Solubilities of Pt and Rh in a haplobasaltic silicate melt at 1300 deg C

摘要

The solubilities of Platinum (Pt) and Rhodium (Rh) in a haplobasaltic melt (anorthite-diopside eutectic composition) have been determined experimentally by using the mechanically assisted equilibration technique at 1300 deg C, as a function of oxygen fugacity (10~(-12) < fO_2 <= 1 bar), imposed by CO-CO_2, N_2-H_2-H_2O, Ar-O_2, and air gas mixtures. Samples were analyzed by sample nebulization (SN) inductively coupled plasma-mass spectrometry and, using some of these samples as standards, also by laser ablation (LA) inductively coupled plasma-mass spectrometry. The latter is a true microanalytical technique that allows small-scale sample heterogeneity to be detected. At each oxygen fugacity step, a time-series of samples was taken, to demonstrate that the solubilities converge on a constant value. In addition, solubilities were measured after both increasing and decreasing the imposed fO_2. The results fall into three groups, according to oxygen fugacity. At high fO_2s, (fO_2>=10~(-2) bars), samples are homogenous at all sampling scales. Both Pt and Rh predominantly dissolve in the silicate melt as 2+ species, with some evidence for Pt~(4+) and Rh~(3+) at the highest fO_2s studied (air and pure O_2). From these data, we obtained the following expressions for the solubilities of Pt and Rh: Pt/ppb=2100(fO_2)+10980(fO_2)~(1/2) Ph/ppb=68630(fO_2)~(3/4)+31460(fO_2)~(1/2) At fO_2<10~(-5) bars, the true solubilities of Pt and Rh appear to be obscured by Pt-Rh micronuggets, which remain suspended in the melt despite stirring on time scales of 10~3 h, resulting in samples that are heterogenous on the laser sampling scale. Samples at intermediate fO_2 (10~(-2) to 10~(-5) bars) are affected by the micronugget problem on the sampling scale of the conventional SN-inductively coupled plasma mass spectrometry, but these can be filtered out by analyzing on the laser sampling scale.