Rare Earth elements (REE) are gaining importance due to their increasing industrial applications and usefulness as petrogenetic indicators. REE-sulfate complexes are some of the most stable REE aqueous species in hydrothermal fluids, and may be responsible for REE transport and deposition in a wide variety of geological environments, ranging from sedimentary basins to magmatic hydrothermal settings. However, the thermodynamic properties of most REE-sulfate complexes are derived from extrapolation of ambient temperature data, since direct information on REE-sulfate complexing under hydrothermal conditions is only available for Nd, Sm and Er to 250 °C (Migdisov and William-Jones, 2008, 2016). We employed ab initio molecular dynamics (MD) simulations to calculate the speciation and thermodynamic properties of yttrium(III) in sulfate and sulfate-chloride solutions at temperatures and pressures up to 500 °C and 800 bar. The MD results were complemented by in situ X-Ray Absorption Spectroscopy (XAS) measurements. Both MD and XAS show that yttrium(III) sulfate complexes form and become increasingly stable with increasing temperature (≥200 °C). The MD results also suggest that mixed yttrium-sulfate-chloride complexes (that cannot be distinguished from mixtures of chloride and sulfate complexes in XAS experiments) form at ≥ 350 °C. Two structures with two different Y(III)-S distances (monodentate and bidentate) are observed for Y(III)-sulfate bonding. The formation constants, derived via thermodynamic integration, for the Y(III) mono- and di-sulfate complexes parallel the trends for those of Nd, Sm and Er determined experimentally to 250 °C. The derived formation constants were used to fit revised Helgeson-Kirkham-Flowers equation-of-state parameters that enabled calculation of formation constants for Y(SO4)+ and Y(SO4)2− over a wide range of temperatures and pressures. The presence of sulfate increases the solubility of Y(III) under specific conditions. Since the stability of sulfate is redox sensitive, Y(III) solubility becomes highly redox-sensitive, with rapid precipitation of Y minerals upon destabilisation of aqueous sulfate.
Abstract The evolution of hydrothermal fluids during metasomatic and/or hydrothermal processes is responsible for the formation of ore deposits and associated alteration. In systems with well-developed breccia and fractures, mineral reactions are largely driven by decompression boiling, fluid cooling or external fluid mixing, but in less permeable rocks, elements exchanges occur at fluid-mineral interfaces, resulting in a self-evolved fluid-mineral reaction system. However, the dynamic fluid evolution leading to large-scale (km) alteration remains poorly understood. We observed experimentally that the sequential sodic and potassic alterations associated with mineralization in large ore deposits, in particular Iron Oxide Copper Gold (IOCG) deposits, can occur via a single self-evolved, originally Na-only, hydrothermal fluid, driven by a positive feedback between equilibrium and kinetic factors. Albite formed first upon reaction of sanidine ((K,Na)AlSi3O8) with a NaCl fluid at 600˚C, 2 kbar. However, with increasing reaction time, some of the initially formed albite was in-turn replaced by K-feldspar (KAlSi3O8). Fluorine accelerated the process, resulting in nearly complete back-replacement of albite within 1 day. These experiments demonstrate that potassic alteration can be induced by Na-rich fluids, and pervasive sequential sodic and potassic alterations do not necessarily reflect near-equilibrium, externally-driven changes in fluid alkali contents.
Abstract Phosphorus is an essential element for life, and the phosphorous cycle is widely believed to be a key factor limiting the extent of Earth's biosphere and its impact on remotely detectable features of Earth's atmospheric chemistry. Continental weathering is conventionally considered to be the only source of bioavailable phosphorus to the marine biosphere, with submarine hydrothermal processes acting as a phosphorus sink. Here, we use a novel 29 Si tracer technique to demonstrate that alteration of submarine basalt under anoxic conditions leads to significant soluble phosphorus release, with an estimated ratio between phosphorus release and CO 2 consumption (∑PO 4 3− /∑CO 2 ) of 3.99 ± 1.03 µmol mmol −1 . This ratio is comparable to that of modern rivers, suggesting that submarine weathering under anoxic conditions is potentially a significant source of bioavailable phosphorus to planetary oceans and that volatile‐rich Earth‐like planets lacking exposed continents could develop robust biospheres capable of sustaining remotely detectable atmospheric biosignatures.
The HighPGibbs program is designed to calculate thermodynamic equilibrium of fluid-rock minerals and solid solutions up to depths of lithospheric mantle. It uses the Gibbs free energy minimization function of the HCh package to calculate mineral-fluid equilibrium assemblages. Chemical potentials of minerals are calculated using the equations of states included in HCh; free energy of aqueous species are calculated using the Deep Earth Water model; and activity coefficients of charged species are estimated using the Davies variant of the Debye-Hückel equation. HighPGibbs was applied to calculate nitrogen speciation in eclogite-buffered fluids from 400 to 790 °C and 30 to 54 kbar, to evaluate the mobility of nitrogen in subducting oceanic crust. Regardless of whether the protolith was altered (and oxidized) or not, N(aq) or NH(aq) are the predominant form of nitrogen in the slab-fluids at sub-arc temperatures, especially in cases of moderate or hot geotherms. Given that molecular nitrogen is highly incompatible in silicate minerals, the simulation indicates that nitrogen (as NH) in silicate minerals can be liberated during metamorphic devolatilization. The majority of nitrogen in subducting crusts can be unlocked during slab devolatilization and eventually expelled to the atmosphere via degassing of arc magmas. Therefore, oceanic crusts recycled to deep earth will be depleted in nitrogen compared to the newly formed crust at spreading centers. As a result of the long-term mantle convection, large proportions of the bulk silicate earth may have suffered nitrogen extraction via subduction, and this may account for the nitrogen enrichment in the Earth’s atmosphere.
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