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

Abstract

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.

Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements

“…Nevertheless, the large apparent range of partition coefficients for HSEs, even at the elevated temperatures accompanying higher pressures in larger bodies (9)(10)(11), is not consistent with the chondritic (i.e. primitive solar system) patterns of HSEs in both the terrestrial and martian mantles, and the similarities in absolute abundances between the two bodies (12).…”

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

“…In addition to the presence of large metal and sulfide globules, some experiments (especially in IHPV) show abundant submicrometric globules of metallic melt homogeneously distributed in the silicate melt. We interpret these small globules as the result of inefficient coarsening of the metal globules (Ertel et al, 2006;Malavergne et al, 2016). …”

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

“…If this is true, it may lead to important implications for the gold budget of arc magmas rising up through the mantle wedge and the arc crust. A few other ore-forming metals (Mo, Pt, Te, Se,…) as well as sulfur have been previously studied to estimate the pressure effect on their solubility in silicate melts (e.g., Mavrogenes and O'Neill, 1999;Ertel et al, 2006;Rose-Weston et al, 2009;Burkemper and Agee, 2010), but all experiments were performed at much higher pressures, some of these studies aiming to constrain early Earth evolution and core-mantle differentiation. Here, we aim to evaluate the ability of the silicate melt to incorporate gold during partial melting of a deep source (upper mantle or lower crust), saturated with sulfide phases or not, and see how the initial gold budget of a primary melt will evolve during its adiabatic ascent through the arc crust.…”