Geochimica et Cosmochimica Acta最新文献

{"title":"Mineralogical and isotopic constraints on early, high-temperature events and reservoirs recorded in the interior of a Type B Ca-Al-rich inclusion from the reduced CV3 chondrite Vigarano","authors":"Jangmi Han ,&nbsp;Changkun Park ,&nbsp;Ming-Chang Liu ,&nbsp;Nozomi Matsuda ,&nbsp;Justin I. Simon ,&nbsp;Lindsay P. Keller","doi":"10.1016/j.gca.2025.07.020","DOIUrl":"10.1016/j.gca.2025.07.020","url":null,"abstract":"<div><div>A coordinated mineralogical and oxygen and Al-Mg isotopic study of a Type B Ca-Al-rich inclusion (CAI) from the reduced CV3 chondrite Vigarano was carried out using electron microprobe analysis, transmission electron microscopy, and secondary ion mass spectrometry. This CAI, a once molten igneous object, shows heterogeneous oxygen isotopic compositions among its constituent mineral phases. Spinel is uniformly ¹⁶O-rich with Δ¹⁷O ≤ −23 ‰. The Δ¹⁷O values of Al,Ti-diopside in the CAI core range from −11 ‰ to −18 ‰, which are positively correlated with its TiO₂ contents, whereas diopside in the Wark–Lovering (WL) rim is ¹⁶O-rich with Δ¹⁷O = −23 ‰. Melilite in the CAI mantle shows Δ¹⁷O values ≤ −10 ‰ only when Åk_&lt;15 but becomes more ¹⁶O-poor (Δ¹⁷O = −3 ‰ to −9 ‰) when Åk_&gt;15. These correlated chemical and oxygen isotopic variations were likely established during crystallization of melilite and Al,Ti-diopside from a partial melt, while spinel preserves the original ¹⁶O-rich composition of the CAI precursor. We infer that a partial melt was isotopically evolving from ¹⁶O-rich to ¹⁶O-poor during melilite crystallization, then back to ¹⁶O-rich during Al,Ti-diopside crystallization via exchange with different gas reservoirs of ¹⁶O-poor and ¹⁶O-rich compositions during heating event(s). Our Al-Mg isotopic measurements of the CAI core and mantle define a single isochron with an inferred initial ²⁶Al/²⁷Al ratio of (4.93 ± 0.18) × 10⁻⁵, indistinguishable from that of the WL rim. This indicates that multiple high-temperature events and oxygen isotope exchange with isotopically distinct gas reservoirs occurred rapidly during the CAI formation.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 25-40"},"PeriodicalIF":4.5,"publicationDate":"2025-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144713206","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Revisiting NH4+–Na+ cation exchange selectivity on smectites: Implications for ammonium/ammonia distribution and speciation in icy planetesimals, Ceres, and Enceladus","authors":"Hiroto Tokumon ,&nbsp;Yohei Noji ,&nbsp;Keisuke Fukushi ,&nbsp;Yasuhito Sekine","doi":"10.1016/j.gca.2025.07.021","DOIUrl":"10.1016/j.gca.2025.07.021","url":null,"abstract":"<div><div>A key step in understanding prebiotic chemistry in the Solar System is to predict and reconstruct the speciation and solid–liquid partitioning of inorganic nitrogen species, such as ammonium and ammonia, in icy planetesimals—including C-type asteroids, the dwarf planet Ceres, and Saturn’s moon Enceladus. Smectite, a common constituent of these bodies, can regulate the chemical behavior of NH₄⁺ through cation exchange reactions. Accurate reconstruction of ammonium and ammonia speciation and distribution therefore requires appropriate selectivity coefficients for these exchange processes. In this study, we measured the Na⁺–NH₄⁺ selectivity coefficients (K_Na_{_}_NH4) of montmorillonite and saponite under varying initial NH₄⁺ and Na⁺ concentration, solid concentration, and pH. Cation exchange was confirmed by stoichiometric NH₄⁺ uptake and Na⁺ release. Montmorillonite exhibited log K_Na_{_}_NH4 ranging from −0.06 to 0.41, while saponite showed systematically lower values, from −0.46 to 0.07, likely reflect a difference in hydration retention capacity between the two smectites. Selectivity coefficients for both smectites showed a pH dependence with a maximum around pH 8, and well-described by second-order polynomial fits. Speciation modeling incorporating these coefficients demonstrates that NH₄⁺ interlayer occupancy and the aqueous concentrations of NH₄⁺ and NH₃ are highly sensitive to pH, salinity, and water–rock ratio under plausible geochemical conditions. Modeling results suggest that the aqueous solutions surrounding the Ryugu and Bennu samples during aqueous alteration were highly alkaline (pH &gt; 9.5), favoring NH₃ over NH₄⁺ in solution and resulting in limited NH₄⁺ retention on solids. In the ancient Ceres ocean, NH₄⁺ was abundant in solution due to moderately alkaline conditions (pH ∼ 8) and a high water–rock ratio. For Enceladus, the results indicate that its rocky core may serve as a reservoir of NH₄⁺, with up to 60–70 % of total NH₄ (NH₃ + NH₄⁺) in Enceladus present as interlayer NH₄⁺. These findings provide a quantitative framework for interpreting nitrogen speciation in icy Solar System bodies, including Europa, and their returned or observed materials.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 188-201"},"PeriodicalIF":5.0,"publicationDate":"2025-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890484","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":"Experimental constraints on stable potassium (K) isotope fractionation during phase separation in NaCl–KCl–H2O and KCl–H2O systems: implications for the K isotope composition of seafloor hydrothermal vent fluids","authors":"Soisiri Charin ,&nbsp;Guy N. Evans ,&nbsp;Xinyang Chen ,&nbsp;Yanlu Xing ,&nbsp;Tianyu Chen ,&nbsp;William E. Seyfried Jr ,&nbsp;Xin-Yuan Zheng","doi":"10.1016/j.gca.2025.07.014","DOIUrl":"10.1016/j.gca.2025.07.014","url":null,"abstract":"<div><div>Phase separation is a ubiquitous process in marine hydrothermal systems, significantly influencing the chemical and isotopic composition of vent fluids. Understanding its effects on elemental partitioning and isotopic fractionation is essential for using vent fluid chemistry as an accurate predictor of the chemical and physical conditions at depth in the oceanic crust. Here, we report the first laboratory experimental study investigating stable potassium isotope (⁴¹K/³⁹K) fractionation during phase separation of alkali (Na, K) chloride fluids under temperature and pressure conditions relevant to natural hydrothermal systems. Two distinct fluid compositions were tested in our experiments at 400 °C, including a mixed NaCl–KCl solution with a starting Na/K molar ratio of 10, and a pure KCl solution. This compositional difference allows for evaluating the role of Na on K isotope fractionation between coexisting vapor and liquid phases. For the NaCl–KCl–H₂O system, all experiments showed preferential enrichment of light K isotopes in the vapor phase during phase separation, yielding an average K isotope fractionation factor of −0.12 ‰ (±0.04 ‰, 1SD). In sharp contrast with the NaCl–KCl–H₂O experiments, the Na-free KCl–H₂O experiments revealed no measurable K isotope fractionation during phase separation. These results suggest that Na plays a critical role in modifying K bonding environments, likely through the formation of multi-cation polynuclear species in the concentrated liquid phase. The coexistence of the multi-cation polynuclear species in the liquid and compositionally simpler K free ions or KCl°(aq) ion pairs in the vapor may account for the observed K isotope fractionation in the NaCl–KCl–H₂O system. The absence of Na in the KCl–H₂O system prevents significant differences in K bonding environments between vapor and liquid phases, thereby precluding measurable K isotope fractionation. Using the K isotope fractionation factor quantified here, a Rayleigh fractionation model can successfully explain the previously unexplained low δ⁴¹K values (∼−0.8 ‰) reported for vapor-dominated fluids at Main Endeavour Field (NE Pacific Ocean). These results highlight the importance of considering phase separation alongside seawater interaction with the oceanic crust to fully understand K isotope variations in hydrothermal systems.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 13-24"},"PeriodicalIF":4.5,"publicationDate":"2025-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144703121","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":"Spatial evidence of cryptic methane cycling and methylotrophic metabolisms along a land–ocean transect in salt marsh sediment","authors":"Sebastian J.E. Krause ,&nbsp;Rebecca Wipfler ,&nbsp;Jiarui Liu ,&nbsp;David J. Yousavich ,&nbsp;DeMarcus Robinson ,&nbsp;David W. Hoyt ,&nbsp;Victoria J. Orphan ,&nbsp;Tina Treude","doi":"10.1016/j.gca.2025.07.019","DOIUrl":"10.1016/j.gca.2025.07.019","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Methylotrophic methanogenesis in the sulfate-rich zone of coastal and marine sediments couples with anaerobic oxidation of methane (AOM), forming the cryptic methane cycle. This study provides evidence of cryptic methane cycling in the sulfate-rich zone across a land–ocean transect of four stations–two brackish, one marine, and one hypersaline–within the Carpinteria Salt Marsh Reserve (CSMR), southern California, USA. Samples from the top 20 cm of sediment from the transect were analyzed through geochemical and molecular (16S rRNA) techniques, in-vitro methanogenesis incubations, and radiotracer incubations utilizing &lt;sup&gt;35&lt;/sup&gt;S-SO&lt;sub&gt;4&lt;/sub&gt;, &lt;sup&gt;14&lt;/sup&gt;C-mono-methylamine, and &lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;. Sediment methane concentrations were consistently low (3 to 28 µM) at all stations, except for the marine station, where methane increased with depth reaching 665 µM. Methanogenesis from mono-methylamine was detected throughout the sediment at all stations with estimated CH&lt;sub&gt;4&lt;/sub&gt; production rates in the sub-nanomolar to nanomolar range per cm&lt;sup&gt;3&lt;/sup&gt; sediment and day. 16S rRNA analysis identified methanogenic archaea (&lt;em&gt;Methanosarcinaceae&lt;/em&gt;, &lt;em&gt;Methanomassiliicoccales&lt;/em&gt;, and &lt;em&gt;Methanonatronarchaeacea&lt;/em&gt;) capable of producing methane from methylamines in sediment where methylotrophic methanogenesis was found to be active. Metabolomic analysis of porewater showed mono-methylamine was mostly undetectable (&lt;3 µM) or present in trace amounts (&lt;10 µM) suggesting rapid metabolic turnover. In-vitro methanogenesis incubations of natural sediment showed no linear methane buildup, suggesting a process limiting methane emissions. AOM activity, measured with &lt;sup&gt;14&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, overlapped with methanogenesis from mono-methylamine activity at all stations, with rates ranging from 0.03 to 19.4 nmol cm&lt;sup&gt;−3&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;. Geochemical porewater analysis showed the CSMR sediments are rich in sulfate and iron. Porewater sulfate concentrations (9–91 mM) were non-limiting across the transect, supporting sulfate reduction activity (1.5–2,506 nmol cm&lt;sup&gt;−3&lt;/sup&gt; d&lt;sup&gt;−1&lt;/sup&gt;). Porewater sulfide and iron (II) profiles indicated that the sediment transitioned from a predominantly iron-reducing environment at the two brackish stations to a predominantly sulfate-reducing environment at the marine and hypersaline stations, which coincided with the presence of phyla (Desulfobacterota) involved in these processes. AOM activity overlapped with sulfate reduction and porewater iron (II) concentrations suggesting that AOM is likely coupled to sulfate and possibly iron reduction at all stations. However, 16S rRNA analysis identified anaerobic methanotrophs (ANME-2) only at the marine and hypersaline stations while putative methanogens were found in sediment across all stations. In one sediment horizon at the marine station, methanogen families (&lt;em&gt;Methanosarcinaceae&lt;/em&gt;, &lt;em&gt;Methanosaetaceae&lt;/em&gt;, &lt;em&gt;Methan","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 53-71"},"PeriodicalIF":5.0,"publicationDate":"2025-07-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144780762","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":"Significant calcium isotope fractionation driven by partial dissolution of subducting carbonate in slab-derived fluids","authors":"Wen-Ning Lu ,&nbsp;Yongsheng He ,&nbsp;Shuguang Li ,&nbsp;Yang Wang (王阳) ,&nbsp;Qingshang Shi ,&nbsp;Shan Ke ,&nbsp;Shui-Jiong Wang","doi":"10.1016/j.gca.2025.07.017","DOIUrl":"10.1016/j.gca.2025.07.017","url":null,"abstract":"<div><div>The burial of carbon from Earth’s surface to its deep interior is vital for sustaining the planet’s habitability, however, uncertainties in the dissolution behavior of subducting carbonates make the efficiency of such a process debatable. In this study, we investigate the dissolution behavior of carbonates in subduction zones by analyzing Ca isotope compositions of ultrahigh-pressure (UHP) marbles from the Sulu orogen, China. Our results reveal a <em>δ</em>^44/42Ca range that extends from protolith-like values (∼0.46 ‰) to significantly lighter ones (0.19 ‰). This variation, correlating positively with ⁸⁷Sr/⁸⁶Sr(i) (0.7038–0.7075), indicates a role of infiltration of external fluids, likely derived from dehydration during the amphibolite to eclogite transition of mafic lithologies with variably low ⁸⁷Sr/⁸⁶Sr(i). Simulations on fluid-carbonate interaction processes at the constrained conditions illustrate that Ca isotope variations of marbles were dominantly governed by dissolution. With heavy Ca isotopes being preferentially enriched in the fluids in equilibrium, <em>δ</em>^44/42Ca of the residual carbonates could have decreased progressively with increasing degrees of dissolution. Utilizing a Rayleigh fractionation model, we estimate that negligible dolomitic marbles versus a substantial proportion—approximately 76 ± 15 %—of Ca-rich marbles have undergone dissolution. Our study illustrates that Ca-rich carbonates can be efficiently dissolved if slab-derived fluids are present, even at continental subduction zones where the slabs are commonly considered to be “dry”. This dissolution process has transferred an isotopically mantle-like Ca flux upward, and left behind residual calcium carbonate with notably lighter <em>δ</em>^44/42Ca. Our findings provide evidence for the contrast dissolution behaviors of calcium versus magnesium carbonates in the subduction channel, and help to explain why light Ca isotope anomalies are commonly identified among mantle-derived magmas far away from magmatic arcs.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 41-52"},"PeriodicalIF":4.5,"publicationDate":"2025-07-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144713211","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":"Theory of classical kinetic isotope effects in evaporation","authors":"Shiori Inada ,&nbsp;Tetsuya Hama ,&nbsp;Shogo Tachibana","doi":"10.1016/j.gca.2025.07.018","DOIUrl":"10.1016/j.gca.2025.07.018","url":null,"abstract":"<div><div>Isotopic fractionation resulting from kinetic isotope effects (KIEs) in evaporation is a key to investigating high-temperature evaporation events in the early Solar System. The magnitude of the KIEs is represented by the kinetic isotope fractionation factor <span><math><mrow><mi>α</mi></mrow></math></span>, which is predicted as <span><math><mrow><mi>α</mi><mo>=</mo><msup><mrow><mfenced><mrow><msub><mi>m</mi><mi>H</mi></msub><mo>/</mo><msub><mi>m</mi><mi>L</mi></msub></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></mrow></math></span> (<span><math><mrow><msub><mi>m</mi><mi>L</mi></msub><mo>/</mo><msub><mi>m</mi><mi>H</mi></msub></mrow></math></span>: the mass ratio of the isotopic evaporated gas species) to a first approximation based on the Hertz-Knudsen equation. However, the experimentally measured <span><math><mrow><mi>α</mi></mrow></math></span> are often closer to 1 than this prediction to various degrees. In this study, we investigated the reason for this observation based on the transition state theory. To evaluate the classical (high-temperature) limit of <span><math><mrow><mi>α</mi></mrow></math></span>, which is given by the isotopic ratio of the imaginary frequencies representing the evaporative motion at the transition state, we constructed a simple model for the vibrational normal mode analysis. In this model, we included the interaction of the evaporating species with the condensed phase surface and the degrees of freedom of atoms in the condensed phase. The present theory clarified the relationship between the magnitude of the evaporative KIEs and the properties of the potential energy surface: the classical limit of <span><math><mrow><mi>α</mi></mrow></math></span> becomes closer to 1 than <span><math><mrow><msup><mrow><mfenced><mrow><msub><mi>m</mi><mi>H</mi></msub><mo>/</mo><msub><mi>m</mi><mi>L</mi></msub></mrow></mfenced></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></mrow></math></span> due to the effect of the condensed-phase degrees of freedom when there exists a potential energy barrier, which is related to unstable interaction between the evaporating species and the condensed phase surface. This result is consistent with the previous experimental data and provides general insights into classical KIEs in chemical reactions.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 172-187"},"PeriodicalIF":5.0,"publicationDate":"2025-07-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890483","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":"Water incorporation in garnet under mantle transition zone conditions: A unique infrared absorption band","authors":"Kai Zhang,&nbsp;Xiaozhi Yang","doi":"10.1016/j.gca.2025.07.015","DOIUrl":"10.1016/j.gca.2025.07.015","url":null,"abstract":"<div><div>Garnet is a critical mineral constituent of the mantle transition zone, where it is in general pyrope-rich (though with a Si-rich majorite component). The water (hydroxyl) storage of garnet in the mantle transition zone has been studied by several experiments, however, the incorporation mechanism remains obscure. We experimentally examined the incorporation of water in garnet under mantle transition zone conditions, by carrying out H-annealing experiments at 15–21 GPa, 1300–1500 °C and Fe-FeO buffered redox conditions. Three natural, <em>gem</em>-quality, pyrope-rich (single crystal) garnet samples with different chemical compositions were chosen as the starting materials, and coexisting assemblages and fluids in the experiments were also controlled. The species and amount of water in the samples were analyzed by infrared spectroscopy. The infrared spectra of the recovered garnet samples consistently display a similar single asymmetric band at ∼ 3630 cm⁻¹, regardless of significant differences in their compositions, original spectral shapes (e.g., band positions or patterns) and experimental conditions. It is expected that this band is a unique feature for water in garnet in the mantle transition zone. The band is essentially the same as the one observed for end-member pure pyrope, and is probably dominated by the well-accepted hydrogarnet substitution. By comparing with available annealing experiments at lower pressures and temperatures where the band positions of water in various garnet samples are essentially the same as those in their starting crystals, we suggest that pressure and temperature are two important factors in affecting the band patterns. The high pressure and temperature corresponding to the mantle transition zone may reshape the local bonding environments of H in garnet in a way different from that at relatively low pressures and temperatures, leading to the unique band at ∼ 3630 cm⁻¹. Under otherwise comparable conditions, the water contents of different garnet samples exhibit complex correlations with other components, due to their composition-regulated incorporation, and the most profound dependences are observed for FeO and MgO, with less or no dependences for others. Very likely, Fe and Mg are also important in the water incorporation; however, the general single band at ∼ 3630 cm⁻¹ indicates that the infrared signals by Fe and Mg substitutions are superimposed with the hydrogarnet substitution.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 162-171"},"PeriodicalIF":5.0,"publicationDate":"2025-07-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144890482","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":"Adsorption and coarsening of gold nanoparticles in iron (oxyhydr)oxides lead to high-grade gold mineralization","authors":"Feng-Xia Huang ,&nbsp;Ya-Fei Wu ,&nbsp;Katy Evans ,&nbsp;Anthony E. Williams-Jones ,&nbsp;Xin-Fu Zhao ,&nbsp;Yong Wei ,&nbsp;Wei Feng ,&nbsp;Jian-Wei Li","doi":"10.1016/j.gca.2025.07.013","DOIUrl":"10.1016/j.gca.2025.07.013","url":null,"abstract":"<div><div>Many hydrothermal gold systems within Earth’s crust develop high-grade ores that contain gold-rich iron (oxyhydr)oxides including hematite (α-Fe₂O₃) and goethite (α-FeOOH). The origin of these iron (oxyhydr)oxides and their role in gold enrichment, however, remain unclear. This study presents detailed micrometer- to nanometer-scale mineralogical and compositional studies of gold-rich iron (oxyhydr)oxides in a high-grade gold deposit at Jinying, China. Our results show that nanoparticles of goethite and gold formed from oxidizing fluids, which might be derived from interactions between magmatic-hydrothermal fluid and hematite-rich wall rocks and infiltration of oxygenated meteoric water. Goethite nanoparticles formed acicular crystals via aggregation and oriented attachment and then transformed into porous hematite via solid-state transformation. Gold nanoparticles that were initially adsorbed on the goethite surfaces via electrostatic interaction and surface adsorption were released during the transformation of goethite into hematite. The released gold was re-enriched within the small pore in hematite and grew into irregular micron-sized particles via aggregation and Ostwald ripening. These results highlight the previously undocumented impact of the formation and transformation of iron (oxyhydr)oxides on gold accumulation in forming some types of high-grade gold mineralization related to iron formation. This study provides significant new insights into the enrichment of gold nanoparticles in iron (oxyhydr)oxides that are common in a variety of oxidizing ore-forming environments.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 1-12"},"PeriodicalIF":4.5,"publicationDate":"2025-07-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144664931","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":"Dominant environmental variables affecting brGMGT-derived temperature proxies differ in lake surface sediments and sediment cores","authors":"Jie Wu, Qiangqiang Kou, Caiming Shen, Xinxin Wang, Liping Zhu, Huan Yang, Shucheng Xie","doi":"10.1016/j.gca.2025.07.012","DOIUrl":"https://doi.org/10.1016/j.gca.2025.07.012","url":null,"abstract":"Branched glycerol monoalkyl glycerol tetraethers (brGMGTs), a suite of orphan membrane lipids structurally similar to branched glycerol dialkyl glycerol tetraethers (brGDGTs), have been proposed as promising tools for quantitative paleotemperature reconstruction in lakes. However, the applicability of brGMGT-based temperature proxies and the influence of salinity and dissolved oxygen (DO) concentration on brGMGT distributions remain largely unexplored, limiting their use as reliable and quantitative paleoclimate proxies in sediment cores. To address these uncertainties, we examined brGMGT concentrations and distributions in surface sediments from West China lakes with varying temperatures and salinities, as well as transects in Lake Yangzonghai featured by a wide gradient of DO concentration. Our results show that brGMGTs are abundant and primarily autochthonous in lake surface sediments, with minimal contributions from surrounding soils. Temperature was found to be the primary factor influencing brGMGT distributions. The brGMGT-based proxy brGMGTII, newly proposed in this study, shows significant correlations with mean annual air temperature (MAAT) in surface sediments from both West China and East African lakes (R<ce:sup loc=\"post\">2</ce:sup> = 0.90 and 0.91, respectively; both <ce:italic>p</ce:italic> &lt; 0.01), underscoring its potential as paleotemperature indicator. BrGMGTII calibrations for temperature reconstructions were established for these regions, yielding RMSE (Root mean squared error) values of 2.8 °C and 2.6 °C, respectively. Additionally, high salinity appears to reduce the methylation degree of brGMGTs, whereas low DO concentration may increase the relative abundance of brGMGTs vs. brGDGTs, though DO shows a complex effect on brGMGTI and brGMGTII. Further analysis of brGMGTs in a Holocene sediment core from Lake Yangzonghai in China reveals that these brGMGT-derived temperature proxies inferred unrealistic temperature trends and absolute values. This is likely due to (1) non-temperature factors, particularly DO concentration, which may exert a stronger influence on brGMGT distributions in sediment core than temperature itself; and (2) the unique mechanism by which brGMGTs respond to temperature, i.e., shifts in bacterial communities between high- and low-temperature brGMGT producers rather than a homeoviscous adaptation of membrane lipids.","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"38 1","pages":""},"PeriodicalIF":5.0,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144622045","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":"Nickel isotopic variations in Ni–Cu sulfide deposits: from theoretical predictions to natural observations","authors":"Zhuang Ma ,&nbsp;Shui-Jiong Wang ,&nbsp;Wenzhong Wang ,&nbsp;Matthew J. Brzozowski ,&nbsp;Yun Zhao ,&nbsp;Shengchao Xue ,&nbsp;Ruipeng Li ,&nbsp;Shangguo Su ,&nbsp;Jie Yang","doi":"10.1016/j.gca.2025.07.008","DOIUrl":"10.1016/j.gca.2025.07.008","url":null,"abstract":"<div><div>Extremely light and highly variable δ^60/58Ni values have been observed in sulfide-bearing rocks globally, making Ni isotopes a potential tracer for the genesis of magmatic sulfide deposits. However, the underlying mechanisms that cause the large Ni isotopic variation in sulfides remain unclear. In this study, we first conduct first-principles calculations to determine the reduced partition function ratios of ⁶⁰Ni/⁵⁸Ni (10³lnβ of ⁶⁰Ni/⁵⁸Ni) for major Ni-doped sulfide minerals, including chalcopyrite, pentlandite, pyrite, and pyrrhotite. Our calculations demonstrate that the 10³lnβ of ⁶⁰Ni/⁵⁸Ni values of pyrite and pyrrhotite are sensitive to Ni concentration, and the 10³lnβ of ⁶⁰Ni/⁵⁸Ni values of sulfides increase in the order chalcopyrite ≈ pyrrhotite &lt; pentlandite &lt; pyrite at natural Ni abundance levels. We then investigate the Ni isotopic variability of sulfide minerals from different types of magmatic Ni–Cu sulfide deposits. Despite the significant Ni isotopic variability observed among the different sulfide minerals, the isotopic differences between pyrrhotite and pentlandite fall along the theoretically calculated equilibrium isotope fractionation line. Chalcopyrite exhibits a progressive enrichment in light Ni isotopes as its Fe/(Fe + Cu) ratio decreases, suggesting that its Ni isotopic composition varies with sulfide liquid fractionation over a wide range of temperatures. Sulfides in the Tulaergen deposit (orogenic setting) have systematically lighter Ni isotopic compositions compared to sulfides in the Hulu (orogenic setting) and Jinchuan (intraplate setting) deposits. These differences may be associated with the different mineralizing processes that operated to form these deposits. The parental magma to the Tulaergen deposit was relatively oxidized, leading to the dissolution of a large amount of isotopically light sulfides in the source of the parent melts, whereas the parental magmas to the Hulu and Jinchuan deposits were comparatively reduced and segregated sulfide liquid prior to emplacement in the crust. Our first-principles calculations and measured Ni isotopic compositions of sulfides in ore samples from magmatic sulfide deposits provide foundational information for the future application of Ni isotopes to the petrogenetic assessment of these economically significant critical metal deposits.</div></div>","PeriodicalId":327,"journal":{"name":"Geochimica et Cosmochimica Acta","volume":"404 ","pages":"Pages 150-161"},"PeriodicalIF":5.0,"publicationDate":"2025-07-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144621778","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}