Solubility of the assemblage Pt-PtAs(melt) in basalt with implications for Pt-As complexing and As speciation

English Abstract:

Given the evidence in nature for a strong Pt-As association in magmas, we evaluated the effect of arsenic on platinum solubility in molten silicate. Two different approaches were employed. In the first, vacuum-sealed quartz tubes contained a synthetic basalt analogue with Pt metal +/- arsenide melt encapsulated in a natural chromite crucible. Oxygen fugacity (fO₂) was fixed using solid oxide buffers, corresponding to log fO₂ of -11.65, -7.72 and -4.10, respectively. Experiments were done at 0.1 MPa, 1200^o C for 1 to 4 days, then quenched in water. It was found that As was volatilized from samples and began to contaminate the oxygen buffer, leading to a systematic decrease in Pt and As solubility with time. To mitigate this problem, a second series of experiments (type 2) first equilibrated the basalt sample with (initially) pure Pt in a gas-mixing furnace, with the sample then sealed in a silica ampoule along with arsenide melt, but without the solid oxide buffer. The fO₂ in these experiments was checked using partitioning of vanadium between chromite and melt. All experiments contained Pt-Fe alloy + Pt-arsenide melt, except some type 2 runs in which the Pt-Fe alloy was absent. The dissolved As content of the silicate melt increased significantly with fO₂, varying from 10 to 10,000 ppm, over the fO₂ range investigated. The average Pt content of the As-bearing glasses produced at log fO₂ of -7.72 and -4.10 are 0.16 ppm and 0.18 ppm, respectively, compared to 0.02 ppm and 0.14 ppm Pt in As-free glasses. At log fO₂ of -7.72, the As content of the silicate melt is ~400 ppm, in which the Pt solubility is enhanced by 8-fold. Hence, the molar As/Pt required to affect this “excess” Pt solubility is ~7400. The As content of typical primitive basalts is ~0.1 ppm, compared to 1-10 ppb Pt, resulting in molar As/Pt of ~300-30; this suggests the solubility enhancement by As complexing will be insignificant in natural systems. Importantly, despite high levels of As in two experiments which were saturated in Pt-arsenide melt (60-80 at% Pt), but not Pt metal, each contained Pt contents of <0.01 ppm. Results therefore imply that the Pt level required for Pt-arsenide saturation in molten silicate is low, possibly leading to crystallization of such phases as sperrylite (PtAs₂) in sulfur-poor, As-bearing magmas.