| Noble gas isotopic ratios (most importantly, helium isotopic ratios) measured in phenocrysts of continental basalts, are employed to derive information about magmatic processes. The first two parts of this dissertation use noble gases to characterize specific mantle materials that melted to produce continental basalts.; High 3He/4He ratios, compared to mid-ocean ridge basalts, have been reported in flood basalt lavas from Siberia and India, ocean island basalts from Hawaii, Iceland and Samoa, among others, and volcanic gases sampled at Yellowstone National Park.; Other basalts in the western United States, mostly related to Tertiary extension, inherit their isotopic characteristics either largely from the asthenosphere or from lithospheric mantle beneath the continent which has been isolated from mantle convection for some time. In order to understand the range of variations of helium isotopes in subcontinental lithospheric mantle (SCLM) and their origin, over two dozen basalts from the western United States were collected and analyzed for Sr, Nd, He, Ne and Ar isotopes and major and trace element composition. Correlations among He, Nd and Sr isotopic ratios indicate that both asthenospheric and lithospheric mantle were melted to generate the basalts.; The remaining two chapters of the thesis focus on noble gas isotopes as tracers of magmatic processes. As part of the Basin and Range study, three Late Quaternary lavas from just outside the Long Valley caldera were found to have anomalously low 3He/4He ratios compared to other eastern California basalts.; Finally, noble gas isotopes were measured in peridotite xenoliths from Simcoe, Washington, which are samples of the mantle wedge above the subducting Juan de Fuca Plate. Helium isotopic ratios measured in these xenoliths are significantly lower than those of mid-ocean ridge basalts, and the He isotopic variations mirror variations in Os, Sr, Pb and Nd isotopes measured by other workers. Interaction between a slab-derived fluid or hydrous melt with a low 3He/4He ratio, and asthenospheric mantle can explain the isotopic data. (Abstract shortened by UMI.)... |
