Metasomatism is a source of methane on Mars

MR and SM acknowledge support from NERC standard grant (NE/PO12167/1) and UK Space Agency Aurora grant (ST/T001763/1). DAS acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Geosciences program under Award Number DE-SC0019830 as well as NSF Petrology and Geochemistry Grant Number 2032039.The abundance of inactive Martian volcanic centres suggests that early Mars was more volcanically active than today. On Earth, volcanic degassing releases climate-forcing gases such as H2O, SO2, and CO2 into the atmosphere. On Mars, the volcanic carbon is likely to be more methane-rich than on Earth because the interior is, and was, more reducing than the present-day Terrestrial upper mantle. The reports of reduced carbon associated with high-temperature minerals in Martian igneous meteorites back up this assertion. Here, we undertake irreversible reaction path models of the fluid-rock interaction to predict carbon speciation in magmatic fluids at the Martian crust-mantle boundary. We find methane is a major carbon species between 300 and 800 °C where logfO2 is set at the Fayalite = Magnetite + Quartz redox buffer reaction (FMQ). When logfO2 is below FMQ, methane is dominant across all temperatures investigated (300–800 °C). Moreover, ultramafic rocks produce more methane than mafic lithologies. The cooling of magmatic bodies leads to the release of a fluid phase, which serves as a medium within which methane is formed at high temperatures and transported. Metasomatic methane is, therefore, a source of reduced carbonaceous gases to the early Martian atmosphere and, fundamentally, for all telluric planets, moons, and exoplanets with Mars-like low logfO2 interiors.Peer reviewe

Speciation of adsorbed yttrium and rare earth elements on oxide surfaces

Abstract The distribution of yttrium and the rare earth elements (YREE) between natural waters and oxide mineral surfaces depends on adsorption reactions, which in turn depend on the specific way in which YREE are coordinated to mineral surfaces. Recent X-ray studies have established that Y 3+ is adsorbed to the rutile (1 1 0) surface as a distinctive tetranuclear species. However, the hydrolysis state of the adsorbed cation is not known from experiment. Previous surface complexation models of YREE adsorption have suggested two to four cation hydrolysis states coexisting on oxide surfaces. In the present study, we investigate the applicability of the X-ray results to rare earth elements and to several oxides in addition to rutile using the extended triple-layer surface complexation model. The reaction producing a hydrolyzed tetranuclear surface species , and Yb 3+ on rutile, hematite, alumina and silica over wide ranges of pH and ionic strength. Where adsorption data were available as a function of surface coverage for hematite and silica, an additional reaction involving a mononuclear species could be used to account for the higher surface coverages. However, it is also possible that some of the higher surface coverage data refer to surface precipitation rather than adsorption. The results of the present study provide an internally consistent basis for describing YREE adsorption which could be used to investigate more complex systems in which YREE compete both in aqueous solution and on mineral surfaces with alkaline earths and ligands such as carbonate, sulfate, chloride and organic species, in order to build a predictive adsorption model applicable to natural waters

The relationship between mantle pH and the deep nitrogen cycle

SM is grateful to the Carnegie Institution of Washington for funding this work through the bestowment of a Carnegie Postdoctoral Fellowship. SM also acknowledges the School of Earth and Environmental Science (St Andrews) for providing a start-up fund which assisted in the development of these data. DAS is grateful to grants from the Sloan Foundation through the Deep Carbon Observatory (Reservoirs and Fluxes, and Extreme Physics and Chemistry programs) and a community-building Officer Grant from the Sloan Foundation, as well as support from NSF grants EAR-1624325 and ACI-1550346, and a grant from the W.M. Keck Foundation (The Co-Evolution of the Geo- and Biosphere) Grant #10583-02 to Sverjensky. PHB’s contribution was supported in part by the NSF grant EAR-1144559 (A Petrological and N Isotope Study Of Crustal Recycling Through Time).Nitrogen is distributed throughout all terrestrial geological reservoirs (i.e., the crust, mantle, and core), which are in a constant state of disequilibrium due to metabolic factors at Earth’s surface, chemical weathering, diffusion, and deep N fluxes imposed by plate tectonics. However, the behavior of nitrogen during subduction is the subject of ongoing debate. There is a general consensus that during the crystallization of minerals from melts, monatomic nitrogen behaves like argon (highly incompatible) and ammonium behaves like potassium and rubidium (which are relatively less incompatible). Therefore, the behavior of nitrogen is fundamentally underpinned by its chemical speciation. In aqueous fluids, the controlling factor which determines if nitrogen is molecular (N2) or ammonic (inclusive of both NH4+ and NH30) is oxygen fugacity, whereas pH designates if ammonic nitrogen is NH4+ and NH30. Therefore, to address the speciation of nitrogen at high pressures and temperatures, one must also consider pH at the respective pressure–temperature conditions. To accomplish this goal we have used the Deep Earth Water Model (DEW) to calculate the activities of aqueous nitrogen from 1-5 GPa and 600-1000 °C in equilibrium with a model eclogite-facies mineral assemblage of jadeite + kyanite + quartz/coesite (metasediment), jadeite + pyrope + talc + quartz/coesite (metamorphosed mafic rocks), and carbonaceous eclogite (metamorphosed mafic rocks + elemental carbon). We then compare these data with previously published data for the speciation of aqueous nitrogen across these respective P-T conditions in equilibrium with a model peridotite mineral assemblage (Mikhail and Sverjensky, 2014). In addition, we have carried out full aqueous speciation and solubility calculations for the more complex fluids in equilibrium with jadeite + pyrope + kyanite + diamond, and for fluids in equilibrium with forsterite + enstatite + pyrope + diamond. Our results show that the pH of the fluid is controlled by mineralogy for a given pressure and temperature, and that pH can vary by several units in the pressure-temperature range of 1-5 GPa and 600-1000 °C. Our data show that increasing temperature stabilizes molecular nitrogen and increasing pressure stabilizes ammonic nitrogen. Our model also predicts a stark difference for the dominance of ammonic vs. molecular and ammonium vs. ammonia for aqueous nitrogen in equilibrium with eclogite-facies and peridotite mineralogies, and as a function of the total dissolved nitrogen in the aqueous fluid where lower N concentrations favor aqueous ammonic nitrogen stabilization and higher N concentrations favor aqueous N2. Overall, we present thermodynamic evidence for nitrogen to be reconsidered as an extremely dynamic (chameleon) element whose speciation and therefore behavior is determined by a combination of temperature, pressure, oxygen fugacity, chemical activity, and pH. We show that altering the mineralogy in equilibrium with the fluid can lead to a pH shift of up to 4 units at 5 GPa and 1000 °C. Therefore, we conclude that pH imparts a strong control on nitrogen speciation, and thus N flux, and should be considered a significant factor in high temperature geochemical modeling in the future. Finally, our modelling demonstrates that pH plays an important role in controlling speciation, and thus mass transport, of Eh-pH sensitive elements at temperatures up to at least 1000 °C.PostprintPeer reviewe

Organic Geochemistry of Earth's Upper Mantle Fluids

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Catalytic peptide hydrolysis by mineral surface: Implications for prebiotic chemistry

Abstract The abiotic polymerization of amino acids may have been important for the origin of life, as peptides may have been components of the first self-replicating systems. Though amino acid concentrations in the primitive oceans may have been too dilute for significant oligomerization to occur, mineral surface adsorption may have provided a concentration mechanism. As unactivated amino acid polymerization is thermodynamically unfavorable and kinetically slow in aqueous solution, we studied mainly the reverse reaction of polymer degradation to measure the impact of mineral surface catalysis on peptide bonds. Aqueous glycine (G), diglycine (GG), diketopiperazine (DKP), and triglycine (GGG) were reacted with minerals (calcite, hematite, montmorillonite, pyrite, rutile, or amorphous silica) in the presence of 0.05 M, pH 8.1, KHCO 3 buffer and 0.1 M NaCl as background electrolyte in a thermostatted oven at 25, 50 or 70°C. Below 70°C, reaction kinetics were too sluggish to detect catalytic activity over amenable laboratory time-scales. Minerals were not found to have measurable effects on the degradation or elongation of G, GG or DKP at 70°C in solution. At 70°C pyrite was the most catalytic mineral with detectible effects on the degradation of GGG, although several others also displayed catalytic behavior. GGG degraded $1.5-4 times faster in the presence of pyrite than in control reactions, depending on the ratio of solution concentration to mineral surface area. The rate of pyrite catalysis of GGG hydrolysis was found to be saturable, suggesting the presence of discrete catalytic sites on the mineral surface. The mineral-catalyzed degradation of GGG appears to occur via a GGG ? DKP + G mechanism, rather than via GGG ? GG + G, as in solution-phase reactions. These results are compatible with many previous findings and suggest that minerals may have assisted in peptide synthesis in certain geological settings, specifically by speeding the approach to equilibrium in environments where amino acids were already highly concentrated, but that minerals may not significantly alter the expected solution-phase equilibria. Thus the abiotic synthesis of long peptides may have required activating agents, dry heating at higher temperatures, or some form of phase separation

The importance of carbon to the formation and composition of silicates during mantle metasomatism

Funding: MR, SM and JK acknowledge support from NERC standard grant (NE/PO12167/1) and UK space agency Aurora grant (ST/T001763/1). DAS acknowledges support from NSF Grant #2032039 and DOE Grant #DE-SC0019830. R-14616 and the US NSF (EAR 1624325 and ACI 1550346).Mineral and fluid inclusions in mantle diamonds provide otherwise inaccessible information concerning the nature of mantle metasomatism and the role of fluids in the mass transfer of material through the Earth’s interior. We explore the role of the carbon concentrations during fluid-rock metasomatism in generating the ranges of garnet and clinopyroxene compositions observed in diamonds from the sub-continental lithospheric mantle. We use the Deep Earth Water model to predict the results of metasomatism between silicic, carbonatitic and peridotitic fluids with common mantle rocks (peridotites, eclogites and pyroxenites) at 5 GPa, 1000 °C, across a range of redox conditions (logfO2 = -2 to -4 ΔFMQ), and a wide range of initial carbon concentrations in the metasomatic fluids. Our results show that the predicted compositions of metasomatic garnets and clinopyroxenes are controlled by the initial geochemistry of the fluids and the rocks, with subsequent mineral-specific geochemical evolution following definable reaction pathways. Model carbon-rich, metasomatic fluids that can form diamond (initial C- content > 5.00 molality) result in Mg-rich garnets and clinopyroxenes typical of peridotitic, eclogitic, and websteritic inclusions in diamonds. However, model carbon-poor, metasomatic fluids that do not form diamond can result in Mg-poor, Ca-rich garnets and clinopyroxenes. Such garnets and clinopyroxenes can nevertheless occur as inclusions in diamonds. In our models, the abundance of carbon in the fluids controls the behaviour of the bivalent ions through the formation of aqueous Mg-Ca-Fe-C complexes which directly govern the composition of garnets and clinopyroxenes precipitated during the metasomatic processes. As the C-rich initial fluids can form the higher Mg-eclogitic, peridotitic, and websteritic inclusions in diamonds, these inclusions can be syngenetic (metasomatic) or possibly protogenetic. However, in our models, the relatively Mg-poor, Ca- and Fe-rich eclogitic garnet and clinopyroxene inclusions found in mantle diamonds formed from C-poor fluids that do not form diamonds. These inclusions most likely reflect a metasomatic event prior to being incorporated into their host diamonds, or they could represent protolith-based protogenetic geochemistry. Therefore, the paragenetic groups used to classify diamonds should not be considered a genetic classification, as the role of the fluid geochemistry appears to be more important than the one played by host rock geochemistry.Publisher PDFPeer reviewe

Interactions of ribonucleotides with aluminium and iron oxides under varying environmental conditions

International audienc

Adsorption of ribonucleotides onto aluminum and iron oxides

International audienceMineral surfaces are known to adsorb organic molecules such as nucleic acids [1,2]. They might have concentrated the building blocks of biomolecules in the context of the origin of life, facilitating their polymerization. They also protect them from degradation [3,4] contributing to an extracellular genetic pool used by microorganisms in soils for horizontal gene transfers [5]. Previous work has highlighted the predominant role of the edges of mineral particles in the adsorption of nucleotides [6], implying oxide-like adsorption sites. Here we further investigate the interactions of ribonucleotides with alumina and hematite, as a function of pH, ionic strength and ligand-to-solid ratio. Batch adsorption experiments and surface complexation calculations using the Extended Triple Layer Model allow us to predict the speciation of the surface species, the stoichiometry and thermodynamic equilibrium constants for the adsorption of nucleotides. Both oxides lead to high values of adsorption of nucleotides (> 2 mol/m2). However, at high pH, hematite nanoparticles present a significantly higher adsorption compared to alumina. On alumina surfaces we propose the formation of a monodentate inner-sphere complex at low pH, and a bidentate outer-sphere complex at higher pH, both involving the negatively charged phosphate group [8]. This pH-dependency might have implications for the availability of nucleotides both in the context of the origin of life for polymerization and in modern soils for transfectio