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3.2 d. Experimental determination of the solubility of siderophile elements in a haplobasaltic melt. II. Solubility of Pt and Rh (W. Ertel and D.B. Dingwell, in collaboration with H.St.C. O'Neill/Canberra)

The experiments concerning the solubilities of Pt and Rh in a haplobasaltic melt were extended to a broader range of fO2's: low oxygen fugacities were achieved using CO/CO2 gas mixtures while high oxygen fugacities were achieved using O2-Ar gas mixtures (1, 3, 6, 10%, and finally pure O2). In a final step the fO2 was reversed to a low value of log fO2 = -11.96 using forming gas bubbled through an ice-water mixture. ICP-MS techniques were used for the determination of the Pt and Rh contents of the samples. The results shown in Fig. 3.2-3 confirm previous results about the Pt and Rh solubilities: both show principally the same solubility behaviour as observed by Ir, which appears to be typical for highly siderophile elements. Starting at high oxygen fugacity, a decrease of fO2 results in a decrease of the solubility of both elements. After a minimum solubility of Pt around log fO2 = -6 of around 60 ppb and 25 ppb for Rh, a further decrease of oxygen fugacity results in a sudden increase in solubility up to 160 ppb for Rh and 500 ppb for Pt at an oxygen fugacity of log fO2 = -10. Two data points at lower oxygen fugacities result from experimental problems and cannot yet be explained completely.

The data indicate a formal oxidation state of Pt and Rh of 2+ at high oxygen fugacity. Results from experiments at an oxygen fugacity around 10-12 controlled by forming gas - ice water mixtures (a C-free system) will be measured soon and might provide information on the influence of C on the solubility of highly siderophile elements.

Fig. 3.2-3: The solubilities of Pt and Rh in a haplobasaltic melt. The high oxygen fugacity range indicates a slope of concentration corresponding to dissolved Pt and Rh oxides with formal charge on the Pt and Rh of 2+. At the low oxygen fugacity range the concentrations indicate slopes corresponding to formal charges of 1-.

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