Geochimica et Cosmochimica Acta最新文献

{"title":"Fractionation of highly siderophile elements in forearc magmas: Influence of mantle source and magmatic differentiation","authors":"Ling Chen ,&nbsp;Limei Tang ,&nbsp;Xiaohu Li ,&nbsp;Jie Zhang ,&nbsp;Wei Wang","doi":"10.1016/j.gca.2025.10.003","DOIUrl":"10.1016/j.gca.2025.10.003","url":null,"abstract":"<div><div>Highly siderophile element (HSE) concentrations are presented for gabbroic rocks from the Yap forearc, Western Pacific. These rocks are categorized into cumulate and noncumulate types, exhibiting HSE contents comparable to those found in subduction-related lavas. They are characterized by a platinum-group elements (PGE) pattern showing enrichment in palladium-PGE (PPGE: Rh, Pt, and Pd) relative to iridium-PGE (IPGE: Os, Ir, and Ru). However, some samples exhibit extremely low Pt concentrations, notably lower than typical subduction-related lavas. The Yap cumulate rocks exhibit higher concentrations of IPGE compared to the noncumulate rocks, while they have similar PPGE concentrations. This suggests that IPGE are incorporated into the cumulates through the crystallization of minerals, which may include olivine, sulfide, or alloys. The notably low Cu/Pd ratios observed in the Yap rocks, particularly in the noncumulate rocks, imply that the magmas were likely sulfur-undersaturated during their evolution, which inhibited extensive sulfide crystallization. Therefore, fractionation of PGE in the magma reflects precipitation of IPGE-rich alloys rather than sulfide segregation. Overall, the Yap rocks, particularly the noncumulate rocks, exhibit lower IPGE concentrations than other subduction-related lavas, with some samples showing significantly depleted Pt concentrations, leading to extremely low Pt/Ir and Pt/Pd ratios. This may be attributed to the presence of Pt- and IPGE-rich alloys in the mantle source, as confirmed by the HSE composition of Yap forearc peridotites. The Yap gabbroic rocks and other arc lavas (e.g., from Grenada, Izu-Bonin, and Tonga) are distinct from mid-ocean ridge basalts (MORB) in having lower Cu/Pd ratios, higher total PGE contents, and elevated PPGE/IPGE ratios. These features are attributed to a combination of subduction-zone processes. High oxygen fugacity increases sulfur solubility, inhibiting sulfide saturation and crystallization, which explains the low Cu/Pd ratios. Concurrently, high degrees of mantle melting exhaust sulfides and promote alloy formation in mantle. This liberates PGE into the melt while retaining IPGE in the residue, leading to the observed high PGE contents and fractionated PPGE/IPGE ratios.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"409 ","pages":"Pages 1-15"},"PeriodicalIF":5.0,"publicationDate":"2025-10-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145265075","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":"Abiotic iron oxidation controlled the deposition of Neoproterozoic iron formations","authors":"Zekun Meng, Kang-Jun Huang, Zhenfei Wang, Zhiquan Li, Leslie J. Robbins, Dalton S. Hardisty, Kurt O. Konhauser","doi":"10.1016/j.gca.2025.10.002","DOIUrl":"https://doi.org/10.1016/j.gca.2025.10.002","url":null,"abstract":"Following a billion-year hiatus, iron formations (IFs) briefly re-emerged during Neoproterozoic Snowball Earth glaciations. Unlike their Archean-Paleoproterozoic counterparts, Neoproterozoic IFs (NIFs) are uniquely associated with glaciogenic diamictites and dominated by hematite, yet the drivers of this mineralogical shift remain debated. Here, we present coupled iron-carbon isotope data from Sturtian-aged NIFs in the Fulu Formation (Nanhua Basin, South China). The Fulu NIFs feature alternating bands of euhedral hematite, quartz, feldspar, Fe chlorite, as well as some siderite. The siderite exhibits exceptionally negative δ<ce:sup loc=\"post\">13</ce:sup>C<ce:inf loc=\"post\">carb</ce:inf> values (down to −19 ‰). Isotopic equilibrium modeling supports two hypotheses for this fractionation: (i) a diminished dissolved inorganic carbon (DIC) reservoir influenced by dissimilatory iron reduction (DIR), and (ii) Fe-mediated anaerobic oxidation of methane (AOM) driven by methane diffusion. Both scenarios align with suppressed primary productivity under ice-covered oceans, where microbial activity and organic burial were limited. This low-productivity regime explains the hematite-dominated mineralogy of NIFs, contrasting sharply with the diverse diagenetic phases (e.g., siderite, magnetite) in Archean-Paleoproterozoic IFs. Heavy δ<ce:sup loc=\"post\">56</ce:sup>Fe values (up to 2.03 ‰) further indicate abiotic Fe<ce:sup loc=\"post\">2+</ce:sup> oxidation via meltwater-derived oxygen, rather than biological pathways like photoferrotrophy. Overall, these results support a model of an anoxic Cryogenian ocean with minimal productivity during NIFs deposition, where Fe cycling was governed by glacially mediated redox dynamics.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"98 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145311743","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":"Solubility of anhydrite in supercritical water from 380 ˚C to 625 ˚C and 220 bar to 270 bar","authors":"Pedro M. Rendel,&nbsp;Bruce W. Mountain,&nbsp;Lucjan Sajkowski","doi":"10.1016/j.gca.2024.09.030","DOIUrl":"10.1016/j.gca.2024.09.030","url":null,"abstract":"<div><div>The solubility of anhydrite<span> in deionized water has been determined experimentally from 380 ˚C to 625 ˚C and 220 bar to 270 bar. The experiments were performed using a unique flow-through reactor capable of reaching supercritical conditions for pure water. The results cover the approximate range of temperature and pressure expected to be found in deep geothermal systems where supercritical conditions could be expected. Anhydrite solubility has not been previously well-defined in this region.</span></div><div>The new experimental data are used to define empirical parameters for a new thermodynamic model based on <span><span>Dolejš and Manning (2010)</span></span>, for anhydrite dissolution in silica-bearing aqueous fluids at elevated temperatures and pressures:</div><div><span><math><mrow><mo>-</mo><mi>R</mi><mi>T</mi><mi>ln</mi><mi>m</mi><mo>=</mo><mi>a</mi><mo>+</mo><mi>b</mi><mo>+</mo><mi>e</mi><mi>l</mi><mi>n</mi><msub><mi>ρ</mi><mi>w</mi></msub></mrow></math></span></div><div>where <em>m</em> is the molar concentration mol∙kg⁻¹<em>, R</em> is the gas constant in J mol⁻¹ K⁻¹, and <em>ρ_w</em> is the density of pure water in kg m⁻³. These parameters only apply in solutions containing silica and at fluid densities above 200 kg m⁻³:</div><div><em>a</em> = 14921.46; <em>b</em> = 369.46; <em>e</em> = -48.81.</div><div><em>T</em> is temperature in K.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 82-89"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145326686","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":"Tungsten speciation in hydrothermal fluids","authors":"Yuan Mei ,&nbsp;Weihua Liu ,&nbsp;Qiushi Guan ,&nbsp;Joël Brugger ,&nbsp;Barbara Etschmann ,&nbsp;Coralie Siégel ,&nbsp;Jeremy Wykes ,&nbsp;Rahul Ram","doi":"10.1016/j.gca.2024.06.030","DOIUrl":"10.1016/j.gca.2024.06.030","url":null,"abstract":"<div><div>Understanding the speciation and thermodynamic properties of aqueous tungsten (W) complexes under various conditions is essential for predicting W transport in hydrothermal fluids relevant to ore formation and geothermal systems. Although previous experimental and geochemical modelling studies have provided insights into W solubility in hydrothermal systems, a comprehensive molecular-level understanding of W in hydrothermal fluids remains elusive.</div><div>In this study, we employed <em>ab initio</em> molecular dynamics (MD) simulations to determine the speciation and coordination geometries of W(VI) complexes in NaCl, NaHS, and NaF-bearing brines at temperatures up to 600 °C and pressures up to 2 kbar. These theoretical calculations were complemented by synchrotron <em>in-situ</em> X-ray Absorption Spectroscopy measurements of W(VI) in chloride-, sulfide-, and fluoride-rich solutions under pressures of 600 bar and temperatures ranging from 25 to 429 °C. The speciation and geometrical properties obtained from <em>ab initio</em> MD simulations are in reasonably good agreement with the <em>in-situ</em> X-ray Absorption Spectroscopy data. Our study reveals that W-Cl complexes are not stable, and W is transported as tungstates (H₂WO_4(aq), HWO₄⁻ and WO₄²⁻)in NaCl-rich fluids. In sulfur-rich fluids under near-neutral pH and reduced conditions (sulfide predominant), S²⁻ ions gradually replace O²⁻ in tungstates to form thiotungstate complexes (WO_4-xS_x²⁻, where x = 1, 2, 3, 4). The MD results suggest that fluoride (F⁻) plays a significant role in W transport by forming WO₃F⁻ and WO₃F₂²⁻ complexes, or their hydrated ions. We employed thermodynamic integration to determine the formation constants of the WO₃F⁻ and WO₃F₂²⁻ complexes at temperatures up to 600 °C and 2 kbar, and extrapolated these properties across a broader range of temperatures and pressures. This study underscores the significance of W-F complexes in W transportation in fluoride-bearing, acidic to neutral (pH &lt; 8) hydrothermal fluids. In contrast, W is most likely transported as thiotungstate complexes in sulfur-bearing hydrothermal fluids within a neutral to alkaline pH range (e.g., pH 5–8.5 at 300 °C) under reduced (sulfide-stable) conditions in the Earth’s crust. Existing models for W transport in hydrothermal ore fluids need to consider the influence of W-F and thiotungstate species.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 262-284"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141871519","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":"Reaction calorimetry and structural crystal properties of non-ideal binary rhabdophane solid solutions (Ce1−xREExPO4·nH2O)","authors":"Alexander P. Gysi ,&nbsp;Nicole C. Hurtig ,&nbsp;Hannah Juan Han ,&nbsp;Emma C. Kindall ,&nbsp;Xiaofeng Guo ,&nbsp;Dmitrii A. Kulik ,&nbsp;George Dan Miron","doi":"10.1016/j.gca.2024.04.034","DOIUrl":"10.1016/j.gca.2024.04.034","url":null,"abstract":"<div><div>Rhabdophane is a hydrous phosphate that commonly replaces monazite as a weathering product in critical mineral deposits during the alteration of rare earth elements (REE) bearing carbonatites and alkaline igneous complexes. It is an important host to the light (L)REE (i.e., La to Gd) but the stability and structure of binary solid solutions between the Ce and the other LREE endmembers have not yet been determined experimentally. Here we present room temperature calorimetric experiments that were used to measure the enthalpy of precipitation of rhabdophane (Ce_1−xREE_xPO₄·<em>n</em>H₂O; REE = La, Pr, Nd, Sm, Eu, and Gd). The solids were characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and the role of water in the rhabdophane structure was further determined using thermogravimetric analysis coupled with differential scanning calorimetry. The calorimetric experiments indicate a non-ideal behavior for all of the binary solid solutions investigated with an excess enthalpy of mixing (Δ<em>H</em>^ex) described by a 2- to 3-term Guggenheim parameters equation. The solid solutions were categorized into three groups: (1) binary Ce-La and Ce-Pr which display positive Δ<em>H</em>^ex values with a slight asymmetry; (2) binary Ce-Nd and Ce-Sm which display negative Δ<em>H</em>^ex values with a nearly symmetric shape; (3) Ce-Eu and Ce-Gd which display both negative and positive Δ<em>H</em>^ex values with nearly symmetric shape. The excess Gibbs energy (Δ<em>G</em>^ex) of the solid solutions was further investigated using a thermodynamic analysis approach of aqueous-solid solution equilibria and the optimization programs GEMS and GEMSFITS. The resulting Δ<em>G</em>^ex values combined with the calorimetric Δ<em>H</em>^ex values indicate that there is likely an excess entropy contribution implying important short-range structural modifications in the solid solutions dependent on the deviation of the REE ionic radii from the size of Ce³⁺. These observations corroborate with the trends in the Raman <em>v</em>₁ stretching bands of the PO₄-site. The excess molar volumes determined from X-ray diffraction analysis further indicate an overall asymmetric behavior in all of the studied binary solid solutions, which becomes increasingly important from La to Gd. The pronounced short-range order–disorder occurring in groups 2 and 3 solid solutions mimics some of the behavior observed from previous studies in anhydrous monazite solid solutions. This study highlights the potential to use the chemistry and the structural modifications of rhabdophane as potential indicators of formation conditions in geologic systems and permits improving our modeling capabilities of REE partitioning in critical minerals systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 189-210"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140920055","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":"Ti- and Ba-rich phlogopitic micas in alkaline basic and upper mantle igneous rocks; stoichiometry, stability, and Fe valence estimation reassessed and rationalised","authors":"C. Michael B. Henderson","doi":"10.1016/j.gca.2024.10.011","DOIUrl":"10.1016/j.gca.2024.10.011","url":null,"abstract":"&lt;div&gt;&lt;div&gt;&lt;span&gt;Ti- and Ba-rich tri-octahedral micas occur in fractionated basic igneous rocks, metasomatized mantle peridotites&lt;span&gt;, metamorphosed pelites/carbonates, and hydrothermally altered mineral deposits. Electron microprobe analyses (EMP), with all iron reported as FeO, were widely used in the 1970/80s to interpret Ti and Ba substitution mechanisms, based on 22 O&lt;/span&gt;&lt;/span&gt;&lt;sup&gt;2–&lt;/sup&gt; unit cell calculations, implying that cation vacancies occur in octahedral and/or intersheet sites. In 1996 EMP with chemical and physical analyses for ferric and total Fe, H&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt;O, (OH), and element-specific Fe X-ray Absorption Spectroscopy (both &lt;/span&gt;&lt;em&gt;K&lt;/em&gt; and &lt;em&gt;L&lt;/em&gt;&lt;span&gt;-edges) established valence states for Fe and Ti and cation site occupancies, that ∼50 % O replaces (OH) molecules, and that 24 anion cell formulae show the absence of cation vacancies. Cell formula calculation protocol for phlogopitic micas is refined here and results tested against the stoichiometric formula for vacancy-free phlogopite, &lt;/span&gt;&lt;sup&gt;XII&lt;/sup&gt;K&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;VI&lt;/sup&gt;Mg&lt;sub&gt;6&lt;/sub&gt;&lt;sup&gt;IV&lt;/sup&gt;[Si&lt;sub&gt;6&lt;/sub&gt;Al&lt;sub&gt;2&lt;/sub&gt;]O&lt;sub&gt;20&lt;/sub&gt;(OH)&lt;sub&gt;4&lt;/sub&gt;&lt;span&gt;. Hypothetical sheet silicate compositions, calculated with fixed contents of vacancies linked to particular mixed-valence element substitutions, confirm that reliable unit cell formulae for natural mica solids require that each stoichiometric vacancy must be accounted for. If reliable estimates for ‘excess O’ (denoted &lt;/span&gt;&lt;sup&gt;W&lt;/sup&gt;O&lt;sup&gt;2−&lt;/sup&gt;) are assigned to EMP analyses, the proportion of the oxy-mica component in a mica solid solution can be defined. This approach is tested using published analyses for Ti- and Ba-rich biotites from fractionated basic and ultramafic volcanic igneous rocks (oxymica range 2.5–45 %; TiO&lt;sub&gt;2&lt;/sub&gt;&lt;span&gt; up to 14 %; BaO up to 23 %), upper mantle peridotites (equivalent values 7–18 %; 6 %; 0.7 %), and metasomatised upper mantle (2–37 %; 9 %; 23 %). Enrichments of Ti and Ba in micas are clearly linked to the extra oxygen charge required to neutralise the more highly charged Ba&lt;/span&gt;&lt;sup&gt;2+&lt;/sup&gt; and Ti&lt;sup&gt;4+&lt;/sup&gt; replacing K&lt;sup&gt;+&lt;/sup&gt; and Mg&lt;sup&gt;2+&lt;/sup&gt;.&lt;/div&gt;&lt;div&gt;Substitution mechanisms involving Ba, Ti, and Fe&lt;sup&gt;3+&lt;/sup&gt; in ideal phlogopite involve coupled inter-site/inter-valence interactions so both site-ordering and valence-balance between the different cation sites must be accounted for. The cation exchange 2&lt;sup&gt;XII&lt;/sup&gt;K&lt;sup&gt;+&lt;/sup&gt; + 4&lt;sup&gt;VI&lt;/sup&gt;(Mg&lt;sup&gt;2+&lt;/sup&gt; + Fe&lt;sup&gt;2+&lt;/sup&gt;) + 4&lt;sup&gt;IV&lt;/sup&gt;Si&lt;sup&gt;4+&lt;/sup&gt; ↔ &lt;sup&gt;XII&lt;/sup&gt;Ba&lt;sup&gt;2+&lt;/sup&gt; + 3&lt;sup&gt;VI&lt;/sup&gt;Ti&lt;sup&gt;4+&lt;/sup&gt; + 4(Al&lt;sub&gt;total&lt;/sub&gt; + Fe&lt;sup&gt;3+&lt;/sup&gt; + Cr&lt;sub&gt;total&lt;/sub&gt;)&lt;sup&gt;3+&lt;/sup&gt; is used here so that valence and site order can be considered together. Compositional variations show well-defined trends towards high-oxymica content and vacancy-free Ti and Ba mica end-members, rather than to oxy-free end-members with essential vacancies. Average compositions fo","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 57-81"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673571","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":"Gold(I)-bisulfite complexation in hydrothermal nanodroplets: A molecular dynamics study","authors":"Wallace C.H. Hui,&nbsp;Kono H. Lemke","doi":"10.1016/j.gca.2025.01.015","DOIUrl":"10.1016/j.gca.2025.01.015","url":null,"abstract":"<div><div><span>Water nanodroplets present a unique environment for gold hydrothermal transport, with fluid properties in aqueous nanodroplets distinct from bulk liquid and vapor phases. By performing classical and </span><em>ab initio</em><span> molecular dynamics simulations, we have probed the stability of water nanodroplets (H</span>₂O)<em>_n</em> (<em>n</em><span> = 100, 1000) at 25 °C and 100 °C. The solvation and complexation of gold(I)-bisulfite AuHSO</span>₃<span> in nanodroplet environments were also examined, with a particular focus on surface and interior solvation<span>. Classical TIP4P/2005 molecular dynamics simulations reveal extreme densities in the interior of (H</span></span>₂O)₁₀₀ and (H₂O)₁₀₀₀ nanodroplets compared to droplet surface regions. At 25 °C, the interior region of (H₂O)₁₀₀ exhibits fluctuating densities at 1.016–1.079 g/cm³, with two maxima at 1.079 g/cm³ and 1.074 g/cm³, corresponding to pressures of ∼ 2.23 kbar and 2.07 kbar, respectively; Reduced densities are predicted for the larger (H₂O)₁₀₀₀ systems, these being 1.013 g/cm³ (25 °C, 370 bar) and 0.968 g/cm³ (100 °C, 220 bar). The outer regions, on the other hand, featured densities intermediate between saturated liquid and vapor conditions, as part of a transition from liquid to vapor-like densities at the edge of the droplet. Born-Oppenheimer molecular dynamics simulations at 100 °C show that the gold(I)-bisulfite complex H₂O-AuHSO₃<span> maintains a near linear solvation structure (θ</span>_O-Au-S = 172°-174°) in bulk aqueous fluids and at surface and interior sites of (H₂O)₁₀₀ nanodroplets. Distance constrained simulations reveal that, upon extension of the gold(I)-bisulfite Au-S contact (equilibrium <em>r</em>_Au-S = 2.3 Å), HSO₃⁻<span> is displaced by a water molecule, forming a two-water solvation shell around Au</span>⁺<span>. Thermodynamic integration gives gold(I)-bisulfite dissociation energies (ΔG) of 17.65 ± 0.37 kcal/mol (bulk), 20.22 ± 0.38 kcal/mol (nanodroplet surface), and 18.31 ± 0.31 kcal/mol (nanodroplet interior). Our </span><em>ab initio</em> molecular dynamics results demonstrate that water nanodroplets are stable at hydrothermal conditions and would play an important role in the speciation and transport of gold in volcanic and hydrothermal vapors.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 317-325"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143462870","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 solubility of La hydroxide and stability of La3+ and La hydroxyl complexes at acidic to mildly acidic pH from 25 to 250 °C","authors":"Kevin Padilla,&nbsp;Alexander P. Gysi","doi":"10.1016/j.gca.2024.12.006","DOIUrl":"10.1016/j.gca.2024.12.006","url":null,"abstract":"<div><div><span>The mobility of rare earth elements<span> (REE) in natural hydrothermal systems can be assessed using geochemical modeling, which requires reliable thermodynamic data of relevant aqueous species. In this study, we evaluate the controls of pH and temperature on La speciation and the role of hydroxyl complexes in REE transport at hydrothermal conditions. Batch-type hydrothermal solubility experiments were conducted using synthetic La hydroxide powders equilibrated in perchloric acid-based aqueous solutions at temperatures between 150 and 250 °C and starting pH of 2 to 5. The La hydroxide solubility is retrograde with temperature and displays a strong pH dependence with a decrease in La concentrations from acidic to mildly acidic pH spanning between 3 and 5 orders of magnitude (e.g. log La molality of −2.5 to −7.2 at 250 °C). Thermodynamic optimizations using GEMSFITS allow to retrieve the standard partial molal Gibbs energies for the La</span></span>³⁺ aqua ion and the formation constants for the La hydroxyl species (i.e., LaOH²⁺, La(OH)₂⁺, La(OH)₃⁰<span>) between 25 and 250 °C. A comparison between the experimentally derived thermodynamic properties with the calculated values from the Helgeson-Kirkham-Flowers equation of state parameters indicates an increased divergence with temperature. Discrepancies in standard partial molal Gibbs energies range between ∼ 1 − 12 kJ/mol and result in a predicted La hydroxide solubility differing by up to 3 orders of magnitude at 250 °C. Speciation calculations indicate a higher stability of La</span>³⁺ and LaOH²⁺<span> over the other La hydroxyl species in the studied pH range of 3.4 to 6. The optimized thermodynamic properties for La aqueous species have important implications for modeling the solubility of REE minerals such as monazite and the mobility of REE in hydrothermal systems.</span></div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 175-188"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142841533","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":"Stability and structure of aqueous Sb (III) chloride complexes from 25 °C to 250 °C at 25 MPa at high chloride ion concentrations","authors":"Avinaash A. Persaud,&nbsp;Swaroop Sasidharanpillai,&nbsp;Jenny S. Cox,&nbsp;Peter R. Tremaine","doi":"10.1016/j.gca.2024.12.013","DOIUrl":"10.1016/j.gca.2024.12.013","url":null,"abstract":"<div><div><span><span>Despite their importance in modelling the formation of antimony-containing hydrothermal ore deposits, only a few values for the thermodynamic properties of the antimony(III) chloride and sulfide complexes have been reported above ambient conditions. The existing </span>thermochemical<span><span> data, which extend to ∼250 °C, and the structures of the equilibrium antimony chloride and hydroxy chloride complexes, are largely based on solubility data and </span>EXAFS<span> studies. In the present study, polarized Raman spectroscopy<span> with pressure-controlled, high-pressure fused-silica capillary cells was used to identify the complexes of antimony present in highly-concentrated aqueous lithium chloride solutions (&gt;2 mol·kg</span></span></span></span>⁻¹<span><span>) from 25 to 250 °C at 25 MPa. Vibrational band assignments were made by comparison with computational studies using Gaussian 16 (B3LYP, IEFPCM </span>solvation model), which showed that SbCl</span>₆³⁻ (octahedral), SbCl₄⁻ (see-saw) and SbCl₃⁰ (trigonal pyramidal) are the predominant aqueous antimony species up to 250 °C. Quantitative speciation data from the solvent-subtracted, reduced isotropic spectra were used to determine the stepwise formation constants, <em>K</em>₃,₄ and <em>K</em>₄,₆<span>, together with the Lindsay-Meissner-Tester activity coefficient model. Attempts to fit </span><em>K</em>₄,₆ to the ionic-strength suggested that the predominant species of SbCl₆³⁻<span> under these conditions is an ion pair, LiSbCl</span>₆²⁻, whose formation constant is expressed as <em>K</em>₄,_6Li. The experimental equilibrium concentrations and the fitted values of <em>K</em>₃,₄ and <em>K</em>₄,_6Li show that the equilibria shift with increasing temperature, such that SbCl₄⁻<span> becomes the dominant species in concentrated solutions at 250 °C. This study includes new evidence based on Raman band<span> assignments to resolve the controversy of whether the dominant species at high alkalinities and lower temperatures is SbCl</span></span>₆³⁻ or SbCl₅²⁻.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 148-163"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145326685","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":"Potential for formation of methylated thioarsenates in geothermal environments","authors":"Ketao Yan ,&nbsp;Qinghai Guo ,&nbsp;Luxia Wang ,&nbsp;Yi Liu ,&nbsp;Britta Planer-Friedrich","doi":"10.1016/j.gca.2024.10.012","DOIUrl":"10.1016/j.gca.2024.10.012","url":null,"abstract":"<div><div><span>Geothermal waters typically have elevated arsenic (As) concentrations. Various As species have been identified, including methylated thioarsenates, which present a high environmental and health risk due to high mobility and toxicity. Upon discharge from hot springs, temperature and geogenic sulfide excess decrease, while redox potential increases. The combined effects of those parameters on activities of sulfate-reducing bacteria and </span>methanogens<span><span> and thereby on extent of As methylation<span> versus thiolation is currently unknown. Here, we incubated sediments from alkaline hot springs and outflow channels at the Tengchong geothermal region in southwestern China, where inorganic and methylated thioarsenates had been detected, at temperatures between 20 to 90 °C with different initial aqueous As and sulfur species. Results from field samples and incubation experiments showed methylated thioarsenates, but no methylated oxyarsenates, implying quantitative thiolation even at low sulfide concentrations. </span></span>Enrichment cultures<span> derived from one sediment with quantitative formation of dimethyl-dithioarsenate (DMDTA) at 55 °C, showed a dominance of inorganic trithioarsenate at 35 °C and monomethylated thioarsenates at 55 °C when incubated with two other sediments. The rate-limiting step was microbially mediated As methylation<span> from arsenite<span>, which was not observed at 20 °C or &gt;= 75 °C, in contrast to thiolation of already methylated arsenates. Addition of free sulfide and thiosulfate to incubation experiments promoted formation of inorganic or monomethylated high-thiolated arsenates, while sulfate promoted full methylation and formation of DMDTA, probably due to continuous low supply of microbially produced sulfide. Since DMDTA is more mobile and toxic than inorganic and monomethylated thioarsenates, understanding constraints to its formation is especially important for future risk assessment.</span></span></span></span></div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"406 ","pages":"Pages 119-133"},"PeriodicalIF":5.0,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142673654","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}