OS ISOTOPE SYSTEMATICS IN THE AZORES - IMPLICATIONS FOR MANTLE PLUME SOURCES

Os isotopes were measured in 25 Holocene alkali basalts and trachybasa lts from 6 islands in the Azores Archipelago. Extreme variations in Os -187/Os-188 signatures extending to very radiogenic values (up to 0.19 5) are found in samples with less than 20 pg/g Os. This trend is simil ar to that found in St. Helena, and is attributed to minor assimilatio n of marine sediment [1]. In contrast, a relatively limited range in O s-187/Os-188 (0.128-0.137) characterizes the basalts with greater than 20 pg/g Os, This range in Os-187/Os-188 is inferred to represent the Os isotopic signature of the Azores plume. High Os concentration sampl es from the island of Sao Miguel range in Os-187/Os-188 from 0.131 to 0.137, a surprisingly limited variation given the very large ranges on this island of Sr, Nd, Pb, He and Th isotopic signatures [2-5]. This is consistent with a model in which the Sao Miguel EMII signature is p roduced by high degree plume melts which mix with low degree melts of shallowly residing subcontinental lithospheric mantle [2], although an origin due to sediment recycling cannot be ruled out based on the Os isotope data. The Azores and many other plumes appear to be characteri zed by a relatively narrow range in Os isotopic composition despite va riable Pb isotopic compositions, The Os isotopic compositions of these plumes are, in general, more radiogenic than depleted MORE mantle and than any chondrite groups, and indicate that plumes contain an additi onal. source of radiogenic Os, Because Os is a highly compatible eleme nt, it is likely that Os only records large percentage crustal recycli ng, such as that inferred for the end-member HIMU islands [1,6,7]. We propose that the radiogenic Os isotope signatures in other plumes are due to the incorporation of radiogenic lower mantle, perhaps ultimatel y due to an outer core contribution [8,9].