Geochimica et Cosmochimica Acta最新文献

{"title":"Assessing the fidelity of shallow-water carbonates as records of the Ni isotope composition of surface seawater","authors":"Eva J. Baransky ,&nbsp;Dalton S. Hardisty ,&nbsp;John M. Rolison ,&nbsp;Laura E. Wasylenki","doi":"10.1016/j.gca.2025.06.021","DOIUrl":"10.1016/j.gca.2025.06.021","url":null,"abstract":"<div><div>Nickel is a bioessential metal that is used in enzymes important to the C, N, and O cycles, and changes in its marine abundance and bioavailability may have affected the evolutionary trajectory of early life. Changes over time in the Ni isotope composition (δ⁶⁰Ni) of surface seawater, which reflects biological demand for Ni, could allow for the reconstruction of the dynamics of Ni demand over Earth’s history, but this approach would require geologic records of surface seawater. Here, we investigate the fidelity of shallow-water carbonates as a record of the Ni isotope composition of surface seawater by determining how Ni is first partitioned into natural carbonates and then how post-depositional processes influence the Ni signal. Our samples come from the Great Bahama Bank, which is a well-studied, modern carbonate platform often used to study ancient platforms. We found that Ni is fractionated from seawater upon incorporation into carbonates capturing shallow (&lt;18 cm), recent deposition (0.1 ‰–0.4 ‰ lighter than seawater). Variation among these [Ni] and δ⁶⁰Ni values may be controlled by variation in mineral proportions. Meteoric diagenesis shifts δ⁶⁰Ni to lower values, which we attribute to isotopically light meteoric fluids. In contrast, carbonates that experienced sediment-buffered marine diagenesis with respect to Ca isotopes and Sr/Ca ratios do not appear to differ in δ⁶⁰Ni values from sediments generally representative of their initial deposition. The sensitivity of δ⁶⁰Ni to diagenetic reset in these samples appears comparable to the sensitivities of Ca isotopes and Sr/Ca ratios, to first order. Thus, in general, carbonates that experienced sediment-buffered marine diagenesis with respect to these elements may hold the most promise as a record of the δ⁶⁰Ni of coeval surface seawater. Additionally, we use our results to infer that the fraction of Ni removed from seawater into carbonates is less than 10 % of the total Ni output from the global oceans and incorporation of this Ni sink into global biogeochemical models will only have a minor impact on the modeled modern Ni budget.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"402 ","pages":"Pages 16-31"},"PeriodicalIF":4.5,"publicationDate":"2025-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144516025","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":"Storage capacity and partitioning of sulfur during silicate melting of the Earth’s shallow upper mantle and the evolution of S/Dy during MORB-source melting","authors":"Anna C. Johnson ,&nbsp;Rajdeep Dasgupta ,&nbsp;Proteek Chowdhury ,&nbsp;Maitrayee Bose","doi":"10.1016/j.gca.2025.06.022","DOIUrl":"10.1016/j.gca.2025.06.022","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Sulfur (S) in the mantle has conventionally been assumed to be stored exclusively in accessory sulfide or sulfate phases, with the S content of primary mantle melts sourced entirely from these S-rich minerals. In contrast, recent works have argued that nominally S-free, major silicate mantle minerals such as clinopyroxene (cpx), orthopyroxene (opx), plagioclase (pl), and garnet (gt) may contain 10′s of ppm sulfur. However, the experimentally determined mineral-melt partition coefficients for sulfur (&lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;min/melt&lt;/sup&gt;) vary significantly between studies from as low as 10&lt;sup&gt;-3&lt;/sup&gt; to as high as 10&lt;sup&gt;-1&lt;/sup&gt;. To contribute to this debate and to better understand the implications of the silicate mineral S-source for equilibrium mantle melting, here we present a series of new and existing silicate mineral-melt equilibria experiments with sulfur introduced to the system, and calculate the &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;min/melt&lt;/sup&gt; for these experiments. The reported experiments span hydrous and anhydrous basaltic starting compositions with 0.04–6.08 wt% S added as sulfide or sulfate at 1.0–3.0 GPa and 1200–1350 °C. The S content of partial melts was measured using electron microprobe, while that of silicate minerals was measured using NanoSIMS. The high spatial resolution and depth profiling enabled by NanoSIMS reveals abundant S-rich hotspots throughout samples, which increase in frequency with experimental S content; we interpret these as sulfide and sulfate nano-inclusions. When hotspots are avoided, the S-carrying capacities of both sulfide- and sulfate-saturated experiments are ≤2.44 ppm for calcic cpx, ≤1.13 ppm for gt, 0.80 ppm for pl, 0.48 ppm for pigeonite (pgt), and 0.55 ppm for opx. The sulfur capacity of the sulfide- and sulfate-saturated minerals exhibits an inverse correlation with experimental temperature, but does not appear to be affected by S speciation or pressure. At sulfide-added conditions, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;cpx/melt&lt;/sup&gt; is ≤0.04190, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;gt/melt&lt;/sup&gt; is ≤0.01081, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;pl/melt&lt;/sup&gt; is 0.00074, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;opx/melt&lt;/sup&gt; is 0.00039, and &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;pgt/melt&lt;/sup&gt; is 0.00034. At sulfate-added conditions, the increased solubility of S&lt;sup&gt;6+&lt;/sup&gt; in the coexisting silicate melt results in reduced partition coefficients of &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;cpx/melt&lt;/sup&gt; ≤0.00105 and &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;S&lt;/sub&gt;&lt;sup&gt;gt/melt&lt;/sup&gt; of 0.00019. These newly calculated values are orders of magnitude lower than previously published values, a consequence of the low mineral S content measured here. Finally, we apply our new partition coefficients and those of prior work to model the evolving S/Dy during adiabatic decompression melting of a sulfide-saturated MORB-source mantle. We find that minimal partitioning of S into silicate minerals, as observed in our experiments, has negligible impact on the S/Dy of MORBs. Importantly, we ","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 20-36"},"PeriodicalIF":4.5,"publicationDate":"2025-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144516027","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":"A Mesozoic high-elevation plateau in eastern North China? Evidence from detrital zircons","authors":"Ziyi Guo, Ming Tang, Hehe Jiang, Xuyang Zheng","doi":"10.1016/j.gca.2025.06.019","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.019","url":null,"abstract":"High-elevation plateaus form as the culmination of tectonic and magmatic processes, and they influence surface processes such as erosion and sedimentation. There are signs indicating high topographies in eastern North China (ENC) during the Mesozoic, coinciding with the reactivation and dramatic destruction of the North China Craton. However, the topographic evolution of ENC and how this evolution may be related to the North China Craton destruction are unclear. To constrain the topographic evolution in ENC, we measured Eu anomalies (a proxy for crustal thickness) in ∼5000 detrital zircons from modern river sands and ancient sandstones to reconstruct the crustal thickness evolution of ENC in the Mesozoic. Our results show that the ENC crust thickened to ∼60–70 km, similar to that of Tibet, from the early-middle Jurassic through the early Cretaceous, followed by a gradual thinning process during the late Cretaceous. Our crustal thickness results are consistent with whole-rock La/Yb results and geologic records that are indicative of crustal contraction and extension. We further note that a similar thickening trend is seen in multiple regions within ENC, which leads us to propose the existence of a vast Mesozoic plateau in ENC. Additionally, we find that the proportion of Mesozoic zircons in sandstones gradually increases with depositional age, reaching a peak in the Late Cretaceous, and then decreases rapidly in younger samples. This pattern likely reflects changes in erosion rates controlled by evolving topography, and thus records the growth and collapse of a plateau. Notably, the peak in Mesozoic detrital zircon proportion occurs approximately 50 Myr after the maximum uplift period of the ENC plateau, suggesting a time lag between uplift and sedimentary response. Based on our results, we model the erosion processes of the plateau and find that this time lag can be readily explained by the uplifting history reconstructed from our detrital zircon Eu anomaly data if the climate was largely arid. The intense thickening of the ENC crust was likely driven by the low-angle subduction of the Izanagi plate, with additional contributions from surrounding circum-craton orogenies. At ∼130 Ma, the rollback of the Izanagi plate likely resulted in both the collapse of the plateau and the destruction of the North China Craton.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"27 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144516026","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":"Radon and radium isotope signatures on a massive ice- and permafrost-rich coastline","authors":"Lauren Kipp ,&nbsp;Gwénaëlle Chaillou ,&nbsp;Markus Kienast ,&nbsp;Joseph Tamborski ,&nbsp;Dustin Whalen","doi":"10.1016/j.gca.2025.06.014","DOIUrl":"10.1016/j.gca.2025.06.014","url":null,"abstract":"<div><div>Rapid warming in the Arctic is degrading permafrost and massive ice deposits, which releases solutes previously trapped in ice and frozen soils into groundwater, rivers, and the coastal ocean. Radium and radon isotopes are enriched in groundwaters and may therefore have utility as tracers for the delivery of permafrost-derived solutes to the ocean, particularly if massive ice- or permafrost-derived meltwaters have a distinct isotopic ratio. The ratio of radium isotopes within massive ice deposits can also be used to constrain timescales of ice segregation, as the four isotopes will decay with known rates once removed from contact with sediments. Here we present the first measurements of radium and radon in massive ice deposits along the Beaufort Sea coast in the Northwest Territories, Canada. Measurable levels of short-lived radium and radon isotopes indicate relatively recent ice segregation and/or inputs from sediment grains frozen within the ice lenses; the radium-224/radium-228 ratio is used to identify ice samples that must have experienced segregation within the last five years, and desorption experiments provide evidence that sediment within the ice can support the radium activities observed in older ice. Short- to long-lived isotope ratios were low in ice and increased in meltwater and groundwater, while coastal ocean ratios were similar to those of the nearby Mackenzie River. Thus, we find that the radium isotope ratio of massive ice is not preserved during transport, and the river is the main control on the coastal radionuclide distribution in this region. A distinctly low (&lt;1) radium-224/radium-228 ratio was measured in massive ice deposits, similar to previous observations in permafrost, suggesting that this low ratio may be characteristic of cryogenic environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 166-181"},"PeriodicalIF":4.5,"publicationDate":"2025-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337613","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":"Evaporation kinetics of silicon carbide in a low-pressure H2-H2O gas mixture: Implications for the survivability of presolar silicon carbide grains in the protosolar disk","authors":"Daiki Yamamoto ,&nbsp;Aki Takigawa ,&nbsp;Lily Ishizaki ,&nbsp;Ryosuke Sakurai ,&nbsp;Yuki Inoue ,&nbsp;Junji Yamamoto ,&nbsp;Sota Arakawa ,&nbsp;Shogo Tachibana","doi":"10.1016/j.gca.2025.06.010","DOIUrl":"10.1016/j.gca.2025.06.010","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Presolar silicon carbide (SiC) grains found in primitive extraterrestrial materials would preserve the pre-accretion thermal history of dust in the protosolar disk. Three series of evaporation experiments of SiC were conducted at total pressures of 0.5 and 2.5 Pa of H&lt;sub&gt;2&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O gas mixture with controlled H&lt;sub&gt;2&lt;/sub&gt;/H&lt;sub&gt;2&lt;/sub&gt;O ratios of ∼ 52–140 and temperatures of 1523–1779 K. The STEM-EDS and Raman spectroscopic analyses of the heated samples indicated the absence of an oxide layer on the sample surface; however porous carbon-rich layers were occasionally observed. This suggests that the evaporation of SiC under the experimental conditions proceeded without the formation of a protective steady-state SiO&lt;sub&gt;2&lt;/sub&gt; layer. Under all the experimental conditions, the evaporation flux (&lt;em&gt;J&lt;/em&gt;) has little/no dependence on temperatures typically higher than ∼1610–1670 K, while larger temperature dependences were observed at lower temperatures. The little/no temperature-dependence of &lt;em&gt;J&lt;/em&gt; suggests that the evaporation reaction rate is controlled by the gaseous supply of H&lt;sub&gt;2&lt;/sub&gt;O to the SiC surface under low-pressure conditions prevailing in the protosolar disk. The overall reaction rates would be limited by the surface chemical reactions in the large-temperature dependent regime. The large activation energies in this regime obtained in this study compared with those reported from the previous studies are likely associated with the transition regime from the SiC evaporation without continuous SiO&lt;sub&gt;2&lt;/sub&gt; formation to that accompanied by the SiO&lt;sub&gt;2&lt;/sub&gt; formation.&lt;/div&gt;&lt;div&gt;The survivability of presolar SiC grains was then compared with that of presolar amorphous silicate grains. We found that the lifetime of 0.1–1 μm-diameter SiC grain in the protosolar disk would have little/no temperature dependence at temperatures higher than ∼1500–1700 K, whereas it has a large temperature dependence at lower temperatures. The survival of these presolar SiC grains during the formation of igneous calcium-aluminum-rich inclusions would largely depend on the heating conditions of high-temperature events. Effective SiC evaporation reaction would occur at ∼1200–1400 K, whereas oxygen isotopic signatures of 0.1 μm-diameter presolar amorphous silicate grains would be erased at ∼ 600–800 K in the accreting ptotosolar disk. At temperatures lower than ∼ 600–700 K, the presolar silicate/SiC number ratio normalized to its initial ratio increases with increasing the heliocentric distance from the Sun (&lt;em&gt;r&lt;/em&gt;), reaching values of ∼ 0.7–0.9 at &lt;em&gt;r&lt;/em&gt; &gt; 4–5 au if the particles were released at ∼ 6–12 au. Assuming that the primitive interplanetary dust particles (IDPs) have an initial presolar silicate/SiC ratio of ∼ 6, the IDP-normalized ratios in primitive carbonaceous chondritic meteorites are in the range of ∼ 0.15–1. The high normalized presolar silicate/SiC ratios (&gt;∼0.7) in meteorites imply that precurso","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"402 ","pages":"Pages 388-409"},"PeriodicalIF":4.5,"publicationDate":"2025-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337641","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":"Initial 244Pu/238U ratios and search for presolar SiC in Ca-Al-rich inclusions from CV3 chondrites using noble gas and trace element abundances","authors":"Daisuke Nakashima, Jon M. Friedrich, Ulrich Ott","doi":"10.1016/j.gca.2025.06.016","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.016","url":null,"abstract":"Noble gas isotopes and trace element abundances in five Ca-Al-rich inclusions (CAIs) from two CV3 chondrites (Allende and Axtell) were analyzed. The noble gases consist of spallogenic, radiogenic, fission, and trapped components. The old U/Th-<ce:sup loc=\"post\">4</ce:sup>He ages of the CAIs (4.0 – 5.4 Ga) suggest no significant loss of radiogenic <ce:sup loc=\"post\">4</ce:sup>He and, by inference, no significant disturbance of the initial (<ce:sup loc=\"post\">244</ce:sup>Pu/<ce:sup loc=\"post\">238</ce:sup>U) ratios, (<ce:sup loc=\"post\">244</ce:sup>Pu/<ce:sup loc=\"post\">238</ce:sup>U)<ce:inf loc=\"post\">0</ce:inf>, which are derived using concentrations of <ce:sup loc=\"post\">244</ce:sup>Pu-fission <ce:sup loc=\"post\">136</ce:sup>Xe. The abundances of rare earth elements and U in the CAIs suggest variable formation temperatures, which is reflected in variable (Pr/<ce:sup loc=\"post\">238</ce:sup>U)<ce:inf loc=\"post\">0</ce:inf> ratios. The (<ce:sup loc=\"post\">244</ce:sup>Pu/<ce:sup loc=\"post\">238</ce:sup>U)<ce:inf loc=\"post\">0</ce:inf> ratios of the CAIs are variable from 0.0103 ± 0.0010 to 0.0419 ± 0.0031, which correlate with the (Pr/<ce:sup loc=\"post\">238</ce:sup>U)<ce:inf loc=\"post\">0</ce:inf> ratios. The correlation suggests Pu-Pr-U fractionation during CAI formation. From the intersection between the correlation line and the calculated early Solar System Pr/<ce:sup loc=\"post\">238</ce:sup>U ratio of 9.27, the <ce:sup loc=\"post\">244</ce:sup>Pu/<ce:sup loc=\"post\">238</ce:sup>U ratio before Pu-Pr-U fractionation in the CAI formation region is calculated as 0.0108 ± 0.0051, which is similar to those derived using other Solar System materials such as chondrites, achondrites, chondrules, and terrestrial zircons. We thus suggest that the initial <ce:sup loc=\"post\">244</ce:sup>Pu/<ce:sup loc=\"post\">238</ce:sup>U ratio has been spatially homogeneous in the inner part of the early solar nebula including the innermost solar nebula, where CAIs formed.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"42 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337636","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":"Equilibrium Fe isotope fractionation between olivine, pyroxene, spinel and MORB glass: Implications for mantle partial melting to generate MORBs","authors":"Nicole X. Nie ,&nbsp;Anat Shahar ,&nbsp;Peng Ni ,&nbsp;Richard W. Carlson ,&nbsp;Steven B. Shirey ,&nbsp;Corliss K. Sio ,&nbsp;Justin Y. Hu ,&nbsp;Andrew Regula ,&nbsp;Kelsey Prissel ,&nbsp;Michael Y. Hu ,&nbsp;Jiyong Zhao ,&nbsp;Barbara Lavina ,&nbsp;Esen E. Alp","doi":"10.1016/j.gca.2025.06.011","DOIUrl":"10.1016/j.gca.2025.06.011","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Primitive mid-ocean ridge basalts (MORBs) exhibit Fe isotopic compositions heavier than the upper mantle by +0.074 ± 0.028 ‰ for δ&lt;sup&gt;56&lt;/sup&gt;Fe. The processes responsible for this isotopic difference remain unclear. Modeling of Fe isotope fractionation during mantle partial melting requires reliable equilibrium Fe isotope fractionation factors between minerals and melts, for which consistent data are still lacking. In this study, we used Nuclear Resonant Inelastic X-ray Scattering (NRIXS) technique to measure Fe force constants for a MORB glass (ALV 519-4-1) and natural mantle minerals (olivine, orthopyroxene, clinopyroxene, and spinel) to determine the equilibrium Fe isotope fractionation factors between them. The force constants determined in this study, in increasing order, are 167 ± 26 N/m for spinel, 175 ± 17 N/m for olivine, 176 ± 20 N/m for MORB glass, 205 ± 26 N/m for clinopyroxene, and 219 ± 36 N/m for orthopyroxene.&lt;/div&gt;&lt;div&gt;We evaluated the previously proposed mechanisms for the heavy Fe isotopic composition of MORBs, including (i) mantle partial melting, (ii) mantle lithological heterogeneity, with pyroxenite in the source, (iii) mantle metasomatism by low-degree melts, and (iv) fractional crystallization of olivine from melts. For (i), we used the pMELTS program to simulate adiabatic decompression melting of mantle peridotites, and calculated Fe isotope fractionation based on Fe&lt;sup&gt;3+&lt;/sup&gt;–Fe&lt;sup&gt;2+&lt;/sup&gt; equilibrium-controlled fractionation, where Fe&lt;sup&gt;3+&lt;/sup&gt; forms stronger bonds and is more incompatible than Fe&lt;sup&gt;2+&lt;/sup&gt;. At 10 wt% peridotite melting, corresponding to MORB generation, only +0.03 ‰ Fe isotope fractionation between the melt and the original bulk composition (&lt;span&gt;&lt;math&gt;&lt;mrow&gt;&lt;msup&gt;&lt;mi&gt;Δ&lt;/mi&gt;&lt;mn&gt;56&lt;/mn&gt;&lt;/msup&gt;&lt;mi&gt;F&lt;/mi&gt;&lt;mi&gt;e&lt;/mi&gt;&lt;mo&gt;=&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;msup&gt;&lt;mi&gt;δ&lt;/mi&gt;&lt;mn&gt;56&lt;/mn&gt;&lt;/msup&gt;&lt;mi&gt;F&lt;/mi&gt;&lt;mi&gt;e&lt;/mi&gt;&lt;/mrow&gt;&lt;mrow&gt;&lt;mi&gt;m&lt;/mi&gt;&lt;mi&gt;e&lt;/mi&gt;&lt;mi&gt;l&lt;/mi&gt;&lt;mi&gt;t&lt;/mi&gt;&lt;/mrow&gt;&lt;/msub&gt;&lt;mo&gt;-&lt;/mo&gt;&lt;msub&gt;&lt;mrow&gt;&lt;msup&gt;&lt;mi&gt;δ&lt;/mi&gt;&lt;mn&gt;56&lt;/mn&gt;&lt;/msup&gt;&lt;mi&gt;F&lt;/mi&gt;&lt;mi&gt;e&lt;/mi&gt;&lt;/mrow&gt;&lt;mn&gt;0&lt;/mn&gt;&lt;/msub&gt;&lt;/mrow&gt;&lt;/math&gt;&lt;/span&gt;) was produced, insufficient to account for the observed MORB-upper mantle difference. For (ii), melting of pyroxenites yields smaller Fe isotope fractionation than melting of peridotites, making it unlikely the cause for the MORB-upper mantle isotopic difference. For (iii), both the Fe&lt;sup&gt;3+&lt;/sup&gt;/ΣFe ratio and the δ&lt;sup&gt;56&lt;/sup&gt;Fe of melts increase with the degree of partial melting, indicating that low-degree melts are not isotopically heavy enough to significantly alter the isotopic composition of lithospheric mantle through metasomatism. For (iv), equilibrium isotope fractionation between olivine and melt is near zero. These results suggest that equilibrium Fe isotope fractionation alone cannot explain the MORB isotopic signature, highlighting the potential role of kinetic isotope fractionation. Using a diffusion model, we calculated kinetic Fe and Mg isotope fractionations associated with (iv) olivine cry","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"403 ","pages":"Pages 130-151"},"PeriodicalIF":4.5,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337642","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":"Examining the Effect of Heat from the Yellowstone Plume on the Release of Helium from the Crust","authors":"D.T. Halford, R. Karolytė, P.H. Barry, R.L. Tyne, D.J. Hillegonds, E.O. Adeniyi, D.V. Bekaert, M.W. Broadley, M.R. Hudak, K.G. Lloyd, B. Marty, C.J. Ramirez, A.M. Seltzer, C.J. Ballentine","doi":"10.1016/j.gca.2025.06.015","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.015","url":null,"abstract":"Helium (He), which is an irreplaceable resource in low-carbon technologies, medical applications, and various science and engineering sectors, is currently being explored as a primary resource. The correlation between crustal helium (He) release and heat flow in certain geologic environments (e.g., mid-ocean ridge and continental hydrothermal systems) is well established, but few studies have evaluated how past igneous processes influence current gas release/storage from crustal rocks. Here, we report bulk gas and noble gas geochemistry data (n = 43) gathered from thermal springs in and around the Yellowstone National Park (YNP), USA. Samples outside the YNP (near the plume track) are dominantly N<ce:inf loc=\"post\">2</ce:inf>-rich, while most samples collected within the active caldera area are CO<ce:inf loc=\"post\">2</ce:inf>-rich. Samples outside the YNP typically have much lower <ce:sup loc=\"post\">3</ce:sup>He/<ce:sup loc=\"post\">4</ce:sup>He than those within the caldera (i.e., near the current plume head). We explore the relationship between thermal aureoles and He isotopic signatures using heat flow data coupled with bulk gas and noble gas geochemistry data. Data are used to determine gas origins, to understand fluid flow in a regional context, and to assess how different environments impact He release from crustal minerals. Models indicate that advection is the dominant process controlling heat and volatile loss from mantle to crustal systems from the Yellowstone Caldera. In contrast, the influence of conduction/boiling of crustal hydrothermal fluids is more substantial for samples outside of the Yellowstone Caldera. Helium-4 is enriched in the samples which are frontal and near the eruptive center, likely due to recent crustal degassing of <ce:sup loc=\"post\">4</ce:sup>He accumulated over long periods in the underlying craton. Ultimately, He and other volatiles are released due to tectonic activity and/or they are enriched as other gases partition out of groundwater (i.e., gas stripping from groundwater). However, elevated heat flow zones likely constitute poor He retention zones. We propose a twofold approach to help identify preferential zones of He release: 1) focusing on areas that are distal from active igneous zones (i.e., areas that have not been fully degassed) with localized moderate heat flow to release trapped crustal He, and 2) utilizing isotope models to constrain groundwater interactions (i.e., migration and accumulation potential).","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"16 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337638","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":"Phosphate behavior during submarine hydrothermal alteration of ca. 3.455 Ga basaltic seafloor rocks from Pilbara, Western Australia","authors":"Yuya Tsukamoto, Takeshi Kakegawa","doi":"10.1016/j.gca.2025.06.013","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.013","url":null,"abstract":"Phosphorus is an essential element for various biomolecules and biochemical processes. A phosphate-poor ocean model is popular for the Archean oceans, because of the potential absence of continental land masses and phosphorus scavenging by BIF. On the other hand, recent studies have suggested the possibility of a high phosphorus flux from Archean submarine hydrothermal activity, yielding phosphate-rich oceans. Here, we performed mineralogical and geochemical analyses for the 3.455 Ga Apex Basalt in ABDP#1 core from East Pilbara Terrane of the Pilbara Craton, Australia, to examine whether Archean submarine hydrothermal activity could play the role of a phosphate source to contemporary oceans.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"47 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144337640","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":"Kinetics of methanethiol oxidation by oxygen under aqueous conditions","authors":"Irina Zweig, Alexey Kamyshny Jr.","doi":"10.1016/j.gca.2025.06.009","DOIUrl":"https://doi.org/10.1016/j.gca.2025.06.009","url":null,"abstract":"Methanethiol is one of the most abundant volatile organic sulfur compounds in natural aquatic systems and one of the main products of dimethylsulfoniopropionate decomposition. This study focuses on the kinetic parameters of the reaction of methanethiol and its deprotonated form, methanethiolate, with dissolved oxygen in aqueous solutions at various reactant concentrations, pH, and temperatures. The reaction proceeds through two distinct pathways: a slow reaction between protonated methanethiol and oxygen under acidic to neutral conditions, and a fast reaction between methanethiolate and oxygen under basic conditions. At the environmentally relevant pH and concentrations, the reaction order with respect to methanethiol is 2.2 ± 0.4 for the protonated form and 1.6 ± 0.2 for the deprotonated form, while in both cases it is 1.0 ± 0.3 with respect to oxygen. Dimethyl disulfide was the only product detected in both reaction pathways. The ratio between the consumption rates of oxygen and methanethiolate was approximately 1:4, while the ratio of oxygen to methanethiol consumption rates was close to 1:2. This implies that dimethyl disulfide is not the only product of methanethiol oxidation. The half-life of methanethiol in oxic water column at 25 °C and typical marine methanethiol concentrations of 0.02–2 nM was estimated to be 80 to 1200 years. Rates of chemical oxidation of MT in the surface waters are lower than the rates of its photooxidation and degassing. Thus, the contribution of the chemical oxidation of MT in marine systems to its budget is negligible but should be considered for the MT-rich aphotic hydrothermal and limnic waters. In systems with methanethiol concentrations exceeding 1 mM, such as bioreactors, chemical oxidation rather than microbial decomposition is likely the primary mechanism for methanethiol removal, even at low oxygen levels. Under fully oxic conditions, the rate of chemical oxidation of methanethiol is expected to surpass the rate of microbial degradation observed under anoxic conditions typically utilized in these reactors.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"43 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-06-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144304964","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}