Chemical Geology最新文献

{"title":"Internally consistent thermodynamic properties of Nd chloride aqueous species and Nd3+ derived from hydrothermal solution calorimetry at 25–150 °C","authors":"Yerko Figueroa Penarrieta ,&nbsp;Alexander P. Gysi","doi":"10.1016/j.chemgeo.2025.123192","DOIUrl":"10.1016/j.chemgeo.2025.123192","url":null,"abstract":"<div><div>Rare earth elements (REE) are important critical elements used in the high-technology and green energy industries. Hydrothermal fluid-rock interaction and the stability of aqueous chloride complexes play an important role in the mobilization and deposition of REE in geologic systems. Geochemical modeling can be used to interpret the mobility of REE in these hydrothermal fluids. However, the accuracy of modeling predictions strongly depends on the underlying thermodynamic properties for REE aqueous species. In this study, hydrothermal solution calorimetry was used to measure the standard partial molal enthalpy of solution (Δ_sol<em>H</em>°) of Nd chloride solid (NdCl₃·6H₂O) from 25 to 150 °C in order to determine the standard thermodynamic properties of Nd chloride aqueous species. The experiments were conducted in aqueous hydrochloric acid-based aqueous solutions with an initial pH of 2 and varying ionic strength (0.08–0.42 mol/kg), at which conditions the Nd³⁺ and Nd chloride species are both stable. In the studied temperature range, the measured Δ_sol<em>H</em>° values differ by 35 kJ/mol in comparison to thermodynamic predictions based on available literature data. These large discrepancies have important implications for geochemical modeling because the thermodynamic properties of REE aqueous species are derived from the properties of REE chloride solids. To address this problem, the standard enthalpy of formation (Δ_f<em>H</em>°) of NdCl₃·6H₂O(s) and aqueous NdCl²⁺ species were optimized to reproduce the experimentally derived Δ_sol<em>H</em>° values and keeping internal consistency with the recently updated thermodynamic properties of Nd³⁺. The outcome of this study permits deriving a set of recommended internally consistent thermodynamic properties for the Nd aqueous species which improve thermodynamic modeling of REE mobilization in acidic NaCl-bearing aqueous fluids.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123192"},"PeriodicalIF":3.6,"publicationDate":"2025-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145690055","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Rapid recycling of sediment-derived granites in accretionary orogens: Implications from the Alataw Mountains, NW China","authors":"Jiyuan Yin ,&nbsp;He Huang ,&nbsp;Ilya Bindeman ,&nbsp;Chiara Maria Petrone ,&nbsp;Mike Fowler ,&nbsp;Andrew C. Kerr ,&nbsp;Zaili Tao ,&nbsp;Tao Wang ,&nbsp;Min Sun ,&nbsp;Guochun Zhao ,&nbsp;Wenjiao Xiao","doi":"10.1016/j.chemgeo.2025.123181","DOIUrl":"10.1016/j.chemgeo.2025.123181","url":null,"abstract":"<div><div>The origin of S- and I-type granites in accretionary orogens remains contentious due to complex geochemical and isotopic signatures, challenging traditional granite classifications. We present new zircon U-Pb-Hf-O, quartz O isotope and whole-rock geochemical data from two-mica (S-type) and biotite (nominally I-type) granites in the Alataw Mountains, northwest China, showing both are derived from sediment-dominated protoliths. Zircon U<img>Pb dating indicates emplacement at 303–298 Ma, with differing inherited ages of 443–397 Ma and 1404–811 Ma. The two-mica granites were derived from metapelitic sources and are strongly peraluminous, while the biotite granites are metaluminous to weakly peraluminous with many “I-type” characteristics. Both types exhibit high SiO₂, low MgO, elevated zircon δ¹⁸O, high quartz δ¹⁸O and high Δ^’17O values consistent with sedimentary sources, but depleted εNd(t) and εHf(t) values that require a juvenile origin. Phase equilibrium modelling indicates that these granites formed through water-fluxed partial melting of variable Carboniferous sedimentary rocks. Thus, crustal melts appear juvenile and S-type granites appear to be I-type. Our findings highlight the critical role of rapid crustal recycling via sediment melting during the reworking of juvenile crust within arc-related accretionary orogens and suggest the need for caution when considering granite origins based on traditional typological classifications.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123181"},"PeriodicalIF":3.6,"publicationDate":"2025-12-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145690052","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Clumped isotope constraints on secondary mineral formation and thermal overprinting in the Michigan Basin, Southwestern Ontario","authors":"Benjamin R. Fosu ,&nbsp;Josué J. Jautzy ,&nbsp;Duane C. Petts ,&nbsp;Ian D. Clark ,&nbsp;Tom A. Al","doi":"10.1016/j.chemgeo.2025.123189","DOIUrl":"10.1016/j.chemgeo.2025.123189","url":null,"abstract":"<div><div>Sections of Michigan Basin sediments were exposed to hydrothermal fluids during the Ordovician. Secondary calcite and dolomite occur as fracture-filling veins/vugs and replacement minerals in host Ordovician limestones – which occur at depth beneath the Bruce Nuclear site (near Tiverton) in southwestern Ontario. To provide insights on the evolution of diagenetic fluids responsible for secondary mineral growth and the effects of temperature-dependent alteration in the basin, clumped isotope geothermometry was applied to these carbonates. Ordovician calcite in veins/vugs have δ¹⁸O_C values of +20.0 to +23.4 ‰ (VSMOW) and are estimated to have formed at 65–83 °C from various waters (δ¹⁸O_w = +0.1 to +4.4 ‰) including Ordovician to modified (an evolved ¹⁸O-enriched) seawater, and mixed seawater and basinal brines at depth. Calcite veins/vugs in the lower Cambrian units formed at 74–91 °C, have lower δ¹⁸O_C values (+14.9 to +17.0 ‰) and precipitated from ¹⁸O-depleted hydrothermal brines (δ¹⁸O_w = −4.6 to −3.1 ‰). Calcite in the matrix of host limestones yielded higher apparent temperatures (T(Δ₄₇) = 49–82 °C)) that are inconsistent with precipitation from Ordovician seawater and burial diagenesis of the sediments but instead, reflect a solid-state isotope reordering due to a Late Devonian-Mississippian rift-related basinal heating. Diagenetic dolomitization in the basin occurred during shallow to intermediate burial, where dolomite primarily replaces calcite in the limestone matrix or occur as secondary infill in fractures and vugs. The clumped isotopic signature in dolomite is more resistant to low-temperature heating than calcite, whose ∆₄₇ composition reflects a 367–322 Ma regional heating event at 125–150 °C during burial, in agreement with, but more precise than previous estimates based on fluid inclusion microthermometry in secondary quartz and saddle dolomite.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123189"},"PeriodicalIF":3.6,"publicationDate":"2025-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145690050","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Reconstruction of oceanic paleosalinity during the Late Ordovician in the inner Yangtze Sea of South China","authors":"Qiang Gu ,&nbsp;Fengcun Xing ,&nbsp;Karem Azmy ,&nbsp;Gesheng Wang ,&nbsp;Xinying Liu ,&nbsp;Xi Wei ,&nbsp;Ziqi Liu ,&nbsp;Aishi Liang","doi":"10.1016/j.chemgeo.2025.123190","DOIUrl":"10.1016/j.chemgeo.2025.123190","url":null,"abstract":"<div><div>The Ordovician-Silurian Transition (OST) was a key geological period, which recorded the Late Ordovician Mass Extinction (LOME), volcanic eruption, oceanic anoxia, rapid sealevel fluctuations, glacial event and carbon isotope excursion. Recent studies also identify a late Katian extinction pulse as part of the LOME. During the OST, the inner Yangtze Sea (IYS) was tectonically restricted, but the controlling factors of salinity heterogeneity in the basin and their impact on LOME remain debated. Carbonate sedimentation dominated the IYS in the Late Ordovician, thus complicating continuous salinity reconstructions because most paleosalinity proxies are calibrated for shales and are less readily applied to carbonates. Based on carbon-isotope stratigraphy, we used geochemical proxies (e.g., elemental and the new proxy “excess B\") to assess the relative salinity variations in the IYS during the OST. The carbonate salinity proxies revealed a paleosalinity trend like that observed in shale (Decreases and then increases near the Hirnantian). Combining multiple paleoenvironmental proxies and previously published data, we evaluated the contribution of paleosalinity changes to LOME. Our δ¹³C_org and δ¹³C_carb records the Guttenberg positive isotope carbon excursion (GICE) and the Hirnantian Isotopic Carbon Excursion (HICE). The paleosalinity proxies, along with others, allowed the division of the OST into four Units where salinity was controlled by sealevel changes, the Kwangsian Orogeny, and climate-induced changes in terrestrial runoff. The reduction in species and abundance supports the occurrence of the Katian Extinction in the IYS. The expansion of anoxic seawater was not the only factor controlling the LOME, but salinity likely has played a crucial role.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123190"},"PeriodicalIF":3.6,"publicationDate":"2025-12-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145689922","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Redox evolution of carbon- and sulphur-bearing clinopyroxenites as proxies of deeply recycled crustal material","authors":"Nadia Malaspina ,&nbsp;Falko Langenhorst ,&nbsp;Kilian Pollok ,&nbsp;Valerio Cerantola ,&nbsp;Mara Murri ,&nbsp;Carolina Longa ,&nbsp;Danilo Bersani ,&nbsp;Alessandra Montanini","doi":"10.1016/j.chemgeo.2025.123173","DOIUrl":"10.1016/j.chemgeo.2025.123173","url":null,"abstract":"&lt;div&gt;&lt;div&gt;Fossil subduction zones are key to studying the deep geochemical cycles of C, O, and S. We analysed graphite-sulphide-bearing garnet clinopyroxenites from the External Ligurian (Northern Apennines, Italy), which serve as indicators of deep recycling of subducted crust. These rocks are among the only three known pyroxenite occurrences worldwide (along with Beni Bousera, Morocco, and Ronda, Spain) that formed through the crystallisation of eclogite-derived melts (&lt;em&gt;P&lt;/em&gt; ≥ 3 GPa and 1100 °C), following a prolonged recycling history in off-craton mantle. In particular, a MOR-type heterogeneous gabbroic sequence was recycled into the mantle as eclogite 1.5–1.0 Ga ago, then partially melted and crystallised in the convective mantle, followed by subsolidus re-equilibration and exhumation. We analysed the redox state of garnets and clinopyroxenes associated with graphite and sulphides in garnet clinopyroxenites, that crystallised from a liquid produced by partial melting of recycled eclogite. Electron Energy Loss Spectroscopy within a transmission electron microscope and Synchrotron micro-Mössbauer analyses revealed heterogeneities in Fe&lt;sup&gt;3+&lt;/sup&gt;/Fe&lt;sup&gt;2+&lt;/sup&gt; distribution and its partitioning among mineral phases (Fe&lt;sup&gt;3+&lt;/sup&gt;/ΣFe error is 0.03 for Electron Energy Loss Spectroscopy and 0.01–0.03 for Synchrotron micro-Mössbauer).&lt;/div&gt;&lt;div&gt;The analysed clinopyroxenites display three generations of clinopyroxenes: unexsolved crystals included in garnet cores with Fe&lt;sup&gt;3+&lt;/sup&gt;/ΣFe = 0.16–0.38 (representative of eclogite-derived melt crystallisation in the asthenosphere), clinoenstatite exsolution-bearing grains with Fe&lt;sup&gt;3+&lt;/sup&gt;/ΣFe = 0.03–0.10 (related to a first stage of exhumation in the lithospheric mantle), and Al-poorer rims without Fe&lt;sup&gt;3+&lt;/sup&gt; (related to the final stage of exhumation). In contrast, garnets have Fe&lt;sup&gt;3+&lt;/sup&gt;/ΣFe-poor cores (&lt;0.03) and slightly higher ratios in the rims (0.04–0.07). When considered together with the markedly higher Fe&lt;sup&gt;3+&lt;/sup&gt; contents in the earliest clinopyroxene generation, this pattern is consistent with a pressure–temperature–dependent partitioning of ferric iron from garnet to clinopyroxene during cooling from 1100 to 950 °C along the exhumation path. &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; calculations suggest a variation from more oxidised samples (ΔFMQ = −1.25 to 0) to more reduced ones (ΔFMQ = −4.2 to −1.6) at 3 GPa. At 1.5 GPa, ΔFMQ ranges from −1.2 to −0.6 down to &lt; −5, indicating that graphite may have formed through reduction of a previously oxidised carbon phase. The oxidation state variations are linked to sub-solidus decompression, and not to S-C-related redox reactions, describing a closed system with no fluid/melt-rock interaction. Our results show that sulphur plays no role in controlling the redox state of these graphite-bearing mantle rocks, even over prolonged geological histories, and that variations in the redox state of carbon and iron in garnet and clinopyroxen","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"700 ","pages":"Article 123173"},"PeriodicalIF":3.6,"publicationDate":"2025-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145645562","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Barite crystallization at the freezing point: Cryogenic mineral assemblage from cave glaciated in late Pleistocene","authors":"Stanislava Milovská ,&nbsp;Rastislav Milovský ,&nbsp;Monika Orvošová ,&nbsp;Sergii Kurylo ,&nbsp;Juraj Šurka ,&nbsp;Pavel Herich ,&nbsp;Frank Bellmann ,&nbsp;Anna Gibalová ,&nbsp;Lukáš Ackermann ,&nbsp;Lucia Kopčanová ,&nbsp;Muhammad Shahzeb","doi":"10.1016/j.chemgeo.2025.123172","DOIUrl":"10.1016/j.chemgeo.2025.123172","url":null,"abstract":"<div><div>A cryogenic mineral assemblage was investigated in a cave that experienced widespread glaciation in Late Pleistocene. Barite and opal crystallized alongside calcite during the progressive water freezing in cave chambers within thawing permafrost. Barite inclusions evolved from prismatic nuclei into complex spherulitic aggregates, occasionally shaped by now-vanished constraints on growth, presumably imposed by ice, CO₂, and possibly CO₂ clathrate. Clathrate formation at 0 °C requires pressures above 10 bar, a condition that can be met in freezing water pools through pressure buildup from volumetric expansion. Dispersed nanoparticulate silica is identified as a disordered opal variety. Stable strontium isotope ratios proved useful in constraining the provenance of the parental water and confirm its autochthonous seepage origin within the limestone massif. Geochemical modelling indicates early oversaturation and precipitation of barite and silica, often preceding calcite, suggesting that microscopic cryogenic minerals may persist even where macroscopic calcite is absent. These findings deepen our understanding of sub-zero mineral precipitation with potential implications for other terrestrial and extraterrestrial cryogenic environments.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123172"},"PeriodicalIF":3.6,"publicationDate":"2025-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145651135","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Crustal growth and modification of the Muzidian Gneiss complex revealed by detrital zircon from modern river sediments","authors":"Xiao-Fei Qiu ,&nbsp;Xi-Run Tong ,&nbsp;Da Wang ,&nbsp;Xin Deng ,&nbsp;Shan-Song Lu","doi":"10.1016/j.chemgeo.2025.123171","DOIUrl":"10.1016/j.chemgeo.2025.123171","url":null,"abstract":"<div><div>The Muzidian Gneiss Complex located in the Yangtze Block, South China Craton was recently shown to contain &gt;4.0 Ga zircon grains and magmatic rocks as old as ∼3.8 Ga. Our understanding is limited regarding the crustal growth and modification history of this complex. In order to understand the crustal evolution history of the early Archean Muzidian Gneiss Complex and its relationship to the surrounding broader Yangtze Block, we focused on the sediments in five local streams and rivers across the Muzidian area where the Muzidian Gneiss Complex was located. Here we report 899 U-Pb dates and 620 Lu<img>Hf isotope measurements of detrital zircons in river sediments. The detrital zircon grains from these rivers show generally similar U<img>Pb age patterns, yielding two major age peaks at ∼2.5–2.4 Ga and ∼ 140–120 Ma, and a few minor groups at 3.8–3.2 Ga, 2.1–1.9 Ga, 780–650 Ma, and 450–400 Ma, respectively.</div><div>Zircon Lu<img>Hf isotopic data suggest dominant crustal reworking over most of the Archean from an ultimate Hadean or Eoarchean magmatic source, and the input of juvenile materials mixing with pre-existing ancient continental crust since the Neoarchean and Paleoproterozoic. The initial Hf isotopic compositions of the Mesozoic zircon grains in the studied river sediments can be projected to those of the Eoarchean and Mesoarchean zircon grains, suggesting a likely reworked origin for the young zircon grains. These observations collectively suggest that the proportion of continental crust formed in the early Archean is likely greater than what is shown in the U<img>Pb age distribution of detrital zircons. Based on our new data, we propose that the Hadean to Eoarchean rocks in the Muzidian Gneiss Complex likely represent a continental nuclei in the present-day South China Craton, which was long-lived and continuously re-melted to form younger rocks around it during the Archean. Significant input of depleted mantle derived, juvenile materials was not a component in the crustal growth and modification of the Muzidian Gneiss Complex until the Neoarchean, which may correspond to the accretion and cratonization of the broader north Yangtze Block of the South China Craton.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"701 ","pages":"Article 123171"},"PeriodicalIF":3.6,"publicationDate":"2025-11-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145613837","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Iron and nitrate-driven anaerobic methane oxidation in methane seep sediments of the Laptev Sea","authors":"D. Purgina ,&nbsp;Yu Moiseeva ,&nbsp;B. Wild ,&nbsp;N. Guseva ,&nbsp;E. Gershelis ,&nbsp;O. Gustafsson ,&nbsp;A. Khvashevskaya ,&nbsp;S. Pakhomova ,&nbsp;V. Kholodov ,&nbsp;O. Dudarev ,&nbsp;D. Kosmach ,&nbsp;D. Chernykh ,&nbsp;T. Polivanova ,&nbsp;I. Gangnus ,&nbsp;A. Kurilenko ,&nbsp;A. Domaniuk ,&nbsp;E. Yakushev ,&nbsp;I. Semiletov","doi":"10.1016/j.chemgeo.2025.123170","DOIUrl":"10.1016/j.chemgeo.2025.123170","url":null,"abstract":"<div><div>The East Siberian Arctic Shelf (ESAS) is a significant source of atmospheric methane, and the coastal Laptev Sea near the Lena River Delta is a documented hotspot for methane seepage. While anaerobic oxidation of methane (AOM) acts as a critical biofilter, the specific pathways and electron acceptors driving this process in the unique river-influenced settings of the Arctic remain poorly understood. The Lena River delivers substantial terrigenous nutrients to this region, contributing approximately 10 % of the total riverine nitrogen and 17 % of the iron input to the Arctic Ocean. This study examines the hypothesis that these nutrient fluxes play a crucial role in regulating AOM, a key process mitigating methane emissions. We compared the geochemical conditions at active methane seep sites and background areas, with a special focus on nitrogen and iron compounds in bottom and pore waters.</div><div>Our geochemical investigation reveals three key findings: (1) Methane concentrations were substantially elevated at seep sites (up to 3118 nM) compared to background areas (as low as 5.5 nM) and showed negative correlations with nitrate (NO₃⁻) and iron (Fe) concentrations. (2) A clear decoupling of the Fe<img>Mn cycles at seep sites, which was absent in background sediments, indicates the preferential utilization of reactive iron oxides in AOM (Fe-AOM). (3) A severe depletion of bioavailable nitrogen species was observed in seep zones, where nitrate and nitrite concentrations were 5.5-fold and 2-fold lower, respectively, than in background areas, suggesting active nitrate/nitrite-dependent AOM (N-AOM).</div><div>These findings demonstrate that on the nutrient-rich, river-influenced Laptev Sea shelf, AOM is primarily driven not by canonical sulfate reduction, but by the reduction of nitrogen species and iron. This newly identified coupling between riverine nutrient fluxes and AOM pathways represents a critical, yet previously overlooked, mechanism that could significantly suppress methane emissions from climate-sensitive Arctic shelves. To our knowledge, this study provides the first comprehensive investigation of nitrogen and iron transformation mechanisms coupled with methane oxidation at the water-sediment interface in the cold seeps of the coastal Laptev Sea shelf.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"699 ","pages":"Article 123170"},"PeriodicalIF":3.6,"publicationDate":"2025-11-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145609523","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Formation of natural IrO2 through Os–Ir–Ru alloy oxidation","authors":"Anton Kutyrev ,&nbsp;Olga Filimonova ,&nbsp;David W. Saxey ,&nbsp;Anna Vymazalová ,&nbsp;Katy Evans ,&nbsp;Vadim S. Kamenetsky ,&nbsp;Didier Wermeille ,&nbsp;Denis Fougerouse ,&nbsp;Steven M. Reddy","doi":"10.1016/j.chemgeo.2025.123167","DOIUrl":"10.1016/j.chemgeo.2025.123167","url":null,"abstract":"<div><div>We studied a rare platinum-group element (PGE)-bearing phase occurring as inclusions in Pt<img>Fe alloys from placer deposits, previously referred to as “iridium oxide,” but lacking confirmation of actual Ir oxide. X-ray absorption spectroscopy confirms that the studied Ir<img>O phase contains genuine IrO₂, with iridium in a formal oxidation state of 4+ and an Ir<img>O bond distance of 1.95 ± 0.02 Å, matching synthetic rutile-type IrO₂. Atom probe tomography reveals that IrO₂ forms thin films along boundaries between metallic PGE-rich domains, which are separated by oxide-rich regions composed of Ca, Mn, Fe, K, and V oxides. Mass-balance constraints indicate that Rh, Ru, and Pt occur mainly as metals, while Os is partly oxidized. Formation of IrO₂ from Os–Ir–Ru alloys requires extremely oxidizing conditions, estimated at approximately fayalite–magnetite–quartz (FMQ) + 5 to +10. While such conditions could potentially develop during hydrothermal alteration, there is limited geological evidence for such high fO₂ values in natural environments. The coexistence of iridium oxide and metallic Ru contradicts thermodynamic predictions, which suggest Ru should oxidize prior to Ir. This discrepancy indicates that conventional equilibrium thermodynamics, where pressure, temperature, and chemical potential are the controlling intensive variables, may not predict natural redox behaviour. Alternative mechanisms, including kinetically controlled reactions, extremely limited length-scales of equilibrium, or a role for electrochemical work, may permit localized Ir oxidation. The results provide rare in-situ evidence for PGE oxidation and Os<img>Ir mobility within the lithosphere, with implications for PGE distribution and Os isotope systematics in mantle and crustal rocks.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"700 ","pages":"Article 123167"},"PeriodicalIF":3.6,"publicationDate":"2025-11-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145593662","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 Ge/Si partitioning during organic-mediated silica precipitation","authors":"Silvina Slagter ,&nbsp;Dan Asael ,&nbsp;Noah J. Planavsky ,&nbsp;Alida Perez-Fodich ,&nbsp;Lidya G. Tarhan","doi":"10.1016/j.chemgeo.2025.123156","DOIUrl":"10.1016/j.chemgeo.2025.123156","url":null,"abstract":"<div><div>Ediacara-style fossils represent Earth's earliest ecosystems of complex macroscopic organisms, preserved as three-dimensional casts and molds within sandstone beds. Differences in Ge/Si ratios between grains and cements in fossiliferous sandstones from the Ediacara Member of South Australia have been used to support early silica cementation as a key factor in moldic preservation. However, experimental studies tracking Ge/Si partitioning during decay-associated silicification are lacking. Here, we employ an experimental framework to further investigate the utility of Ge/Si ratios as a fossilization proxy. We conducted fossilization experiments using marine invertebrate animals, aquatic plants, and silica-rich artificial seawater to track the fate of Ge throughout the early stages of tissue silicification. We found that experimentally precipitated amorphous silica associated with decaying organisms is characterized by Ge/Si values of 0.25–2.07 μmol mol⁻¹, and displayed an overall increase during each experiment, concurrent with Ge incorporation into silica precipitating onto the experimental carcasses. These results help build a framework for using Ge/Si ratios to track fossilization processes and highlight the role of early silica precipitation in preserving soft tissues. Comparison of our experimental data with Ge/Si signatures from Ediacara fossiliferous sandstones improves our understanding of early diagenetic silicification and enables more robust reconstructions of the mechanisms responsible for the preservation of Earth's earliest animal communities.</div></div>","PeriodicalId":9847,"journal":{"name":"Chemical Geology","volume":"699 ","pages":"Article 123156"},"PeriodicalIF":3.6,"publicationDate":"2025-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145575318","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}