Geochimica et Cosmochimica Acta最新文献

{"title":"Clay minerals and the stability of organic carbon in suspension along coastal to offshore transects","authors":"Michael Fettweis ,&nbsp;Saumya Silori ,&nbsp;Rieko Adriaens ,&nbsp;Xavier Desmit","doi":"10.1016/j.gca.2025.03.003","DOIUrl":"10.1016/j.gca.2025.03.003","url":null,"abstract":"<div><div>Particulate (POC) and dissolved organic carbon (DOC) concentration, clay mineral content and composition and the suspended particulate matter (SPM) concentration have been analyzed in water samples taken along transects from the high turbid nearshore to the low turbid offshore on the North Sea shelf. The suspended POC has been classified into a mineral-associated (POC_mineral), a slowly degrading (POC_slow) and a fresh fraction (POC_fresh). The POC_mineral has been estimated based on the clay mineral composition and on literature data of the mineral specific surface area per g and the OC content per specific surface area. It consists of organic molecules adsorbed onto mineral surfaces and is thereby the most refractory fraction. The POC_fresh content (% of POC_fresh in SPM) has been calculated using the semi-empirical POC-SPM model of Fettweis et al. (2022) and is intrinsically labile. The POC_slow content is refractory with variable rates of degradation. The total POC content of the SPM was between 2 and 11%, from which about 0.3–6.6% (0.1–2.1%) was POC_fresh in spring (resp., winter). The POC_mineral content was between 0.4% and 1.1% and decreased towards the offshore, meaning that the POC offshore is less refractory than nearshore. The organic molecules adsorbed onto clay minerals, are dynamically exchanging with the DOC, and thus influencing its fate and concentration. However this process is not sufficient to explain the increasing POC/DOC ratio with increasing SPM concentration, which is further explained by primary production, advection and diffusion, density gradients and seabed erosion. Our results highlight the difficulty and the necessity of estimating the respective sample-POC and DOC concentrations, fluxes and fates along SPM concentration gradients in coastal zones. This is needed as organo-mineral interactions influence the vertical dynamics and horizontal transport of SPM and have an impact on particles and organic carbon fluxes.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 229-237"},"PeriodicalIF":4.5,"publicationDate":"2025-03-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666341","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":"Helium isotopes in geothermal fluids reveal off-rift plume degassing and localized seismicity-induced processes in North Iceland","authors":"Carolina Dantas Cardoso ,&nbsp;Raphaël Pik ,&nbsp;Antonio Caracausi ,&nbsp;Sæmundur Ari Halldórsson ,&nbsp;Andri Stefánsson ,&nbsp;Laurent Zimmermann ,&nbsp;Guillaume Paris ,&nbsp;Andrea Ricci ,&nbsp;Hreinn Hjartarson","doi":"10.1016/j.gca.2025.03.004","DOIUrl":"10.1016/j.gca.2025.03.004","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Iceland is a location of geological interest due to the combination of upwelling mantle plume and divergent plate boundary, which resulted in the formation of its extensive surface area (&gt;100,000 km&lt;sup&gt;2&lt;/sup&gt;) that rises above sea-level. This unique setting facilitates assessing the role of the underlying mantle plume and tectonic activity on crust-forming processes. Helium isotopes provide a useful tool in this regard, as they can identify physical processes and resolve deep and shallow fluid sources in the crust. In Iceland, the highest &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He for geothermal fluids are found in Vestfirðir with values up to 29 R&lt;sub&gt;a&lt;/sub&gt; (where R&lt;sub&gt;a&lt;/sub&gt; is the &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He of air), more than 110 km away from current active rift zones. Such locations are key to understand the extent of mantle degassing processes associated with the high buoyant Icelandic mantle plume. Other off-rift regions, such as most of North Iceland, have not been extensively investigated, despite the widespread presence of geothermal activity. Although North Iceland has been volcanically inactive for the past 0.8 Ma, severe earthquake hazards associated with mature and partially on-land transform zones have occurred, rendering the monitoring of the full tectonic-hydrogeochemical system of societal importance. Our study in North Iceland aimed to (i) assess temporal variations in helium isotopic signatures in low-T geothermal water and their relationship with regional earthquakes, (ii) diminish the helium isotope data gap in geothermal fluids of this region, and (iii) elucidate both local and regional processes controlling the He isotope systematics in this region as a case study for other off-rift contexts on Earth. In order to achieve these goals, we report helium isotope time series data collected from June 2020 to October 2022 from a borehole in Hafralækur, Aðaldalur valley (95 samples collected on a near-weekly basis), along with an isotope survey (δ&lt;sup&gt;2&lt;/sup&gt;H-&lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He-δ&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;TDIC&lt;/sub&gt;-δ&lt;sup&gt;18&lt;/sup&gt;O-δ&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt;) of North Iceland geothermal fluids (T &lt; 130 °C, n = 36 samples). The results indicate a large regional variability in helium isotope ratios (4 to 27 R&lt;sub&gt;a&lt;/sub&gt;) that is comparable to the entire range evident in geothermal fluids across Iceland (∼1 to 29 R&lt;sub&gt;a&lt;/sub&gt;) where the maximum &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He signature is among the highest measured in geothermal fluids from oceanic and continental hotspots globally. Several processes, both on regional and local scales, are needed to account for this large range: (i) influence of a deeply-derived mantle flux evidenced by a high &lt;sup&gt;3&lt;/sup&gt;He/&lt;sup&gt;4&lt;/sup&gt;He mantle component, degassing via fault systems, (ii) release of local radiogenic helium components, potentially associated with seismic events along the Dalvík Lineament, and (iii) local groundwater mixing, for example evident at the Hafralækur sit","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 12-31"},"PeriodicalIF":4.5,"publicationDate":"2025-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143629448","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":"Mineral-melt calcium isotope fractionation factors constrained using ab initio molecular dynamics simulations and their implications to calcium isotope effects during partial melting in the upper mantle","authors":"Yonghui Li ,&nbsp;Justin Hardin ,&nbsp;Wenzhong Wang ,&nbsp;Zhongqing Wu ,&nbsp;Shichun Huang","doi":"10.1016/j.gca.2025.02.032","DOIUrl":"10.1016/j.gca.2025.02.032","url":null,"abstract":"<div><div>To better constrain the Ca isotope effect during partial melting of Earth’s mantle, we used <em>ab initio</em> molecular dynamic simulations to calculate the equilibrium mineral-silicate melt Ca isotope fractionation factors for the major Ca-bearing minerals of the upper mantle (orthopyroxene, clinopyroxene, olivine, and garnet), as well as plagioclase. We found that mineral-melt Ca isotope fractionation factors are dependent on pressure, temperature, and mineral major element compositions, but not the silicate melt composition. Specifically, our calculations show that under equilibrium, clinopyroxene has a slightly heavier Ca isotope composition compared to silicate melt, consistent with the inference of published research that studied the Ca isotope effects during basaltic magma evolution.</div><div>We then utilized the calculated mineral-melt Ca isotope fractionation factors to model the Ca isotope effects on both silicate melts and residues during partial melting of spinel peridotite at 1–2 GPa, garnet peridotite at 3–7 GPa, and garnet pyroxenite/eclogite at 2–5 GPa. Our model predicts that silicate melts only have <em>δ</em>^44/40Ca up to 0.12 lower than their source value, consistent with previous estimates. Importantly, partial melts of spinel peridotite, garnet peridotite, and garnet pyroxenite/eclogite exhibit overlapping <em>δ</em>^44/40Ca values, if their mantle sources have the same <em>δ</em>^44/40Ca. Partial melting alone cannot explain the full range of <em>δ</em>^44/40Ca values observed in natural basalts, and at least part of this variation must reflect <em>δ</em>^44/40Ca variation of their mantle sources and/or other processes such as crystal fractionation.</div><div>Our calculations show that melting residues always have <em>δ</em>^44/40Ca higher than their mantle source, with the highest <em>δ</em>^44/40Ca at 0.40 higher than their source value. This range is much smaller than that observed in natural ultramafic rocks that might represent melting residues. In addition, the range and direction of inter-mineral Ca isotope fractionation factors predicted in our modeled residues for the mineral pairs orthopyroxene-clinopyroxene and garnet-clinopyroxene are much more restricted than those observed in natural ultramafic rocks, including peridotites and pyroxenites/eclogites. Therefore, most natural ultramafic rocks have likely experienced more complicated petrogeneses than partial melting.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"396 ","pages":"Pages 51-70"},"PeriodicalIF":4.5,"publicationDate":"2025-03-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666342","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 fate of nitrogen in deep magma oceans","authors":"Ekanshu Mallick ,&nbsp;Kelsey Prissel ,&nbsp;Kevin Righter ,&nbsp;Colin R.M. Jackson","doi":"10.1016/j.gca.2025.02.020","DOIUrl":"10.1016/j.gca.2025.02.020","url":null,"abstract":"<div><div>Nitrogen is important in planetary evolution because it is essential to life and the most abundant element in Earth’s atmosphere. Here, we investigate how core formation affects the distribution of N within accreting terrestrial planets. We conducted laser-heated diamond anvil cell experiments (LH-DAC) over a wide range of high pressure–temperature-compositional (PTX) conditions (38–103 GPa, 2728–5609 K, −1.95 to −1.03 ΔIW, 0.5–3.7 NBO/T) to study nitrogen partitioning in metal-silicate systems. Combining our data with existing low and high PT results, we developed a nitrogen partitioning model applicable from early accretion to extreme PT stages associated with giant impacts. We test the robustness of our model by accurately predicting nitrogen partitioning in a multi-anvil experiment conducted independently at 15 GPa, 2573 K with oxygen fugacity of −2.5 ΔIW. Our model shows that increasing pressure, oxygen fugacity, and N concentration in the alloy make nitrogen more siderophile, while increasing temperature, oxygen and silicon contents in the alloy, and the SiO₂ content of the silicate melt make nitrogen less siderophile. Application of our model to core formation conditions under oxidized and reduced scenarios suggest that nitrogen can be siderophile or lithophile under low PT conditions but exhibits a neutral partitioning at high PT conditions (&gt; 100 GPa, 5000 K) over a wide range of bulk planet compositions. Using our model, along with partitioning models for S and C, we examine how core formation scenarios can fractionate C/N and S/N ratios in the BSE. Our model suggests that backreaction of volatile rich cores from reduced, smaller impactors (sub-Mars-sized) within deep magma oceans can impart a wide range of C/N and S/N ratios on the magma ocean. We find that the amount of silicate entrainment has a strong control on elemental fractionations imparted to the magma oceans. Elevated C/N and S/N ratios are associated with larger degrees of silicate entrainment, and vice versa. Thus, Earth’s apparent depletion of N may relate to its volatiles being reprocessed within deep magma oceans, possibly during the end stages of accretion.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"394 ","pages":"Pages 298-318"},"PeriodicalIF":4.5,"publicationDate":"2025-03-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666413","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":"Mixed messages: Unmixing sedimentary molecular distributions reveals source contributions and isotopic values","authors":"Pratigya J. Polissar ,&nbsp;A.Tyler Karp ,&nbsp;William J. D’Andrea","doi":"10.1016/j.gca.2025.03.001","DOIUrl":"10.1016/j.gca.2025.03.001","url":null,"abstract":"<div><div>Plant leaf waxes and their isotopic composition are important tracers of ecological, environmental, and climate variability, with strong preservation potential in sedimentary archives. However, they represent an integrated, and often complicated, signal of vegetation and hydrology within a watershed. Here, we report a new approach for examining complex mixtures of <em>n</em>-alkanes in sediments and their isotope values: non-negative matrix factorization (NMF). NMF identifies the endmembers in a mixture from the integrated <em>n</em>-alkane data and provides quantitative information on the relative importance of those endmembers across samples. We apply this approach to a synthetic dataset and two previously published datasets to illustrate its uses. Our application of NMF to re-analyse previously published data reveals new insights into past climate and ecological change. We demonstrate that NMF allows a user to 1) identify potential mixing problems, 2) evaluate which specific compounds in a mixture carry the isotope signal that can best address a given scientific objective, 3) determine compound concentrations after excluding contributions from particular endmember sources, and 4) calculate isotope values of different sources. NMF provides a quantitative approach for evaluating the influence of endmember mixing on molecular concentrations and isotope values within a dataset. The re-analysis of two published datasets reveals new quantitative insight into Holocene Arctic climate and Neogene vegetation change.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"396 ","pages":"Pages 122-134"},"PeriodicalIF":4.5,"publicationDate":"2025-03-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666344","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":"Fe-Mg interdiffusion in orthopyroxene: Complex interdependencies of temperature, composition and oxygen fugacity","authors":"Maria A. Dias ,&nbsp;Ralf Dohmen ,&nbsp;Harald Behrens","doi":"10.1016/j.gca.2025.03.002","DOIUrl":"10.1016/j.gca.2025.03.002","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Knowledge of Fe-Mg interdiffusion coefficients (&lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt;) in orthopyroxene (opx) is relevant for diffusion chronometry as well as for thermometers based on Fe-Mg exchange between opx and other common mafic minerals. We extended the existing data set for &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt; to quantify the effect of the molar fraction of Fe, &lt;em&gt;X&lt;/em&gt;&lt;sub&gt;Fe&lt;/sub&gt; = Fe/(Fe + Mg) = 0.1 to 0.4, and oxygen fugacity, &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; = 10&lt;sup&gt;-7&lt;/sup&gt; to 10&lt;sup&gt;-11&lt;/sup&gt; Pa, in the temperature range, &lt;em&gt;T&lt;/em&gt; = 950–1100 °C, where we build up on our recently developed experimental and analytical approach. Thin film diffusion couples using different natural opx with different trace element contents were annealed in vertical gas mixing furnaces. The diffusion profiles were extracted by acquiring backscattered electron images on foils cut from the samples using a focused ion beam-scanning electron microscope. We found complex interplays between the effect of &lt;em&gt;T&lt;/em&gt;, &lt;em&gt;X&lt;/em&gt;&lt;sub&gt;Fe&lt;/sub&gt; and &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; on &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt;. The effect of &lt;em&gt;X&lt;/em&gt;&lt;sub&gt;Fe&lt;/sub&gt; increases with &lt;em&gt;T&lt;/em&gt; for &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; = 10&lt;sup&gt;-7&lt;/sup&gt; to 10&lt;sup&gt;-9&lt;/sup&gt; Pa but decreases with &lt;em&gt;T&lt;/em&gt; for more reducing conditions. For &lt;em&gt;T&lt;/em&gt; &gt; 1000 °C and &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; &gt; 10&lt;sup&gt;-10&lt;/sup&gt; Pa, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt; depends on &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; and &lt;em&gt;X&lt;/em&gt;&lt;sub&gt;Fe&lt;/sub&gt;. For these conditions &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt; is described by an activation energy of 284 ± 19 kJ/mol. For &lt;em&gt;T&lt;/em&gt; = 950 °C to 1000 °C and for &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; &lt; 10&lt;sup&gt;-10&lt;/sup&gt; Pa, &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt; seemingly ceases to depend on &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt;, indicating a change of the diffusion mechanism, and is described by an activation energy of 246 ± 78 kJ/mol. Based on these diffusion data, we propose a qualitative point defect model for opx where the majority point defects changes over the explored Fe content, &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; and &lt;em&gt;T&lt;/em&gt; conditions from vacancies on the metal site/electron holes to vacancies on the metal site/Fe&lt;sup&gt;3+&lt;/sup&gt; to Mg interstitials/electrons, each one responsible for a different effect of &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; on &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt;. Based on the re-evaluation of kinetic data of order–disorder in orthopyroxene we propose that for &lt;em&gt;T&lt;/em&gt; &lt; 950 °C &lt;em&gt;D&lt;/em&gt;&lt;sub&gt;Fe-Mg&lt;/sub&gt; becomes insensitive to &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; and the effect of Fe is relatively constant and smaller than at higher &lt;em&gt;T&lt;/em&gt;. We derived two parameterizations valid for temperatures between 950 and 1100 °C and &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; &gt; 10&lt;sup&gt;-10&lt;/sup&gt; Pa and for the same temperature range but for log &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; ≤ 10&lt;sup&gt;-10&lt;/sup&gt; Pa. In addition, we explore how the new diffusion data would affect the re-equilibration of Fe-Mg during cooling between opx and spinel as well as opx and clinopyroxene. Based on our results, it is likely that the peak composi","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 195-211"},"PeriodicalIF":4.5,"publicationDate":"2025-03-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666343","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":"The role of dissolved sulfide in controlling copper speciation in basaltic melts","authors":"Antonio Lanzirotti ,&nbsp;Michelle Muth ,&nbsp;Elisabet Head ,&nbsp;Matthew Newville ,&nbsp;Molly McCanta ,&nbsp;Paul J. Wallace ,&nbsp;Zoltan Zajacz","doi":"10.1016/j.gca.2025.02.037","DOIUrl":"10.1016/j.gca.2025.02.037","url":null,"abstract":"<div><div>This study evaluates changes in copper (Cu) speciation that occur in sulfate-dominated basaltic and andesitic magmas equilibrated at oxygen fugacities (<em>f</em>O₂′s) above the nickel-nickel oxide (NNO) buffer. Cu K-edge microfocused X-ray absorption fine structure spectroscopy (XAFS) data are presented from both natural and synthetic silicate glasses. Natural samples analyzed include olivine-hosted melt inclusions from tephra of mafic cinder cones in the Lassen segment of the Cascade arc (USA) and from the Michoacán-Guanajuato volcanic field (Mexico) as representative samples from melts equilibrated at <em>f</em>O₂ &gt; NNO. A comparison with melts equilibrated at <em>f</em>O₂ &lt; NNO is provided by analysis of olivine-hosted melt inclusions from Kīlauea Volcano. Data are also presented from copper- and sulfur-bearing synthetic hydrous andesitic glasses synthesized over a range of <em>f</em>O₂, from roughly NNO-2 to NNO+2. The Cu spectroscopy data from the natural and synthetic glasses show two dominant Cu species, Cu¹⁺ oxides (referred to here as Cu–O) and Cu¹⁺ sulfides (referred to here broadly as Cu-S, but not precluding Cu-Fe-S species). The relative proportion of each species present correlates with the relative concentration of dissolved sulfide in the melt. Synthetic sulfur-bearing glasses equilibrated at NNO-1.2 were found to contain exclusively Cu-S species. Sulfur-bearing experimental glasses equilibrated at NNO-0.5 give calculated Cu–O/(Cu–O + Cu-S), defined here as the “Cu–O fraction”, of &lt; 0.10, whereas sulfur-bearing glasses synthesized at NNO+0.6 and NNO+1.8 give calculated Cu–O fraction &gt; 0.96. Natural melt inclusions from Lassen and Kīlauea show a bimodal distribution in Cu–O fraction, with overlapping ranges, of 0.14–0.77 for Lassen and 0.18––0.78 for Kīlauea. Michoacán-Guanajuato inclusions yield Cu–O fractions of 0.68–0.91. The difference in the calculated proportions of Cu–O to Cu-S species appear correlated with available sulfide in the melt. As relative S^2- concentrations decrease, the dissolved Cu species in the melt evolves from dominantly Cu-S to Cu–O. This includes melts equilibrated at <em>f</em>O₂’s where S⁶⁺ is the dominant S species. At intermediate sulfide abundances both species appear to coexist. Thermodynamic modeling of the Cu speciation in these silicate glasses suggests that speciation of Cu as a CuFeS₂ melt species (akin to chalcopyrite or intermediate solid solution) most accurately predicts the measured Cu species. The modeling suggests that a_FeO in the silicate melt, <em>f</em>O₂ and melt S^2- (expressed as <em>f</em>S₂) are the most important parameters controlling the proportions of Cu–O vs. Cu-S species. Our results provide a new perspective for understanding Cu solubility, transport, and partitioning in magmatic systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 181-194"},"PeriodicalIF":4.5,"publicationDate":"2025-03-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666345","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":"Stable isotope equilibria in the dihydrogen-water-methane-ethane-propane system. Part 2: Experimental determination of hydrogen isotopic equilibrium for ethane-H2 from 30 to 200 °C and propane-H2 from 75 to 200 °C","authors":"Andrew C. Turner ,&nbsp;Roman Korol ,&nbsp;Markus Bill ,&nbsp;Daniel A. Stolper","doi":"10.1016/j.gca.2025.02.033","DOIUrl":"10.1016/j.gca.2025.02.033","url":null,"abstract":"<div><div>The stable isotopic compositions of light <em>n</em>-alkanes, including methane, ethane, and propane, are often used to identify the sources and thermal maturity of natural gas samples. Though stable isotopic compositions of these molecules are commonly assumed to be controlled by kinetic isotope effects, recent studies have proposed both carbon and hydrogen isotopic equilibrium may also occur in some samples. Assessing whether samples are in isotopic equilibrium requires knowledge of light alkane equilibrium fractionation factors over geologically relevant temperatures for formation and storage (up to ∼300 °C). In this study, we report experimental results of hydrogen isotopic equilibrium between ethane and H₂ from 30 to 200 °C and propane and H₂ from 75 to 200 °C. We compare these results with high-level theoretical calculations and provide a preferred polynomial fit to describe equilibrium fractionation factors. Comparison of these fractionation factors with a compilation of ∼500 compiled environmental gas samples supports the proposal that many (∼50%) of these natural gas samples exhibit hydrogen isotopic compositions consistent with having formed in or attained methane-ethane-propane hydrogen isotopic equilibrium over geologically relevant temperatures for formation and storage (50–300 °C).</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"396 ","pages":"Pages 91-106"},"PeriodicalIF":4.5,"publicationDate":"2025-03-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666346","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 overlooked role of mineral pore geometry in the solid-solution partitioning of (poly)oxyanionic metals","authors":"Yu Dai ,&nbsp;Benjamin C. Bostick ,&nbsp;Huihui Du ,&nbsp;Xueyuan Gu ,&nbsp;Guopei Huang ,&nbsp;Shirong Liu ,&nbsp;Lei Song ,&nbsp;Yizhang Liu ,&nbsp;Zengping Ning ,&nbsp;Jing Sun ,&nbsp;Chengshuai Liu","doi":"10.1016/j.gca.2025.02.031","DOIUrl":"10.1016/j.gca.2025.02.031","url":null,"abstract":"<div><div>A high specific surface area (SSA) typically signifies a superior adsorption capacity. Nevertheless, minerals with high SSAs tend to possess tiny pores that may not be accessible to relatively large (poly)oxyanionic metals. Herein, we assessed the adsorption of (poly)oxyanionic metals on 2-line and 6-line ferrihydrite and goethite with distinct SSAs and pore geometries. SSA was estimated by BET isotherm using N₂, while pore geometry was measured by N₂ adsorption isotherms and positron annihilation lifetime spectroscopy. Tungstate and its polymers were chosen as representative (poly)oxyanions. Adsorption experiments were performed with constant mineral surface area, but different contact time, pH, and tungsten concentrations. Unexpectedly, the Langmuir adsorption capacities per unit surface area on 6-line ferrihydrite and goethite were 2–6 and 3–11 times as high as those on 2-line ferrihydrite, respectively. Adsorption on 2-line ferrihydrite was also severely kinetically limited. The varying rates and magnitudes of adsorption were attributed to distinct mineral pore widths: (poly)tungstates could hardly fit within the abundant tiny pores in 2-line ferrihydrite formed by voids between the primary particles/aggregates, but could enter the larger pores in 6-line ferrihydrite and goethite. Consequently, significant decreases of mineral microporosity (&lt;2 nm) were observed following (poly)tungstate adsorption. Tungstate and polytungstate with different hydrated ion radii exhibited similar adsorption behavior, most likely due to the formation of adsorbed polytungstate directly on mineral surface. Our data demonstrate that mineral pore geometry controls the solid-solution partitioning of (poly)oxyanionic metals, which is crucial to comprehend their environmental fate and to mitigate their contamination.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"394 ","pages":"Pages 393-404"},"PeriodicalIF":4.5,"publicationDate":"2025-03-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666347","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":"Structural transformation of manganese oxides induced by the oxidation of As(III) and Mn(II)","authors":"Huan Liu ,&nbsp;Xiangjie Cui ,&nbsp;Xiancai Lu ,&nbsp;Elaine D. Flynn ,&nbsp;Jeffrey G. Catalano","doi":"10.1016/j.gca.2025.02.034","DOIUrl":"10.1016/j.gca.2025.02.034","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Manganese (Mn) oxides are ubiquitous in natural systems, occurring as reactive nanoparticles with high surface area that play key roles in controlling the fate of trace elements and contaminants. These minerals may effectively scavenge arsenite [As(III)] from solution via oxidation and adsorption reactions, generating solid-phase Mn(II/III) as well as dissolved Mn(II). At redox interfaces, dissolved Mn(II) similarly generates solid-phase Mn(II/III) via adsorption and comproportionation reactions with Mn oxides, inducing structural transformations. It is unclear how the co-reaction of As(III) and Mn(II) with Mn oxides at redox interfaces affects both arsenic oxidation and mineral structure. This study investigates the reaction of As(III) and δ-MnO&lt;sub&gt;2&lt;/sub&gt;, a phyllomanganate mineral that has a turbostratic structure similar to natural biogenic Mn oxides, at pH 4, 7, and 8.5. Dissolved Mn(II) is introduced in some systems to investigate the synergistic and competitive effects of its co-reaction with As(III) on the structure of Mn oxides. Results show that partial adsorption of dissolved As after 2 d, with all remaining dissolved As being oxidized to As(V). The adsorption of As increased with decreasing pH. As(III) was fully oxidized to As(V) in the solid phase, and thus also the system, regardless of pH and dissolved Mn(II) concentration. Adsorbed As(V) occurred as a bidentate, binuclear surface complex with a coordination geometry unaffected by chemical conditions. Reactions of As(III) and Mn(II) with δ-MnO&lt;sub&gt;2&lt;/sub&gt; each decreased the average manganese oxidation state of the mineral. When these co-occurred, their effects were largely additive. Both Mn(II) and Mn(III) were generated in the solid phase, with a greater relative proportion of Mn(III) at higher pH. Mn(II) induced a partial transformation of δ-MnO&lt;sub&gt;2&lt;/sub&gt; to nsutite at pH 4, but this was inhibited by As(III) despite being similar to the greater generation of solid-phase Mn(II/III). At pH 7, the reaction with Mn(II) increased layer stacking, with a lesser increase from reaction with As(III) and weak to no change in stacking when dissolved As(III) and Mn(II) initially co-occurred. In contrast, at pH 8.5, reaction with As(III) induced greater layer stacking than dissolved Mn(II), which had little effect, and As(III) and Mn(II) co-existence produced even stronger stacking and initiated a partial conversion of the sheet symmetry from hexagonal to orthogonal. Co-addition of dissolved As(V) and Mn(II) had similar effects as a mixture of dissolved As(III) and Mn(II) at pH 4 and 7 but caused a complete inhibition of structural changes at pH 8.5. This study indicates that the effects of the co-existence of Mn(II) and As(III) on the transformation of Mn oxides are pH-dependent, showing an inhibitory effect at acidic and neutral pH while a synergistic effect under alkaline conditions. This study also suggests that As(III) could act as an important reducing agent, similar to othe","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"395 ","pages":"Pages 166-180"},"PeriodicalIF":4.5,"publicationDate":"2025-03-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143666348","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}