Sulfur saturation limits in silicate melts and their implications for core formation scenarios for terrestrial planets

Holzheid, Astrid; Grove, T. L.

doi:10.2138/am-2002-2-304

Cited by 180 publications

(115 citation statements)

References 28 publications

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“…The authors explain these modeling predictions by compositional and pressure effects on the activities of the sulfide and sulfate species dissolved in the melt. In addition, this hypothesis is also consistent with the fact that the SCSS (Sulfur Concentration at Sulfide Saturation) and the SCAS (Sulfur Concentration at Anhydrite Saturation) decrease with increasing degree of melt polymerization (Holzheid and Grove, 2002;Yang, 2012;Zajacz et al, 2013;Zajacz, 2015). Therefore, the respective dataset of Botcharnikov et al (2011) and Jégo and Pichavant (2012) seem to be mutually consistent to each other.…”

Section: Speciation and Dissolution Of Sulfursupporting

confidence: 80%

Is gold solubility subject to pressure variations in ascending arc magmas?

Jégo

Nakamura

Kimura

et al. 2016

Geochimica et Cosmochimica Acta

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Magmas play a key role in the genesis of epithermal and porphyry ore deposits, notably by providing the bulk of ore metals to the hydrothermal fluid phase. It has been long shown that the formation of major deposits requires a multi-stage process, including the concentration of metals in silicate melts at depth and their transfer into the exsolved ore fluid in more superficial environments. Both aspects have been intensively studied for most of noble metals in subsurface conditions, whereas the effect of pressure on the concentration (i.e., solubility) of those metals in magmas ascending from the sublithospheric mantle to the shallow arc crust has been quite neglected. Here, we present new experimental data aiming to constrain the processes of gold (Au) dissolution in subduction-linked magmas along a range of depth. We have conducted hydrous melting experiments on two dacitic/adakitic magmas at 0.9 and 1.4GPa and ~1000°C in an end-loaded piston cylinder apparatus, under fO 2 conditions close to NNO as measured by solid Co-Pd-O sensors. Experimental charges were carried out in pure Au containers, the latter serving as the source of gold, in presence of variable amounts of H 2 O and, for half of the charges, with elemental sulfur (S) so as to reach sulfide saturation. Au concentrations in melt quenched to glass were determined by LA-ICPMS. When compared to previous data obtained at lower pressures and variable redox conditions, our results show that in both S-free and sulfide-saturated systems pressure has no direct, detectable effect on melt Au solubility. Nevertheless, pressure has a strong, negative effect on sulfur solubility. Since gold dissolution is closely related to the behaviour of sulfur in reducing and moderately oxidizing conditions, pressure has therefore a significant but indirect effect on Au solubility.The present study confirms that Au dissolution is mainly controlled by fO 2 in S-free melts and 3 by a complex interplay of fO 2 and melt S 2-concentration in sulfide-saturated melts, at given temperature. In addition, we propose that the transition from sulfide (S 2-) to sulfate (S 6+ ) species in melt is shifted towards more oxidizing conditions when pressure and the degree of melt polymerization increase. If this is true, this may have important consequences during mantle melting and magma ascent. In particular, if mantle melting occurred in moderately oxidizing conditions, a small degree of partial melting would allow the primary melts to become Au-enriched but the relatively high pressure would move the sulfide-sulfate transition to more oxidizing conditions, making the primary melts saturated with sulfide phases that would sequester gold from the melt. During magma ascent, the decreasing pressure would favour the destabilization of sulfides and the release of gold to the silicate melt. However, at shallow levels, decreasing pressure, magma evolution, and varying redox conditions would be continuously competing to concentrate or fractionate gold.

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Section: Speciation and Dissolution Of Sulfursupporting

confidence: 80%

Is gold solubility subject to pressure variations in ascending arc magmas?

Jégo

Nakamura

Kimura

et al. 2016

Geochimica et Cosmochimica Acta

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“…Several predictive models of SCSS in silicate melts have been previously published and allow prediction of SCSS in magmas relevant to the Earth or Mars (Holzheid and Grove, 2002;Li and Ripley, 2005;Righter et al, 2009;Ding et al, 2014). These models do not reproduce our experimental results ( Supplementary Fig.…”

Section: Modeling Sulfur Solubility In Silicate Meltscontrasting

confidence: 56%

“…For these compositional terms, we used molar fractions normalized to SiO 2 . This takes into account the degree of polymerization of the melt, which plays a role on SCSS (Holzheid and Grove, 2002), and also reduces the errors when applying the model to Mercurian magmas for which elemental ratios (normalized to Si) were measured by MESSENGER. In theory, SCSS could be adequately modeled independently of f O 2 because S in both the silicate melt and the sulfide melt is present as S 2− .…”

Section: Modeling Sulfur Solubility In Silicate Meltsmentioning

confidence: 99%

Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury

Namur

Charlier

Holtz

et al. 2016

Earth and Planetary Science Letters

118

160

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Chemical data from the MESSENGER spacecraft revealed that surface rocks on Mercury are unusually enriched in sulfur compared to samples from other terrestrial planets. In order to understand the speciation and distribution of sulfur on Mercury, we performed high temperature (1200-1750 • C), lowto high-pressure (1 bar to 4 GPa) experiments on compositions representative of Mercurian lavas and on the silicate composition of an enstatite chondrite. We equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions (IW-1.5 to IW-9.4; IW = iron-wüstite oxygen fugacity buffer). Under these oxygen fugacity conditions, sulfur dissolves in the silicate melt as S 2− and forms complexes with Fe 2+ , Mg 2+ and Ca 2+ . The sulfur concentration in silicate melts at sulfide saturation (SCSS) increases with increasing reducing conditions (from <1 wt.% S at IW-2 to >10 wt.% S at IW-8) and with increasing temperature. Metallic melts have a low sulfur content which decreases from 3 wt.% at IW-2 to 0 wt.% at IW-9. We developed an empirical parameterization to predict SCSS in Mercurian magmas as a function of oxygen fugacity ( f O 2 ), temperature, pressure and silicate melt composition. SCSS being not strictly a redox reaction, our expression is fully valid for magmatic systems containing a metal phase. Using physical constraints of the Mercurian mantle and magmas as well as our experimental results, we suggest that basalts on Mercury were free of sulfide globules when they erupted. The high sulfur contents revealed by MESSENGER result from the high sulfur solubility in silicate melt at reducing conditions. We make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and/or that the mantle contains a minor metal phase and combine our parameterization of SCSS with chemical data from MESSENGER to constrain the oxygen fugacity of Mercury's interior to IW-5.4 ± 0.4. We also calculate that the mantle of Mercury contains 7-11 wt.% S and that the metallic core of the planet has little sulfur (<1.5 wt.% S). The external part of the Mercurian core is likely to be made up of a thin (<90 km) FeS layer.

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“…Again this is unexpected based on knowledge of low P-T partitioning experiments, which show large inter-element fractionations (e.g., see review in Walter et al, 2000). These two distinctive features of the HSE have long been interpreted as resulting from a late-addition of oxidized material with chondritic relative abundance levels of the HSE (Kimura et al, 1974;Wänke, 1981;Ringwood, 1984;Wänke et al, 1984;O'Neill, 1991a,b;O'Neill and Palme, 1998;Holzheid and Grove, 2002). The idea is that after core formation ceased, the addition of the 'late veneer' was very efficiently mixed into the mantle, perhaps while in a partially molten state, and that this established the abundances of HSE without significantly perturbing the MSE.…”

Section: Introductionmentioning

confidence: 99%

Experimental study of platinum solubility in silicate melt to 14GPa and 2273K: Implications for accretion and core formation in Earth

Ertel

Walter

Drake

et al. 2006

Geochimica et Cosmochimica Acta

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We determined the solubility limit of Pt in molten haplo-basalt (1 atm anorthite-diopside eutectic composition) in piston-cylinder and multi-anvil experiments at pressures between 0.5 and 14 GPa and temperatures from 1698 to 2223 K. Experiments were internally buffered at $IW + 1. Pt concentrations in quenched-glass samples were measured by laser-ablation inductively coupled-plasma mass spectrometry (LA-ICPMS). This technique allows detection of small-scale heterogeneities in the run products while supplying threedimensional information about the distribution of Pt in the glass samples. Analytical variations in 195 Pt indicate that all experiments contain Pt nanonuggets after quenching. Averages of multiple, time-integrated spot analyses (corresponding to bulk analyses) typically have large standard deviations, and calculated Pt solubilities in silicate melt exhibit no statistically significant covariance with temperature or pressure. In contrast, averages of minimum 195 Pt signal levels show less inter-spot variation, and solubility shows significant covariance with pressure and temperature. We interpret these results to mean that nanonuggets are not quench particles, that is, they were not dissolved in the silicate melt, but were part of the equilibrium metal assemblage at run conditions. We assume that the average of minimum measured Pt abundances in multiple probe spots is representative of the actual solubility. The metal/silicate partition coefficients (D met/sil ) is the inverse of solubility, and we parameterize D met/sil in the data set by multivariate regression. The statistically robust regression shows that increasing both pressure and temperature causes D met/sil to decrease, that is, Pt becomes more soluble in silicate melt. D met/sil decreases by less than an order of magnitude at constant temperature from 1 to 14 GPa, whereas isobaric increase in temperature produces a more dramatic effect, with D met/sil decreasing by more than one order of magnitude between 1623 and 2223 K. The Pt abundance in the Earth's mantle requires that D met/sil is $1000 assuming core-mantle equilibration. Geochemical models for core formation in Earth based on moderately and slightly siderophile elements are generally consistent with equilibrium metal segregation at conditions generally in the range of 20-60 GPa and 2000-4000 K. Model extrapolations to these conditions show that the Pt abundance of the mantle can only be matched if oxygen fugacity is high ($IW) and if Pt mixes ideally in molten iron, both very unlikely conditions. For more realistic values of oxygen fugacity ($IW À 2) and experimentally-based constraints on non-ideal mixing, models show that D met/sil would be several orders of magnitude too high even at the most favorable conditions of pressure and temperature. These results suggest that the mantle Pt budget, and by implication other highly siderophile elements, was added by late addition of a 'late veneer' phase to the accreting proto-Earth.

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Sulfur saturation limits in silicate melts and their implications for core formation scenarios for terrestrial planets

Cited by 180 publications

References 28 publications

Is gold solubility subject to pressure variations in ascending arc magmas?

Is gold solubility subject to pressure variations in ascending arc magmas?

Sulfur solubility in reduced mafic silicate melts: Implications for the speciation and distribution of sulfur on Mercury

Experimental study of platinum solubility in silicate melt to 14GPa and 2273K: Implications for accretion and core formation in Earth

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