Os isotope systematics in the Azores: implications for mantle plume sources

Abstract

Os isotopes were measured in 25 Holocene alkali basalts and trachybasalts from 6 islands in the Azores Archipelago. Extreme variations in 187 Os 188 Os signatures extending to very radiogenic values (up to 0.195) are found in samples with less than 20 pg/g Os. This trend is similar to that found in St. Helena, and is attributed to minor assimilation of marine sediment [1]. In contrast, a relatively limited range in 187 Os 188 Os (0.128-0.137) characterizes the basalts with greater than 20 pg/g Os. This range in 187 Os 188 Os is inferred to represent the Os isotopic signature of the Azores plume. High Os concentration samples from the island of Sao Miguel range in 187 Os 188 Os 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 produced 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 characterized by a relatively narrow range in Os isotopic composition despite variable Pb isotopic compositions. The Os isotopic compositions of these plumes are, in general, more radiogenic than depleted MORB mantle and than any chondrite groups, and indicate that plumes contain an additional source of radiogenic Os. Because Os is a highly compatible element, it is likely that Os only records large percentage crustal recycling, 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 ultimately due to an outer core contribution [8,9].