The effect of Cl on Pt solubility in haplobasaltic melt: Implications for micronugget formation and evidence for fluid transport of PGEs

Platinum solubility was determined in a haplobasaltic, diopside-anorthite melt at 1523 K and 0.2 GPa as a function of oxygen fugacity and chlorine content. Synthetic glass powder of an An₄₂Di₅₈ composition was sealed in a platinum or platinum-iridium alloy capsule and equilibrated with a solid CaCl₂ and MgCl₂ chlorine source, water and the noble-metal capsule. All experiments were run in an internally-heated pressure-vessel equipped with a rapid-quench device with oxygen fugacity controlled by the water content and intrinsic hydrogen fugacity of the autoclave (MnO-Mn₃O₄). Resultant glasses were analyzed by isotope dilution ICP-MS and LA-ICP-MS to determine the solubility and distribution of Pt and assess potential Cl-complexation of Pt in the melt.Experiments with run durations longer than 96 h show Pt solubilities consistent with solubilities determined for the equivalent Cl-free diopside-anorthite system, under the same P-T conditions. These results indicate that chlorine has no discernable effect on Pt solubility and there is no evidence of Pt-Cl complexing in the silicate melt from 0.6 to 2.75 wt% Cl (saturation).However, products from short run duration experiments (<96 h) contain Pt concentrations which are orders of magnitude higher than those of the Pt-free starting glass and of the experimental products of the longer run duration experiments. These anomalously high levels are most pronounced in the shortest experiments and Pt concentration decreases with increasing run duration. It is suggested that this excess platinum is dissolved within the Cl-bearing fluid during the heating stages of the experiment and is left behind as the fluid dissolves into the melt leaving small amounts of Pt as “micronuggets”, increasing the bulk Pt concentration. With increasing run duration the platinum appears to migrate out of the melt, back to the capsule walls, decreasing the amount of Pt contained within the glass. This behavior offers compelling evidence that Cl-bearing fluids have the capacity to transport significant amounts of Pt under magmatic conditions.Mass balance calculations on the excess amount of Pt in the glass (above inherent solubility) in short duration experiments lead to an estimation of Pt concentration in the Cl-bearing fluid ranging from tens to a few hundred ppm, versus ppb levels in the melt. The correspondingly high estimated apparent partition coefficients of 10³-10⁴ suggest that Cl-bearing fluids can be highly efficient at enriching and transporting platinum in mafic magmatic-hydrothermal ore-forming systems. These values strongly contrast with recent experimental results in felsic systems, highlighting the potential importance of melt composition on partitioning, the need for composition specific partitioning experiments, as well as a detailed understanding of Pt distribution in experimental products.