environments alternating between sabkha and coastal salina. The δ¹³C and δ¹⁸O values of Vilobí dolomites (from −6.3 to −0.4‰ and from 0.2 to 2.3‰, respectively) compare well with microbially influenced dolomite from modern sabkha and coastal environments. The dolomite ⁸⁷Sr/⁸⁶Sr ratios (0.70816 to 0.70874) show significant variations and indicate that the coastal sabkha/salina was strongly influenced by seawater. Microbial activity is evidenced by petrography (microbially induced sedimentary structures interpreted as cyanobacterial mats, and spheroidal dolomite with empty bacterial voids), and geochemical data (elemental concentrations enrichment in Fe²⁺ and Mn²⁺, up to 1388 and 605 ppm, respectively; supported by δ⁵⁶Fe values of between −0.16 and 0.44‰). Collectively, these data suggest that the studied dolomite is a primary precipitate that records the geochemistry of the saline depositional environment influenced by microbial activity. The Vilobí Gypsum Unit is here dated to 16.05 Ma and 16.30 Ma (Burdigalian) by correlating the ⁸⁷Sr/⁸⁶Sr values of dolomite and shell of the oysters (shells between 0.70870 and 0.70872) with the global Sr curve. Moreover, dolomite records marine (porewater) values (C-O-Fe-Sr) that can be used to infer the depositional environment and geochemical conditions at the time of deposition. Results of non-traditional isotopes and trace metal concentrations support using dolomite formed in evaporitic environments as an archive of paleoenvironmental conditions.</p></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"356 ","pages":"Pages 66-82"},"PeriodicalIF":5.0,"publicationDate":"2023-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"3019940","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}