HR: 10:20h
AN: V22A-01 INVITED     [Abstracts]
TI: Noble gases and mantle reservoirs: constraints from isotope ratios, degassing fluxes, and noble gas abundances and ratios
AU: * Kurz, M D
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, MS#25, Woods Hole, MA 02543 United States
AU: Jenkins, W J
EM: wjenkins@whoi.edu
AF: Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, MS#25, Woods Hole, MA 02543 United States
AB: Isotopic measurements in volcanic rocks provide important information regarding their sources and provide one of the best `windows' into the mantle, an approach that was pioneered by S.R. Hart and many colleagues. 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. The most unradiogenic helium and neon isotopic signatures are routinely found in the most active volcanic regions, such as Hawaii and Iceland, also suggesting a relationship between noble gases and excess heat and melting. Global isotopic data compilations show that samples with the most unradiogenic helium and neon are associated with Sr, Nd and Pb isotopic compositions that are intermediate between depleted mantle and hypothetical bulk earth mantle, demonstrating that the undegassed reservoirs are not primitive in geochemical composition. This observation, and the common assumption that recycling of ocean crust and sediments can explain most mantle isotopic variations, has led to a challenge of the undegassed mantle hypothesis. One alternative explanation is that helium is more compatible than Th and U in major mineral phases 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 have yielded values that may be higher than Th and U (e.g., Brooker et al., 2003; Parman et al., 2005 in press, for clinopyroxene and olivine). Estimates based on natural basaltic phenocrysts yield significantly lower partition coefficents (i.e. D(He) for olivine/melt < 0.001), so the existing experimental data are far from conclusive. Consideration of simple conceptual models for terrestrial noble gas evolution demonstrates that absolute noble concentrations in the mantle, both for the present day and for the early earth, are crucial unknown parameters. Due to the effects of near-surface degassing, and atmospheric contamination, absolute concentrations are extremely difficult to infer from surficial volcanic gases. Despite tremendous advances in characterizing mantle isotope ratios and trace element abundance patterns, the mantle gas concentrations remain poorly constrained. Within the framework of highly idealized isotopic evolution models, we explore the information content of these diverse observations, and evaluate the contribution of each to our knowledge of earth formation, evolution and structure. Recognizing that the early earth most likely had a huge inventory of light noble gases, a partially degassed/depleted terrestrial mantle could yield the observed noble gas isotopic characteristics. The existence of relatively undegassed mantle reservoirs in the deep earth still provides a plausible explanation for all the observations.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 3621 Mantle processes (1038)
DE: 8430 Volcanic gases
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005