The solubility of molecular hydrogen in silicate melts and the origin of hydrogen in the interiors of terrestrial planets

Experimental determination of N2 solubility in silicate melts and implications for N2–Ar–CO2 fractionation in magmas

Solubility of molecular hydrogen in silicate melts and consequences for volatile evolution of terrestrial planets

The solubility of N2 in basaltic (MORB) and haplogranitic melts was studied at oxidizing conditions (oxygen fugacity about two log units above the Ni–NiO buffer). Under these conditions, N2 is expected to be the only significant nitrogen species present in the melt. Experiments were carried out from 0.1 to 2 GPa and 1000–1450 ˚C using either an externally heated TZM pressure vessel, an internally heated gas pressure vessel or a piston cylinder apparatus. Nitrogen contents in run product glasses were quantified by SIMS (secondary ion mass spectrometry). To discriminate against atmospheric contamination, 15N-enriched AgN3 was used as the nitrogen source in the experiments. According to infrared and Raman spectra, run product glasses contain N2 as their only dissolved nitrogen species. Due to interactions with the matrix, the N2 molecule becomes slightly infrared active. Nitrogen solubility in the melts increases linearly with pressure over the entire range studied; the effect of temperature on solubility is small. The data may, therefore, be described by simple Henry constants Khaplogranite = (1461 ± 26) ppm N2/GPa and KMORB = (449 ± 21) ppm N2/GPa, recalculated to ppm by weight (μg/g) of isotopically normal samples. These equations describe the solubility of nitrogen during MORB generation and during melting in the crust, as we show by thermodynamic analysis that N2 is the only significant nitrogen species in these environments. Nitrogen solubility in the haplogranitic melt is about three times larger than for the MORB melt, as is expected from ionic porosity considerations. If expressed on a molar basis, nitrogen solubility is significantly lower than argon solubility, in accordance with the larger size of the N2 molecule. Notably, N2 solubility in felsic melts is also much lower than CO2 solubility, even on a molar basis. This implies that the exsolution of nitrogen may drive vapor oversaturation in felsic melts derived from nitrogen-rich sediments. We also measured the partitioning of nitrogen between olivine, pyroxenes, plagioclase, garnet, and basaltic melt by slowly cooling MORB melts to sub-liquidus temperatures to grow large crystals. The mineral/melt partition coefficients of nitrogen range from 0.001 to 0.002, and are similar to argon partition coefficients. These new data, therefore, support the assumption that there is little fractionation between nitrogen and argon during mantle melting and that the N2/Ar ratio in basalts and xenoliths is, therefore, representative of the N2/Ar ratio in the mantle source. This assumption is essential for assessing the size of the nitrogen reservoir in the mantle. Our data also show that for an upper mantle oxidation state that is similar to the one observed today, nitrogen outgassing by partial melting is extremely efficient and even low melt fractions in the range of a few percent may extract nearly all nitrogen from the mantle source.

Oxygen fugacity and melt composition controls on nitrogen solubility in silicate melts

Noble gas (He, Ne, and Ar) solubilities in high-pressure silicate melts calculated based on deep-potential modeling

Oxygen fugacity (fO2) is typically controlled in high P-T experiments by using solid-state redox buffer assemblages. However, these are restricted to impose discrete fO2 values, often with significant gaps between neighboring assemblages. Semi-permeable hydrogen membranes (Shaw 1963) are often used in internally heated pressure vessels for more flexible fO2 control in hydrous experiments; however, their implementation in more widely available externally heated pressure vessels has not yet gained space. We propose a prototype molybdenum-hafnium carbide (MHC) pressure vessel apparatus that simultaneously allows rapid quenching and flexible, precise, and accurate redox control via a custom-designed hydrogen membrane. Test runs with two membranes at a time, one imposing and another one monitoring fH2, demonstrated that 95% of the imposed hydrogen pressure was attained inside the pressure vessel within 2 h at 800–1000 °C, after which a steady state equilibrium was established. Furthermore, experiments comparing redox-dependent Cu solubility in silicate melts at fO2 imposed by the fayalite-magnetite-quartz, Re-ReO2, and MnO-Mn2O3 buffers and identical target fO2 imposed by the hydrogen membrane confirmed consistency between the two methods within 0.25 log units fO2 deviation at T = 900 °C and P = 2000 bar. This powerful yet cost-effective and low-maintenance apparatus may open up new pathways for studying redox reactions in hydrous magmas and magmatic fluids. As a proof of concept, we conducted near-liquidus phase-equilibrium experiments with H2O-saturated calc-alkaline basalt and shoshonite melt compositions at five different fO2 values equally distributed between half log unit below the Ni-NiO buffer (NNO-0.5) and NNO+2.7. Most experiments crystallized olivine, clinopyroxene, and Ti-magnetite. The Mg# of the olivine increased with fO2, and the Fe3+/Fetotal ratios in the silicate melt were determined based on Fe(II)-Mg exchange between olivine and melt. The Fe3+/Fetotal ratios in the shoshonite melt were systematically higher by about 0.06 ± 0.01 than those in the calc alkaline basalt melt at identical fO2. The values determined for the basaltic melt were consistent within 1σ error (<0.033 deviation) from those predicted by the equation of Kress and Carmichael (1991). The Fe-Ti exchange coefficient between magnetite and silicate melt increases from 1.73 ± 0.19 (1σ) at NNO –0.5 to +7.12 ± 0.36 at NNO+2.7 for shoshonite and has a similar range for the calc-alkaline basalt.

A striking feature of Mercury's volcanic surface is its high S and low FeO contents, which is thought to be produced by very reducing conditions compared to other terrestrial bodies. Experiments show that S solubility in silicate melts increases to % wt levels for oxygen fugacities lower than three log units below the iron‐wustite (IW) buffer. During magma ocean solidification, large amounts of sulfide could potentially precipitate. This work investigates the effects of primordial sulfide layering on the first 750 Myr of Mercury's mantle dynamics. It is proposed that sulfide layering could have been produced by fractional solidification in the highly reduced Mercury magma ocean (MMO). Such chemical layering implies mantle sources with variable sulfur contents that might have played an important role in early Mercurian magmatism. Our models investigate the production of sulfide‐rich layers and their preservation during post‐MO solid‐state mantle dynamics. An intriguing question is the role of these sulfide‐rich layers on mantle dynamics as they are expected to incorporate a substantial amount of heat‐producing elements (U, Th, and K). We use experimentally determined sulfur solubility in silicate melts to predict the depth at which sulfides precipitate in the MMO. The model produces primordial sulfide layers whose thickness and locations depend upon the oxygen fugacity (fO2) and initial sulfur content (Sinit) of the MMO. Several geodynamic regimes have been identified in the fO2‐Sinit space. This study shows that oxygen fugacity, bulk sulfur content, and sulfide segregation are key for the early thermochemical evolution of Mercury.

The effect of compression on noble gas solubility in silicate melts and consequences for degassing at mid-ocean ridges

Carbon solubility in silicate melts in equilibrium with a CO-CO2 gas phase and graphite

Noble gas solubilities in silicate melts: New experimental results and a comprehensive model of the effects of liquid composition, temperature and pressure

Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (µ-XANES) to obtain their Fe3+/ΣFe profiles near the graphite–melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3–1.9 mm thickness. Even for a “time-zero” experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 µm in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 °C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/ΣFe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/ΣFe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at ~ 1300 °C and ~ 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO −0.9, which agrees well with the measured Fe3+/ΣFe ratios in the basaltic glasses and the estimated oxygen fugacity for graphite and Pt–graphite capsule experiments from a previous study. Future studies are needed to better constrain the kinetics of this process at different temperature, pressure, and in silicate melts of different compositions. The dynamic process of H2 diffusing and reducing Fe3+ to Fe2+ shown in our experiments also provides a potential way to determine the diffusivity and solubility of molecular hydrogen in silicate melts, which are crucial in understanding volatile behaviors on reducing planetary bodies, such as the Moon and Mercury.

Rutile solubility and TiO2 activity in silicate melts: An experimental study

Even though platinum group elements (PGE) solubilities are measured relative to pure metals, the PGE are assumed to dissolve as oxide complexes in silicate melts. PGE-oxide phases are, however, not known in magmatic rocks; in many cases PGE are associated with discrete magmatic phases (alloys, arsenides, bismuthotellurides, antimonides and sulfides). Here, we determine the concentrations of Pt, Pd, S, As, Se and Te in basaltic melts saturated with Fe, Pt or Pd sulfides, arsenides, selenides and tellurides and note that the solubilities of these elements are largely variable and depend on the metal–ligand reservoir in equilibrium. We equilibrated basaltic melts with immiscible Fe, Pt, and Pd sulfide, arsenide, selenide and telluride melts in a piston cylinder apparatus at 1250 °C, 0.5 GPa and relative fO2 of ~ FMQ to FMQ-1.5. The concentrations of S, As, Se and Te in the basaltic melt vary considerably with the metal–ligand reservoir; the highest concentrations are recorded when the ferrous iron cation is the principal metal ligand. When instead Pt-(S/As/Se/Te) or Pd-(S/As/Se/Te) are used, the concentrations of S, As, Se and Te fall drastically. Platinum and Pd increase the activities of semimetals and chalcogenes in the silicate melt more than Fe does. Implications are that Pt and Pd can preferentially form stable associations (fundamental building blocks) with chalcogens and semimetals before the melt attains saturation in Fe-chalcogens or Fe-semimetals. Estimated concentrations of Pt–ligand and Pd–ligand required to saturate silicate melts in some Pt–ligand and Pd–ligand minerals are close to their abundances in the parent magmas of some layered intrusions.

Sulfide-silicate melt partitioning controls the behavior of gold in magmas, which is critical for understanding the Earth's deep gold cycle and formation of gold deposits. However, the mechanisms that control the sulfide-silicate melt partitioning of gold remain largely unknown. Here we present constraints from laboratory experiments on the partition coefficient of gold between monosulfide-solid-solution (MSS) and silicate melt (DAuMSS/SM) under conditions relevant for magmatism at subduction settings. Thirty-five experiments were performed in Au capsules to determine DAuMSS/SM at 950-1050°C, 0.5-3 GPa, oxygen fugacity (fO2) of ∼FMQ-1.7 to FMQ+2.7 (FMQ refers to the fayalite-magnetite-quartz buffer), and sulfur fugacity (fS2) of −2.2 to 2.1, using a piston cylinder apparatus. The silicate melt composition changes from dry to hydrous andesite to rhyolite. The results obtained from electron microprobe and laser-ablation ICP-MS analyses show that the gold solubility in silicate melts ranges from 0.01 to 55.3 ppm and is strongly correlated with the melt sulfur content [S]melt at fO2 of ∼FMQ-1.7 to FMQ+1.6, which can be explained by the formation of complex Au-S species in the silicate melts. The gold solubility in MSS ranges from 130 to 2800 ppm, which is mainly controlled by fS2. DAuMSS/SM ranges from 10 to 14000 at fO2 of ∼FMQ-1.7 to FMQ+1.6, the large variation of which can be fully explained by combined [S]melt and fS2. Therefore, all of the parameters that can directly affect [S]melt and fS2, such as alkali metals, water, FeO, and fO2, can indirectly affect DAuMSS/SM. The mechanisms that control the sulfide-silicate melt partitioning of gold and the other chalcophile elements, such as Ni, Re, and Mo, differ significantly. This is because gold is dissolved mainly as Au-S species in the silicate melts, while the other chalcophile elements are dissolved mainly as metal oxides in the silicate melts. Applying the correlation between DAuMSS/SM and [S]melt to slab melting and arc magmatic differentiation under different redox conditions, we find that ancient to modern slab melts carry negligible to less than 25% of the slab gold to the subarc mantle; however, gold-enrichment can occur in MSS-saturated arc magmas that have differentiated under moderately oxidized conditions with fO2 between FMQ and FMQ+1.6, in particular if the magmatic crystallization follows a fractional crystallization model. We conclude that moderately oxidized magmas with high contents of alkali metals, sulfur, and water, owing to their low DAuMSS/SM and efficient magma-to-fluid transfer of gold and sulfur, have a high potential to form gold deposits.

Zircon saturation in terrestrial basaltic melts and its geological implications