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 mu g/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.
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.
The Ural Mountains host well documented natural hydrogen (H-2) 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 Bulqiz & euml; ophiolite in Albania. In both massifs, H-2-rich seeps (>80-90 vol. % H-2) occur within similar to 300 m of podiform chromitite bodies. At Kempirsay, degassing takes place at low temperatures (14-30 degrees C), and experiments on Kempirsay rocks show that Fe-bearing minerals can generate H-2 at near-ambient conditions. New radiocarbon data from Bulqiz & euml; methane (apparent age similar to 26 ka) demonstrate that associated CH4-H-2 inventories are renewed on 10(4) 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.
A garnet-bearing clast has been identified in martian breccia meteorite NWA 8171, comprising two distinct domains: an andradite-diopside domain, and a K-feldsparaugite 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 andraditerich 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.
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 & centerdot;6H2O) and its pseudomorph, glendonite, maybe 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 Rey & eth;arfj & ouml;r & eth;ur, Iceland. High AT fluxes from deeper sediment layers and low stable carbon isotope (613C) values of the ikaite (-49.8 to-53.8 parts per thousand) 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.
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 s41/39K (-0.291 +/- 0.043 %o), comparable to Bennu and lighter than other CI chondrites and Ryugu, and the lightest s87/85Rb (+0.058 +/- 0.036 %o) yet measured in a CI chondrite. Meanwhile, its s74/70Ge (+0.980 +/- 0.042 %o) and s65/63Cu (+0.197 +/- 0.020 %o) 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 s41/39K and s87/85Rb 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.
The isotope records of short lived Hf-182-W-182 and Sm-146-Nd-142 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 mu W-182-stable W isotope data set for felsic and mafic-komatiitic suites (similar to 3.4-3.0 Ga) from the western Dharwar Craton in India previously analysed for their mu Nd-142. 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 mu W-182 values resembling that of the modern Earth's upper mantle. In contrast, their anomalous Nd-142 isotope compositions requires Sm/Nd fractionation before 4.0 Ga. This implies that the mantle source differentiated only after Hf-182 became extinct (
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.
The trace element (TE) composition of sedimentary pyrite is widely used as a palaeoproxy 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.
Zoning and speciation of arsenic (As) in pyrite are used to infer changes in the chemical composition of hydrothermal fluids and conditions of ore deposit formation. Yet, the processes controlling the distribution and oxidation state of As during pyrite formation are poorly understood. We report the results of experiments designed to test the capacity of pyrite to record changes in fluid composition under dynamic reaction conditions. When pyrite seeds were exposed alternately to As-bearing and As-free fluids, concentric As-rich (<= 6.0 wt. % of As1-) and As-free pyrite overgrowths formed when native S was used as the sulfur source. In contrast, a sodium thiosulfate source produced randomly oriented aggregates of concentrically zoned microparticulate (similar to 1 mu m) As-bearing pyrite (<1.5 wt. % of As2+/3+), failing to record the changes of fluid composition. We demonstrate that by controlling H2S(aq) availability, the source of sulfur affects the degree of pyrite supersaturation under conditions relevant to natural hydrothermal systems, which controls the nucleation rate, crystal growth, As uptake, As oxidation state, and consequently, the ability of pyrite to record individual fluid pulses. This sulfur source effect has significant implications for metal incorporation into pyrite and understanding of the formation of many ore deposits.
It remains unclear how Earth's continental crust became deficient in Cu. Lower crustal cumulates from the Gangdese continental arc present key evidence. These cumulates are Cu-rich and exhibit significant delta 65Cu variability (-2.09 %o to & thorn;0.51 %o), attributed primarily to the fractionation involving Monosulfide Solid Solution (MSS). Combined with the systematic Cu depletion and positive delta 65Cu signatures of the Gangdese differentiated arc magmas and the bulk continental crust, we propose that widespread MSS segregation occurs at the base of continental arcs. This process distinguishes continental arcs from island arcs: in the former, higher pressures promote earlier sulfide saturation, and MSS dominated segregation slows the rate of Cu depletion during differentiation. Ultimately, the continuous foundering of these sulfide-rich mafic cumulates drives the continental crust toward a silica-rich and Cu depleted bulk composition.
Whole rock Nd isotopes are commonly used to assess mantle-crust contributions in magma sources and to constrain the timing of magmatic and metamorphic events. However, such measurements provide limited insights into deep crustal roots, where complex processes (e.g., open system melting, magma hybridisation) may occur. Here, we combine whole rock and in situ Sm-Nd isotopic analyses across a 25-30 km thick crustal section in Calabria (Italy). This section exposes lower crustal granulites and migmatites overlain by mid-crustal post-collisional granitoids, forming a 13 km thick batholith. The lower crust is strongly heterogeneous (whole rock epsilon Nd(i) = -10.5 to & thorn;1.7) with isotopic variability evident from outcrop to grain scale. By contrast, the mid-crustal igneous rocks display remarkable homogeneity with consistent crustal signatures (epsilon Nd(i) approximate to -7). Our results indicate efficient isotopic homogenisation from a 1-2 km thick transition zone at the lower-middle crust boundary, where hybridisation between mafic and felsic magmas is evidenced at the grain scale using Sm-Nd isotopic analyses. A minor mantle contribution was likely involved in the batholith genesis but largely obscured by processes like crustal assimilation and cannot be resolved in the granitoids using the Sm-Nd system.
The compositional variability of mid-ocean ridge basalts (MORBs) stems from a combination of the heterogeneity of the mantle source, magma mixing, and partial crystallisation of magma in the lower crust. These variations have been mainly explored at the global and ridge scales, with only a few studies investigating them at the kilometre scale and below. Here, we focus on a series of tholeiitic to K-rich basalt samples collected during a single submarine dive near the eastern intersection between the Mid-Atlantic Ridge and the Romanche transform fault, in the equatorial Atlantic. The geochemical and petrological variations, attributed to variations in melting conditions, are extreme and consistent with the geological features. Clinopyroxene phenocrysts present in certain K-rich basalts, recorded the history of magma storage. We calculated the crystallisation pressures of clinopyroxenes and compared them to microseismicity depths recorded in the area, thus providing constraints on the lithospheric structure. This multidisciplinary work highlights the interest in exploring the composition of MORBs at high resolution to better understand the construction of the oceanic crust.
The light calcium (Ca) isotope anomaly observed in ocean island basalts (OIBs) compared to mid-ocean ridge lavas has typically been attributed to recycling of carbonate-bearing sediments, partial melting of garnet-rich lithologies or a combination of both. This study presents Ca isotopic data for lavas from Pitcairn Island and nearby seamounts, yielding a s44/42Ca variation (0.30 +/- 0.02 %o to 0.40 +/- 0.02 %o) similar to that found in global OIBs. This s44/42Ca variation cannot be attributed to post-eruption alteration, magmatic differentiation or recycling of carbonate-bearing sediments. Instead, correlations of s44/42Ca with Sr/Nd and Eu/Eu* suggest that the light Ca isotope anomaly in Pitcairn lavas most likely reflects derivation from a pyroxenite source akin to the lower part of the recycled oceanic crust. Modelling suggests that the low s44/42Ca end member can be explained by multiple-stage pyroxenite melting without requiring recycled carbonate-bearing sediments, offering new insights into using Ca isotopes to trace crust-mantle interactions.
Arc magmas have long been considered significantly more oxidised than their ocean island and mid-ocean ridge counterparts, a characteristic widely attributed to infusion of the mantle wedge by fluids from subducted lithologies. However, here we show that at comparable degree of differentiation and sulfur content, arc magmas have comparable oxidation state to ocean island magmas. Our study is based on measurements of Fe3+/& sum;Fe along with major and volatile elements in olivine and plagioclase hosted melt inclusions and matrix glasses from eleven volcanic systems located in arc settings worldwide. Accounting for fractional crystallisation (to MgO = 6 wt. %) we find that all systems lie on a reducing trend accompanying sulfur degassing, from QFM +0.9 (+/- 0.2, 1 sigma) when S > 2000 ppm to QFM -0.2 (+/- 0.6, 1 sigma) when S < 100 ppm (where QFM stands for the Quartz-Fayalite-Magnetite buffer). These findings reconcile the observed discrepancy between the oxidation states of xenoliths in arc magmas and gas emissions from arc volcanoes. We further show that fractional crystallisation influences the redox evolution of arc magmas to a comparable extent as, and sometimes counteracting, sulfur degassing.
