Rhenium and platinum group element abundances correlated with mantle source components in Hawaiian picrites: sulphides in the plume

Core addition and crustal recycling models that seek to explain the radioge nic Os isotopic compositions of primitive Hawaii tholeiites predict distinc tive geochemical consequences for chalcophile and siderophile element abund ances in the mantle plume. To test these models and to improve our understa nding of compositional variability in the Hawaiian plume, the platinum grou p element (PGE) and Re contents of primitive shield picrites from several H awaiian volcanoes were measured. PGE abundances span a large range, from si milar to MORB for a picrite from Koolau, to compositions similar to those o f basaltic komatiites for picrites from Kilauea and Loihi. Re concentration s range from 0.25 to 0.95 ng/g and with a mean of 0.73 ng/g, higher than pr eviously compiled global averages for ocean island basalts (OIB) (0.38 ng/g ) and closer to average MORE (0.98 ng/g) than previously recognised. Some s ubaerial tholeiites, notably from Kilauea and Mauna Kea, have anomalously l ow Re abundances and high Cu/Re ratios, possibly reflecting Re loss upon er uption or during degassing of shallow magma chambers. These data show that the PGE and primary Re contents of primitive Hawaiian picrites are well cor related with isotopic compositions of these lavas, linking the PGE and Re c haracteristics directly with source features of the mantle plume. However, mixing models that describe the isotopic effects of core addition End crust al recycling do not account for the PGE and Re abundances. The range of PGE and Re contents in these lavas does not appear to reflect abundance variat ions in the plume components, but some aspect of the melting process that i s linked to source characteristics of the plume. One possibility is that th e PGE and Re characteristics of Hawaiian tholeiites may reflect variable am ounts or compositions of residual sulphide during melting. In this scenario , the high PGE and Re contents of Kilauea and Loihi picrites may be indicat ing a relatively small amount of residual sulphide during melting, whereas the low PGE and Re contents of Koolau primitive magmas may be indicating gr eater amounts of residual sulphide in the plume. The systematic composition al variations of PGE and Re in primitive tholeiites must be accounted for b y any model for the origin of the Hawaiian plume. (C) 2000 Elsevier Science B.V. All rights reserved.