Noble Gases and Isotope Systematics in Continental and Hot Spot Volcanism

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Helium data now available on small numbers of samples from about 40 of the earth's hotspots, show a well- marked bimodal distribution of isotope ratios greater and less than MORB values. One part of this proposal is for the measurement of 3He/4He ratios on suites of lavas from the Caroline and Marquesas island chains, which are known to have ratios greater that MORB (Carolines) and equal or less than MORB (Marquesas, based on a few samples only). The second, and major proposed effort, is the continuation of studies of the isotopic systematics of three major continental rift valley terrains: the Ethiopian Rift (plus the Rungwe basalts at the southern terminus of the East African Rift), the Snake River Plain with studies also of the Columbia River flood basalts, and the Antarctic basalts from Marie Byrd Land and Victoria Land. The third effort is the continuing study of rare-gas systematics (He-Ne-Ar) in phenocrysts (olivine, cpx) and glass from OIB lavas as applied to the question of the Ne and Ar isotope ratios of the "undepleted mantle" source as compared to atmospheric ratios.