HR: 16:50h
AN: V44B-04    [Abstracts]
TI: Do unradiogenic noble gases in oceanic basalts indicate undegassed deep mantle?
AU: * Kurz, M D
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Institution, Mail Stop #25 Clark Laboratory WHOI, Woods Hole, MA 02543 United States
AB: Unradiogenic helium and neon isotopic compositions, found in some oceanic island volcanoes, have been interpreted by geochemists as evidence for undegassed reservoirs deep in the earth. This presentation will provide a brief review of some of the evidence for and against this standard model. The deep undegassed mantle hypothesis has been challenged by geochemists and geophysists who would prefer to explain geochemical variations by recycling and non-plume processes. One alternative explanation is that helium is more compatible than Th and U during silicate melting, which could result in unradiogenic helium isotopes (high 3He/4He ratios) in ancient depleted sources. Crystal/liquid noble gas partition coefficients are not well known, but recent laboratory studies (e.g., Brooker et al., 2003; see also Parman et al., this meeting) yield values significantly lower than earlier studies which had suggested that noble gases might behave as compatible elements during melting. Estimates based on natural basaltic phenocrysts, compared to co-existing submarine glasses, strongly suggest that helium is more incompatible than Th and U (D for olivine/melt less than 0.001). Therefore, existing data do not support the hypothesis that helium is more compatible than Th and U. Recent studies suggest that helium and neon isotopes are well correlated in submarine oceanic basalt glasses, which suggests coherent evolution of mantle (Th+U)/Ne and (Th+U)/He ratios. The correlations of helium with neon, and helium with solid radiogenic isotopes (Sr,Nd,Pb), provide important arguments against models involving complete decoupling of helium from other elements by storage in ancient lithosperic reservoirs or melting processes. The most unradiogenic helium and neon isotopic signatures are routinely found in the most active volcanic regions, also suggesting a relationship between noble gases and excess heat and melting. The global isotopic data show that unradiogenic helium and neon are most often associated with Sr, Nd and Pb isotopic compositions that are intermediate between depleted mantle and hypothetical bulk earth mantle; this demonstrates that the undegassed reservoirs are not totally primitive in geochemical composition. The early earth most likely had a huge inventory of light noble gases and a partially degassed/depleted terrestrial mantle could yield the observed isotopic characteristics. The existence of relatively undegassed mantle reservoirs in the deep earth still provides the simplest explanation for all the observations. Undegassed reservoirs could exist within the low viscosity lower mantle or the core/mantle boundary.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting