Platinum group elements in mantle xenoliths from eastern China

Abstract

The platinum group element (PGE) composition of a suite of anhydrous spinel peridotite xenoliths from five different localities in eastern China were determined in order to evaluate the geochemical behaviour of these elements during mantle processes. The samples are composed of harzburgites of granoblastic textures, and lherzolites mainly of protogranular and porphyroclastic textures. The total PGE contents vary from 16.20 to 50.7 ppb. Ruthenium, Ir and Pd are correlated with CaO and are interpreted to define a partial melting trend. The positive correlation between Cu and Pd supports this interpretation. However, with the exception of Au, all peridotitic xenoliths display a downward concave PGE pattern, indicating Pt enrichment. This pattern is very different from the flat or negatively sloped PGE pattern observed commonly for mantle xenoliths and peridotitic massifs. The Pt enrichment is ascribed to percolating basaltic magma which is precipitating only small amounts of sulphides. As Pt is less strongly chalcophilic than Pd, it may fractionate from the sulphides which host the Pd, and form Pt-alloys. This process also explains the Au enrichment and the Pt–Au correlation, as Au is also less strongly chalcophilic than Pd. The strong Ir depletion in the protogranular samples is ascribed to Ir retention in the deep mantle due to the high metal/silicate distribution coefficient of Ir at f O 2 = IW −1.