Correlated Os-Pb-Nd-Sr isotopes in the Austral-Cook chain basalts: the nature of mantle components in plume sources

Osmium (Os), strontium (Sr), neodymium (Nd) and lead (Pbf isotopes have bee n measured on a suite of aphyric basalts from 12 islands of the Austral-Coo k island archipelago, an area which exhibits a range in Pb isotope composit ions that encompasses almost the entire range displayed by ocean island bas alts (OIB). Although the samples have Os concentrations (1.69-34.80 ppt) at the lower end of the range measured for GIB, they display a range of initi al Os-187/Os-188 ratios (between 0.1279 and 0.1594) similar to that defined by olivine-phyric, Os-rich GIB. Positive Os-Nd, Os-Pb and negative Os-Sr i sotope correlations are documented, indicating that the isotopic arrays rep resent various proportions of mixing between a HIMU-type end-member represe nted by Mangaia and Tubuai islands and characterized by radiogenic Os and P b isotopic compositions, and an end-member represented by Rarotonga island which is characterized by unradiogenic Os and intermediate Sr, NJ and Pb is otopic compositions. The HIMU signature of the mantle component involved in Tubuai-Mangaia mantle sources requires long-term enrichments of U and Th r elative to Pb and Be relative to Os, without associated increase in Rb/Sr, that are consistent with recycled oceanic crust. The end-member represented by Rarotonga basalts shows Os, Sr, Nd, and Pb isotopic signatures similar to those presumed for the;bulk silicate earth' (BSE), which cannot be obtai ned by mixing the four mantle components (DMM HIMU and EM1 and 2) generally used to circumscribe the Sr-Nd-Pb isotopic data of GIB. The primitive-like isotopic characters of this end-member might be ascribed to the presence o f undepleted material from a lower segment of the mantle in the source of t he Austral-Cook island basalts land more specifically Rarotonga basalts); h owever, such a hypothesis is challenged by both the absence of a primordial He-3 signature and the non-primitive Ce/Pb and Nb/U values for the Austral -Cook island basalts. Alternatively, assuming that the primitive-like isoto pic composition of the Rarotonga samples reflects mixing proportions betwee n the HIMU component and a mantle component characterized by unradiogenic O s, Nd and Pb and radiogenic Sr isotopic composition relative to BSE, the in volvement of recycled, old subcontinental lithosphere in the genesis of thi s component should then be considered, (C) 2001 Elsevier Science B.V. All r ights reserved.