Enrichment of Rh, Ru, Ir and Os in Cr spinels from oxidized magmas: evidence from the Ambae volcano, Vanuatu

Abstract

Experimental studies, performed under oxidized conditions (fO2>QFM+2, where QFM is quartz-fayalite-magnetite oxygen buffer), have shown that Rh, Ru, Ir and Os are strongly compatible with Cr spinel, whereas empirical studies of Cr spinels from ultramafic-mafic rocks suggest that the experimental results may overestimate the partition coefficients. We report laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of platinum-group elements (PGE), Au and Re abundances in Cr spinels from the Ambae volcano, Vanuatu (fO2=QFM+2.5), the Jimberlana layered intrusion, western Australia, and the Bushveld complex, South Africa (fO2~QFM). The results show that Rh and IPGEs (Iridium-group PGE; Ru, Ir, Os) partition strongly into the Cr spinels that crystallized from the oxidized Ambae lavas whereas most of the Cr spinels from the more reduced Jimberlana layered intrusion and the Bushveld complex contain no detectable PGE, Au or Re, with exception of ~10ppb of Ir in some Jimberlana Cr spinels. In the Ambae Cr spinels, Rh, Ru and, to lesser extent Os, are positively correlated with Fe3+, Ni and V. The homogeneous distribution of Rh and IPGEs in LA-ICP-MS time-resolved spectra indicates that these elements are in solid solution in Cr spinels. Pt-Fe alloys occur as inclusions within the Ambae Cr spinels, which indicate that the Ambae melt was saturated with Pt.Our results show that partitioning of Rh, Ru and Ir into Cr spinels increases with increasing oxygen fugacity, which suggests that the high concentrations of these elements in the Ambae Cr spinels are due to the high oxygen fugacity of the host magma. Therefore, Cr spinels may play an important role in controlling the concentrations of Rh and IPGEs during fractional crystallization of oxidized ultramafic-mafic magmas and during partial melting of oxidized arc mantle.