HR: 17:05h
AN: V44B-05    [Abstracts]
TI: The Standard Model for Noble Gases in Mantle Geochemistry: Some Observations and Alternatives
AU: * Meibom, A
EM: meibom@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, 320 Lomita Mall, Stanford University, CA 94305 United States
AU: Sleep, N H
EM: norm@pangea.stanford.edu
AF: Department of Geophysics, Mitchell Building, Stanford University, CA 94305 United States
AU: Zahnle, K
EM: kzahnle@mail.arc.nasa.gov
AF: NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 United States
AU: Anderson, D L
EM: dla@gps.caltech.edu
AF: Seismological Laboratory, MS 252-21 California Institute of Technology, Pasadena, CA 91125 United States
AB: We evaluate the Standard Model of noble gases against a number of observational constraints of relevance to the distribution of noble gases in the Earth's mantle. These constraints include: $1$) the lack of evidence for high $^3$He/$^4$He ratios correlating with high (initial) He concentrations, $2$) that MORB and OIB $^3$He/$^4$He data do not represent two different distributions [$1$], $3$) that systematic global correlations between $^3$He/$^4$He ratios and lithophile isotopic systems are lacking, $4$) that the correlations we do observe are broadly linear, $5$) that large, local geographical $^3$He/$^4$He variations are observed, which are inconsistent with a strongly localized (i.e. plum-stem) flux of high-$^3$He/$^4$He material, and $6$) that dramatic temporal $^3$He/$^4$He variations are observed on very short time scales ($10-100$ years). Non-layered noble gas mantle models, in which the carrier of unradiogenic He is a relatively noble gas-poor phase scattered in the mantle, are more consistent with this set of constraints. We propose that the carrier of unradiogenic noble gases is primarily olivine [$2$]. Olivine-rich lithologies, produced in previous partial melting events, are a natural part of the Statistical Upper Mantle Assemblage (SUMA); a highly heterogeneous assemblage of small-to-moderate scale ($1-100$ km) enriched and depleted lithologies with a wide range in chemical composition, fertility, age and isotopic signatures. The isotopic signatures of oceanic basalts, including noble gases, are obtained by partial melting of the SUMA under slightly different P-T conditions; i.e. different degrees of partial melting and different degrees of homogenization prior to eruption [$3-5$]. Noble gas isotopic systematics do not trace deep mantle components in the source materials of oceanic basalts. They may, however, indirectly indicate potential temperature, as the order in which different mantle lithologies melt depends on pressure. References: [$1$] Anderson, EPSL $193$, $77-82$ ($2001$). [$2$] Brooker et al., Lithos, $73$, S$15$ ($2004$). [$3$] Morgan and Morgan, EPSL $170$, $215-239$ ($1999$). [$4$] Meibom and Anderson, EPSL $217$, $123-139$ ($2003$). [$5$] Ito and Mahoney, EPSL submitted ($2004$).
DE: 8450 Planetary volcanism (5480)
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8147 Planetary interiors (5430, 5724)
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting