The geochemistry of the platinum-group elements in Brazilian and southern African kimberlites

Abstract

Concentrations of platinum-group elements (PGE) and Au have been determined in thirty-four samples of kimberlitic and alkaline igneous rocks (melilitite, carbonatite, and alkali basalt) from Brazil and southern Africa. The intrusions were grouped according to their lithophile element isotopic signatures (Group I, Group II, and transitional) and also their location (cratonic or off-craton) with respect to the Brazilian São Francisco and African Kaapvaal cratons. In contrast to the lithophile elements, the PGE signatures of Kaapvaal cratonic Group I and Group II kimberlites are not strongly fractionated relative to mantle and largely overlap, with Group II showing only a slight enrichment in Pd and Au relative to Group I. However, Kaapvaal cratonic and off-craton kimberlites show sharply contrasting PGE signatures; off-craton kimberlites have lower concentrations of Os, Ir, and Ru, lower Os/Ir ratios, and are more strongly fractionated relative to mantle. A similar distinction was observed between kimberlites and alkaline rocks flanking the margin of the São Francisco craton, where the high concentrations of Os, Ir, and Ru in the kimberlites make the PGE signature appear less fractionated than the alkaline rocks. Cratonic kimberlites have more primitive PGE signatures and higher concentrations of Os, Ir, and Ru than other mantle-derived melts (MORB, OIB, komatiites). The relatively high PGE concentrations and primitive signatures and the generally accepted view that kimberlites are low degree partial melts are difficult to reconcile with existing models of PGE fractionation during mantle melting. Given the spatial association between the primitive PGE signature and Archean cratonic lithosphere, we suggest that it might reflect a PGE component(s) which is/are inherited from the cratonic lithospheric mantle, either from the source region through melting or by post-melting assimilation. Suitable candidates might include: PGE-rich phases exposed in former oceanic podiform chromitites which have been tectonothermally converted to spinel-poor lithospheric peridotites; or fragments of deep mantle, PGE-rich, siderophile heterogeneities which were incorporated into the earliest cratonic lithosphere during extensive plume-driven melting or plate-tectonic-like stacking and underthrusting of oceanic lithosphere. Either process of cratonic development could allow the accumulation of an Os, Ir, and Ru-rich PGE component at depth in the lithospheric mantle. The absence of this component in off-craton kimberlites and shallow alkaline melts is suggested to reflect its concentration at depth in the earliest lithospheric keels.