Variations of platinum group elements in ultramafic xenoliths

Platinum group elements (PGE), gold, and rhenium are the siderophiles which show contrasting geochemical distribution with lithophile elements such as rare earth elements. The PGE abundances are generally very low on the order of ppb or less in felsic rocks, and are relatively high in ultramafic rocks. The PGE pattern is the plot of abundances of PGE and Au, normalized by CI-chondrite value or primitive mantle value, in descending order of melting point, i.e., Os, Ir, Ru, Rh, Pt, Pd, and Au. The pattern shows characteristic varieties on their rock species or tectonic settings. The PGE and Au can be classified into two sub-groups on the basis of their geochemical behavior: The Ir group (I-PGE; Os, Ir, and Ru) and the Pd group (P-PGE; Rh, Pt, Pd, and Au). Compared with the I-PGE, the P-PGEs tend to be disturbed during metasomatism or hydrothermal events. Investigation of the PGE behavior in ultramafic xenoliths and their host rocks would clarify the interactions which xenoliths have experienced in upper mantle materials or during the uplift.