Hf-isotopes in Ocean Island Basalts and Meteorites

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SaltersEAR-0124965This project will focus on Hf-isotope studies of ocean island basalts (OIB) and meteorites with the goal of characterizing the mantle and understanding how heterogeneities (metasomatic component, subducted components etc.) are mixed back into the ambient mantle. The terrestrial variation in Hf and Nd isotopes indicates that the Lu-Hf and Sm-Nd system are well correlated and regression of the array leads to eHf = 1.35 ?eNd + 3 eHf. I propose to test this excellent correlation between Nd and Hf isotopic compositions by analyzing materials from "extreme" isotopic or trace element environments. The Hf and Nd isotope systematics will provide interesting and powerful information in places where the Nd and Hf systems are differently correlated. Lu-Hf-Sm-Nd characteristics of pelagic sediments, oceanic crust as well as carbonatites imply evolution lines that have a different slope than the terrestrial or ocean basalt array. These distinctive trace element and Hf-Nd isotopic characteristics allows for relatively easy recognition of these components in oceanic basalts as well as good estimates on the duration of isolation of these components (mantle heterogeneity). The fact that the oceanic island basalt array is so well defined indicates that processes or components that would move mantle compositions off the array are either insignificant in volume or are mixed back in on relatively short timescales. A first step towards understanding these mixing processes is recognition of the different type of materials (pelagic sediment, oceanic crust etc) and processes (melting, metasomatism etc) in the sources of the basalts. I propose to obtain Hf-isotope data for basalts from the Samoan Island chain, from Koolau volcano, Hawaii, and from Walvis Ridge. A second component of the project is the determination whether Lu/Hf-Sm/Nd fractionation in chondrites is significantly different then the time integrated fractionations in earth as observed by the terrestrial array. Presently there are three different estimates for the isotopic composition of bulk Earth in Hf-Nd-isotope space. The available data indicate that Hf and Nd isotopic compositions of meteorites are correlated and that the meteorite array is at sharp angle with the OIB array. I propose to analyze a suite of meteorites for Hf and Nd isotopic composition and thereby better define the meteorite array.