A Platinum Group Element and Re–Os Isotope Investigation of Siderophile Element Recycling in Subduction Zones: Comparison of Grenada, Lesser Antilles Arc, and the Izu–Bonin Arc

Abstract

The picritic (MgO >13·5%) lavas of Grenada provide a unique opportunity to evaluate the platinum group elements (PGE) and Os isotope compositions of primitive subduction-generated melts. Compared with other arc lavas they have undergone very limited crustal contamination (∼<2%) and only minimal fractionation (earlier calculations indicate that <10% olivine removal from a range of parental melts can explain all major element variations). Despite their primitive nature, the picritic lavas contain very low concentrations of PGE (<0·2 ppb Ir, 1–4 ppb Pd) compared with other high-MgO lavas such as kimberlites, komatiites and plume-related picrites. This is probably due to a combination of lower degrees of partial mantle melting and early removal of PGE with cumulus phases such as olivine, magnetite and sulphide in the subduction systems studied here. Comparison of Grenada samples with those from Izu–Bonin illustrates that although the major element chemistries of Grenada and Izu–Bonin are different (alkalic vs boninitic) their PGE signatures are very similar. Thus, on the basis of the two systems studied, it would appear that melts generated in a subduction regime may have common characteristics. In contrast, Re is markedly depleted in the Grenada picrites compared with the Izu–Bonin boninites, suggesting retention of Re by residual garnet in the Grenada sub-arc mantle wedge. Generation above a subduction zone does not appear to have any significant systematic effect on the PGE signatures of resultant lavas. The potentially more fluid-mobile elements, Os and Pd, do not show major enrichment in relation to the other PGE compared with other tectonic environments. Os isotope analyses of several Grenada picrites reveal variable 187Os/188Os signatures (0·134–0·164), which are even more radiogenic than peridotite xenoliths previously measured from mantle wedges modified by addition of subducted material. Crustal contamination appears unable to explain all of the Os isotope enrichment and thus elevated 187Os/188Os may reflect the modification of the Grenadian sub-arc mantle wedge by addition of small amounts (∼2%) of sediment and/or slab-derived fluids enriched in radiogenic Os.