[1] The existence of an enriched component in the mantle with a pyroxenitic or eclogitic composition and its importance for basalt genesis has been discussed for over two decades. Inferences about the depth of melting as well as the dynamics of melting based on the presence of garnet and the location of the spinelgarnet transition are different if garnet-pyroxenite is present in the peridotitic mantle. Trace element partition coefficients are dependent on composition, and the differences between garnet-pyroxenite and peridotite are large enough to produce significant differences in trace element fractionation between melts derived from these different lithologies. Melts derived from garnet-pyroxenite or eclogite-bearing sources will have small or no 230 Th excesses, which are largely independent of melting and upwelling rate. Melts derived from garnet-peridotite will have significant 230 Th excesses, which are dependent on melting and upwelling rate. We show that the combined hafnium-neodymium-thorium (Hf-Nd-Th) isotope and trace element data can distinguish between melts derived from peridotitic and pyroxenitic or eclogitic sources. We also present new Hf isotope data for Hawaiian basalts and use the combined Hf-Nd-Th isotope and trace element systematics to argue against the existence of garnet-pyroxenite or eclogite in the source of Hawaiian basalts. It is especially the large variation in degree of melting for relatively constant isotopic composition that allows us to rule out garnet-pyroxenite in the source of the Hawaiian basalts.

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We present a new multicollector inductively coupled plasma mass spectrometric (MC-ICP-MS) method for measuring isotopic ratios of 234 U/ 238 U and 230 Th/ 232 Th in volcanic rocks using the Thermo Fisher Neptune. Uranium isotope measurements using SRM U010 as bracketing standard produce agreement within several permil for NBL 112A, REMP-18, as well as for equilibrium rock standards. We also demonstrate that uranium standards are not appropriate for correcting thorium isotopic measurements due to differences between U and Th in both mass bias and mass-dependent ion transmission. Use of the thorium synthetic standard, UCSC ThA, as a bracketing standard produces data that agree well with 'accepted values' for other synthetic Th isotope standards and equilibrium rock standards.

Young rhyolites and associated lavas and magmatic enclaves from the KatmaiEEENovarupta volcanic system (Alaskan Peninsula), and the Crater Lake and Medicine Lake volcanic system (Cascades) were analyzed for U and Th isotope abundances, as well as major and trace element concentrations, to investigate the timescales of the processes that lead to rhyolite generation in continental arcs. Basalts and basaltic andesites typically migrate from the mantle to the surface within several thousand years. Variations in (230 Th)/(232 Th) and (238 U)/(232 Th) ratios with SiO 2 concentrations in intermediate lavas appear to result from crystal fractionation combined with assimilation of recently crystallized magmas. These data also suggest that $10 4 EEE10 5 years of mafic magmatism are required at a volcanic center to generate silicic andesites and dacites. Rhyolite genesis involves varying proportions of crystal fractionation of intermediate magmas and assimilation of crust. The nearequilibrium (238 U)/(230 Th) ratios for all of the rhyolites suggest an average time since U was last fractionated from Th for the constituents making up these rhyolites of 410 5 years. Therefore, the residence times of continental magmas and their entrained crystals appear to increase by a minimum of 2EEE3 orders of magnitude with increasing SiO 2 concentrations from basalt to rhyolite.

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