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South African Journal of Geology; September 2004; v. 107; no. 3; p. 333-340; DOI: 10.2113/107.3.333
© 2004 Geological Society of South Africa
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Article

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

W.D. Maier

Centre for Research on Magmatic Ore Deposits, Dept. of Geology, University of Pretoria, Pretoria 0002, South Africa, email: wdmaier{at}scientia.up.ac.za

Sarah-Jane Barnes

Sciences de la Terre, Université du Québec, Chicoutimi G7H 2B1, Canada email: sjbarnes{at}uqac.uquebec.ca

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 monosulfide 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 alkaline magmas, 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.




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Geological Society, London, Special Publications, January 1, 2005; 248(1): 59 - 70.
[Abstract] [PDF]


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