Osmium Isotopic Constraints on the Nature of the Hawaiian Plume Source

V. C. Bennett, (RSES, The Australian National Univ, Canberra, ACT 0200), M. D. Norman (GEMOC, Macquarie Univ. North Ryde), T. M. Esat (RSES, ANU)

Hawaiian lavas are the melting products of the longest-lived, hottest and most voluminous of the currently active plumes, providing a unique opportunity to study the long-term characteristics of a mantle hotspot. We have determined the Os, Pb, Nd, and Sr isotopic compositions and trace element characteristics of picritic lavas from seven volcanic centres spanning 2 myr in order to investigate the composition of the Hawaiian plume source. Hawaiian picrites exhibit a very large range of Os isotopic compositions, from near MORB source mantle values with gOs=3 (where gOs is the per cent deviation from chondritic values) in the Kilaeua endmember, to much more radiogenic compositions (gOs=14) in the Koolau endmember. Os isotopic compositions are well correlated with other radiogenic isotopes and with many trace element ratios e.g. Ce/Pb, Zr/Nb. In particular, Os isotopes form excellent linear, negative correlations with all Pb isotopes. The Os-Pb isotopic arrays require mixing of two source components with similar incompatible element/compatible element ratios, which we interpret as reflecting two deep mantle peridotitic components.

Ocean island basalts (OIB) encompass a wide range of isotopic characteristics, especially for Pb, which has been partly attributed to the influence of recycled oceanic crust of varying age and composition. Although the database for reliable, i.e. high MgO and high Os concentration, samples from OIB is still limited, the most extreme compositions for several OIB localities (Marcantonio et al, EPSL 1995, Hauri and Hart, EPSL 1993) including our Hawaiian samples, fall into a narrow range characterised by gOs=14 to 18. This is surprising given the large age range and thus huge span of Os isotopic compositions possible in recycled oceanic crust, from gOs=0 in modern MORB to gOs>2800 in 2Ga basalts. The limited Os isotopic range of primitive OIB suggests either a buffering process, or the existence of a distinct, well-mixed, Os component in the plume source region which is sampled in many OIB localities and may reside in the lower mantle.