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Partitioning of Ir between chromite and silicate melt

To understand the fractionation of the platinum-group elements (PGE) during partial anatexis of the Earth’s mantle in S undersaturated conditions we investigate the partitioning behaviour of Ir between silicate melt and chromite. Starting mixture for our experiments is a natural picrite from the Solomon Islands doped with FeCr2O4, Ir2O3 and Li-tetraborate. This mixture was equilibrated in a piston cylinder press at temperatures up to 1700°C and pressures up to 2.5 GPa. Quenched samples consist of euhedral chromites with attached (Ir,Fe)-alloys in a silicate glass matrix.

The glasses and chromites are analyzed for Ir using the LA-ICPMS technique to achieve low detection limits combined with a good spatial resolution (in collaboration with Hugh St. C. O’Neill, ANU, Canberra). Preliminary results show that Ir is compatible in chromite and that the partition coefficient (DIrchromite‑melt) strongly varies with oxygen fugacity (ƒO2). Nevertheless a strong preference for (Ir,Fe)-alloys to crystallize on the surfaces of chromite is apparent. (Os,Ir,Fe)-alloys associated with chromite may therefore also contribute to the observed fractionation of PGE in natural systems.

BSE image chromite

Backscattered electron images of chromites up to 200 mm across crystallized in the silicate melt. Frequently (Ir,Fe)-alloys crystallise on chromite surfaces. (© Raúl Fonseca)

Contacts: Vera Laurenz and Prof. Christian Balhaus