GEMOC ARC National Key Centre

Bennett, V.C.¹, Norman, M.D.², Garcia, M.O.³
1Research School of Earth Sciences, ANU; 2GEMOC, Macquarie; 3Dept of Geology and Geophysics, Univ. of Hawaii

The origins of the distinctive chemical characteristics of OIB, in particular their radiogenic Os isotopic compositions, are currently controversial with principal models focusing either on addition of small amounts of highly fractionated metallic outer core, or incorporation of large amounts of ancient recycled basaltic crust. To provide new constraints on the composition and origin of plume sources we have determined the platinum group element (PGE) and Re concentrations in tholeiitic picrites from the major volcanic centers of the Hawaiian island chain. PGE concentrations were determined by an isotope dilution ICP-MS, Te precipitation procedure, incorporating carius tube followed by HF-HCl dissolution to ensure complete decomposition and spike/sample equilibration. Re was determined by ID ICP-MS following anion exchange preconcentration.

Total PGE contents and PGE patterns for 5 picrites which span the known compositional range of the Hawaiian plume components are well correlated with isotopic and trace element characteristics. Koolau picrites (high 187Os/188Os, low 206Pb/204Pb, high Zr/Nb) have the lowest PGE and Re abundances, and the most fractionated pattern (highest Pt/Ir). Loihi and Kilauea picrites have the highest PGE contents and the least fractionated patterns, and are similar to basaltic komatiites. There is no obvious correlation of PGE abundances or pattern with MgO or phenocryst content. Re concentrations for 14 picrites range from 0.2 to 0.8 ppb and, in contrast to regional OIB compilations, Re/Yb ratios are well correlated with source characteristics (e.g.187Os/188Os, 206Pb/204Pb, Zr/Nb). Koolau picrites have the lowest Re/Yb (0.20 ppb/ppm), whereas the Kilauea and Loihi picrites have the highest Re/Yb (0.76 ppb/ppm).

Neither crustal recycling nor addition of outer core to the plume source simply accounts for the PGE and Re characteristics of Hawaiian picrites. Garnet has been suggested to have significant control on Re abundances in melts, but low Re/Yb in the Koolau picrites is not a consequence of additional garnet from an eclogitic component in the plume. High Pt/Re in the Koolau picrites is compatible with core interaction, but low Re/Yb and total PGE contents in this endmember contradict simple mass balance models for core addition. The PGE and Re/Yb characteristics can be modelled using variable sulfide contents in the plume components, with the Koolau picrites requiring a relatively sulfide-rich (0.1%) source and the Loihi/Kilauea picrites a relatively sulfide-poor (0.02-0.03%) source. The sulfide variations must reflect distinct source compositions and the origin of these variable sulfide contents needs to be accounted for in any model for the Hawaiian plume.