2 MANTLE-PLUME COMPONENTS IN HAWAIIAN PICRITES INFERRED FROM CORRELATED OS-PB ISOTOPES

OCEAN island basalts (OIBs) are thought to result from the melting of plumes rising from a boundary layer in the mantle, such as the core-ma ntle boundary or that between the upper and lower mantle at 660 km dep th(1,2). OIBs display considerable compositional heterogeneity, genera lly attributed to mixing between components such as depleted mantle, s ubducted oceanic crust, continental lithospheric mantle and primitive mantle(3-8), Although the existence of multiple endmembers in the OIB array is now well established, the relationships between crustal recyc ling, plume volcanism and deep mantle structure and composition are co ntroversial, We present isotope data for picritic lavas from seven vol canic centres, sampling 2 million years of Hawaiian plume activity(9), and show that the osmium and lead isotopic compositions form remarkab ly linear, negative arrays. These correlations are indicative of binar y mixing, but are difficult to explain by combining recycled oceanic c rust with depleted mantle. We argue that mixing between two distinct m antle components, within the Hawaiian plume provides a better explanat ion of the isotopic variations of the picrites.