Pt/Pd and Pd/Ir ratios in mantle-derived magmas : a possible role for mantle metasomatism

摘要

We compare concentrations of Pt, Pd, and Ir in mantle-derived magmas, ranging from tholeiitic basalts to komatiitic basalts, komatiites, and various alkaline magmas, and in oceanic and continental settings. The alkaline magmas tend to have higher Pt/Pd ratios, but lower Pd/Ir ratios than most of the other magmas. We suggest this is attributable to different melting conditions in the mantle. Under relatively "dry" melting conditions applicable to tholeiites and komatiites, Pt-alloys and Os-Ir-Ru-Rh-enriched rnonosulfide solid solution (Mss) behave in a refractory manner, resulting in sub-chondritic Pt/Pd and super-chondritic Pd/Ir. Under fluid-rich melting regimes in metasomatised lithospheric mantle sources that may be applicable to the generation of many alkalineudmagmas, the alloys/mss are more fusible, resulting in PGE ratios closer to chondrite. Bushveld magmas and some continental flood basalts also have relatively high Pt/Pd ratios and may thus contain a component of the metasomatised sub-continental lithospheric mantle. Komatiites have relatively low Pt/Pd suggesting that they are derived from a dry mantle source.