The Standard Model for Noble Gases in Mantle Geochemistry: Some Observations and Alternatives

Details The Standard Model for Noble Gases in Mantle Geochemistry: Some Observations and Alternatives The Standard Model for Noble Gases in Mantle Geochemistry: Some Observations and Alternatives

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v44b..05m&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #V44B-05

Physics

8450 Planetary Volcanism (5480), 8121 Dynamics, Convection Currents And Mantle Plumes, 8124 Earth'S Interior: Composition And State (Old 8105), 8147 Planetary Interiors (5430, 5724), 3672 Planetary Mineralogy And Petrology (5410)

Scientific paper

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 3He/4He ratios correlating with high (initial) He concentrations, 2) that MORB and OIB 3He/4He data do not represent two different distributions [1], 3) that systematic global correlations between 3He/4He ratios and lithophile isotopic systems are lacking, 4) that the correlations we do observe are broadly linear, 5) that large, local geographical 3He/4He variations are observed, which are inconsistent with a strongly localized (i.e. plum-stem) flux of high-3He/4He material, and 6) that dramatic temporal 3He/4He 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, S15 (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).

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