 | Supra-subduction zone ophiolites retain hydrogen generation potential after 300 Myr Abstract: The Ural Mountains host well documented natural hydrogen (H2) emissions, yet the Palaeozoic ultramafic complexes are 250–400 Ma old and could have exhausted serpentinisation potential through progressive alteration over geological time. We revisit overlooked Soviet observations from the Kempirsay chromitite district and compare them with recent data from the younger, Jurassic Bulqizë ophiolite in Albania. In both massifs, H2-rich seeps (>80–90 vol. % H2) occur within ∼300 m of podiform chromitite bodies. At Kempirsay, degassing takes place at low temperatures (14–30 °C), and experiments on Kempirsay rocks show that Fe-bearing minerals can generate H2 at near-ambient conditions. New radiocarbon data from Bulqizë methane (apparent age ∼26 ka) demonstrate that associated CH4-H2 inventories are renewed on 104 year time scales. We interpret chromitite bodies and their damage zones as catalytic and hydraulic hubs embedded in a supra-subduction zone ophiolitic architecture that localises serpentinisation and preserves reactive peridotite. This chromitite centred architecture implies that chromitite-bearing mantle slabs can retain hydrogen generation potential for hundreds of millions of years and constitute priority targets for natural hydrogen exploration and stimulated geological hydrogen production. |
 | Water driven iodine degassing from basaltic volcanic systems Abstract: Iodine is a trace constituent of magmas and its volcanic release and their atmospheric implications remain poorly quantified. To better understand and quantify the iodine contribution of volcanic degassing to the atmosphere, we have experimentally monitored iodine degassing from natural basaltic melts in real time at magmatic pressure and temperature for different situations: hydrous systems relevant for a subduction zone context and anhydrous systems relevant for rifts and intraplate volcanisms. We show that for anhydrous systems, when CO2 is the major volatile phase, iodine degassing is not significant whereas for hydrous systems, when water is the major volatile phase, iodine degassing is confirmed to be significant. Together with previous results, we show that the degassing of iodine and bromine is water driven regardless of the composition of the melt (haplogranite or basalt). A maximal effusive volcanic flux of 1.94 Gg/yr iodine is calculated, meaning that volcanic iodine degassing is more important at subduction zones during effusive volcanism than previously thought. |
 | Modern mantle-like μ182W signatures in Paleoarchean rocks from southern India Abstract: The isotope records of short lived 182Hf-182W and 146Sm-142Nd decay systems preserved in Archean crustal rocks that formed from different mantle domains, provide temporal constraints on chemical differentiation processes on Earth during the Hadean eon (>4.0 Ga). These early differentiation events provide potential insights on the geodynamic environment prevailing during the formation of the first continents. We present a μ182W-stable W isotope data set for felsic and mafic-komatiitic suites (∼3.4–3.0 Ga) from the western Dharwar Craton in India previously analysed for their μ142Nd. As one of the oldest cratonic areas on Earth, it preserves key evidence for an incomplete convection and mixing of the Hadean mantle. The results reveal μ182W values resembling that of the modern Earth’s upper mantle. In contrast, their anomalous 142Nd isotope compositions requires Sm/Nd fractionation before 4.0 Ga. This implies that the mantle source differentiated only after 182Hf became extinct (<∼4.50 Ga) or was homogenised before silicate differentiation (∼4.38 Ga) with no anomalous contributions from the late veneer or the core. Compared to other Archean cratons, this further confirms an isotopically heterogeneous mantle early in Earth history. |
 | Temperature dependence of magmatic iron redox speciation to 2100 °C Abstract: The iron redox speciation of silicate melts, quantified by Fe3+/Fe2+ ratios, is of significance to redox evolution during magmatism and is temperature dependent, but exploration of temperature effects has been limited by the narrow range of investigable super-liquidus conditions accessible in gas mixing furnaces (∼1200–1600 °C). This study combines gas mixing and aerodynamic-laser-levitation furnace experiments to quench glasses from a single martian basaltic melt composition at fixed fO2 and 100 kPa from 1250–2100 °C. This 850 °C span allows improved quantification of temperature dependence on Fe3+/Fe2+ ratios. Fe3+/Fe2+ ratios, measured by XANES, are used to calibrate thermodynamic parameterisations. Results indicate that models calibrated from smaller intervals have variable success quantifying the influence of temperature on magmatic Fe3+/Fe2+ ratios. A survey of experimental results suggests that the temperature dependence of Fe3+/Fe2+ does not vary appreciably with melt composition. The new data improve constraints on iron redox systematics during igneous differentiation of martian magmas by quantifying the effect of cooling on magma Fe3+/Fe2+ and, in extrapolation, refine predictions of Fe3+/Fe2+ ratios established in magma oceans at extreme temperatures, by constraining the temperature dependence of Fe3+/Fe2+ at a fixed fO2 relative to melt-alloy equilibrium. |
 | K, Rb, Ge, and Cu isotopes in Oued Chebeika 002 compared with CI chondrites, Ryugu, and Bennu Abstract: The primordial isotopic compositions of moderately volatile elements in CI chondrites may be obscured by aqueous alteration on their parent bodies. Here we report K, Rb, Ge, and Cu isotopic compositions of Oued Chebeika 002 (OC002), a CI chondrite recovered shortly after its fall and thus minimally affected by terrestrial weathering. OC002 exhibits light δ41/39K (−0.291 ± 0.043 ‰), comparable to Bennu and lighter than other CI chondrites and Ryugu, and the lightest δ87/85Rb (+0.058 ± 0.036 ‰) yet measured in a CI chondrite. Meanwhile, its δ74/70Ge (+0.980 ± 0.042 ‰) and δ65/63Cu (+0.197 ± 0.020 ‰) values fall within the CI-Ryugu field. This element specific pattern − K and Rb variable, Ge and Cu uniform − indicates that parent body aqueous alteration selectively fractionated fluid mobile alkalis whereas Ge and Cu isotopes remained largely insensitive. The range of K and Rb isotopic composition in CI chondrites spans nearly the total variation observed across carbonaceous chondrite groups, implying that δ41/39K and δ87/85Rb of the CI end member cannot be uniquely defined. This introduces significant uncertainty when using CI chondrites as the matrix end member to constrain chondrule isotopic compositions for fluid mobile elements. More broadly, our results demonstrate that “pristine” with respect to terrestrial weathering does not equal “primitive” with respect to parent body processing. |
 | Expanding Mars’ lithologic diversity: discovery of a garnet-bearing clast in NWA 8171 Abstract: A garnet-bearing clast has been identified in martian breccia meteorite NWA 8171, comprising two distinct domains: an andradite-diopside domain, and a K-feldspar-augite domain. Similar assemblages occur in terrestrial metamorphic/metasomatic settings like skarns, in alkali igneous rocks; and as secondary phases in carbonaceous chondrites. Mineralogical and textural analyses of the clast reveals a complex history, possibly reflecting multiple crystallisation stages and/or alteration events on Mars. However, as NWA 8171 is a regolith breccia, we also consider if the garnet-bearing clast is extra-martian in origin. Analysis of pyroxene Mn/Fe ratios indicate that augites from the K-feldspar-rich domain match martian values, while diopsides in the andradite-bearing domain are more varied in composition. This variability, together with similarities to metasomatic assemblages from chondritic and terrestrial analogues, suggests that the andradite-rich domain may not comprise primary igneous minerals. This could indicate the clast was altered on Mars in an oxidising metasomatic event, although an extra-martian origin cannot be ruled out. Still, the first identification of garnet in a martian meteorite has major implications. The andradite-bearing clast in NWA 8171 may be the first sample of a garnet-bearing lithology from Mars, representing a previously unidentified martian magma source, alteration process, regolith impactor component, or metamorphic event. |
 | Plagioclase as a magmatic nitrogen reservoir in reduced planetary crusts Abstract: Plagioclase in Earth’s continental crust holds significant nitrogen (N), yet its potential to act as a major N reservoir in other planetary crusts remains unexplored. Here I experimentally determine plagioclase-silicate melt N partitioning at the saturation of N2-rich gas and at pressure, temperature, and redox conditions relevant to reduced crust differentiation. Plagioclase incorporates substantial N (380–2200 μg/g), with concentrations increasing at higher pressure and lower oxygen fugacity (IW−0.7 to IW−1.8). The plagioclase melt N partition coefficients increase from 0.15 to 0.50 as oxygen fugacity decreases, demonstrating that N becomes progressively more lithophile under reducing conditions. These results suggest that early reduced crustal differentiation on stagnant lid bodies such as the Moon and Mars could have established long lived magmatic N reservoirs through plagioclase crystallisation and accumulation. In contrast, Earth’s more oxidised and plate tectonic regime promotes continual redistribution of N between crust, mantle, and atmosphere, resulting in limited retention of magmatic N in the continental crust. |
 | Ikaite precipitation indicates near surface occurrence of methane in an Icelandic fjord Abstract: Climate change driven release of methane (CH4) from polar and sub-polar sediments could accelerate global warming, and tracking CH4 in cold sediments over geological time helps predict future releases. Isotopic signatures of ikaite (CaCO3·6H2O) and its pseudomorph, glendonite, may be used to identify past CH4 in cold environments, as alkalinity (AT) from anaerobic oxidation of methane (AOM) can induce precipitation of this mineral at low temperatures. However, the suitability of ikaite as a proxy for CH4 near the sediment surface remains uncertain, as ikaite linked to modern seeps has only been retrieved from sediment depths of several metres. We report ikaite crystals in surface sediments (0–40 cm depth) in Reyðarfjörður, Iceland. High AT fluxes from deeper sediment layers and low stable carbon isotope (δ13C) values of the ikaite (−49.8 to –53.8 ‰) suggest formation from AOM, while sub-bottom profiling indicates shallow gas below the sampling site. As such, the recovered ikaite provides indirect evidence that CH4 locally reaches shallow sediment layers in the studied fjord, considerably expanding the environmental range of CH4-derived ikaite and substantiating ikaite and glendonite as proxies for cold environment CH4 seeps. |
 | Trace element partitioning in sedimentary pyrite controlled by nanoscale processes Abstract: The trace element (TE) composition of sedimentary pyrite is widely used as a palaeo-proxy for Earth environments, raising a need to better estimate the pyrite-water TE partitioning during sedimentary pyrite formation. By monitoring TE incorporation into pyrite grown in laboratory experiments at ambient temperature, we determined transfer functions that link the TE signature of early diagenetic pyrites formed under anoxic, Fe-rich conditions to the initial TE concentrations of the corresponding precipitating aqueous medium, relevant to sediment porewater. Synchrotron based X-ray fluorescence mapping at the nanometre scale and correlation plots reveal that TE association modes with pyrite progressively deviate from ideal solid solution from Se to Ni, As, and Co, while Cu and Zn display exsolution behaviour. Nanoscale distributions modulate the solid-solution distribution coefficients with a significant dependence on the total TE:Fe ratio of the pyrite precipitation medium. Hence, we provide revised average concentration factors and a ranking of distribution coefficients: Se > Co ≥ Cu ≥ Ni ≥ As ≥ Zn ≥ Mn. These improved estimates, particularly for Co, Mn and Zn partitioning in sedimentary pyrite, offer refined constraints for palaeoenvironmental reconstructions and Earth oxygenation studies. |
 | Geochemical constraints on the use of an amorphous precursor phase by corals Abstract: A growing body of evidence suggests that corals, like many other marine calcifying organisms, may form at least parts of their aragonite skeleton via an amorphous calcium carbonate (ACC) or other metastable precursor phase. This is in apparent conflict with our understanding of coral skeletal chemistry, in that most trace element systems can be modelled with just minor modifications to the inorganic aragonite distribution coefficients (e.g., via Rayleigh fractionation), whereas ACC, and crystalline CaCO3 derived from ACC, has a vastly different chemistry. Via a simple geochemical model, we show that these observations may be reconciled provided either: i) ACC undergoes near complete dissolution and exchange with the calcifying fluid during crystallisation, and/or ii) skeletogenesis via this pathway is of overall minor importance. Comparing the chemical composition of different skeletal components additionally demonstrates that differential degrees of ACC utilisation is unlikely to be the main cause of well documented micro-scale chemical heterogeneity. While important processes remain poorly constrained, in particular the mode and chemical dynamics of ACC crystallisation in biological systems in seawater, our results highlight how biomineral geochemistry can place important constraints on the role of ACC in forming biominerals. |
<< Previous issueNext issue >>
