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Models for noble gases in mantle geochemistry: Some observations and alternatives

By
Anders Meibom
Anders Meibom
Department of Geological and Environmental Sciences, 320 Lomita Mall, Stanford University, California 94305, USAmeibom@mnhn.fr. Current address: Museum National d'Histoire Naturelle, Laboratoire d'Etude de la Matière Extraterrestre, USM 0205 (LEME), Case Postale 52, 57, rue Cuvier, 75005 Paris.
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Norman H. Sleep
Norman H. Sleep
Department of Geophysics, Mitchell Building, Stanford University, California 94305, USA
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Kevin Zahnle
Kevin Zahnle
NASA Ames Research Center, Mail Stop 245-3, Moffett Field, California 94035, USA
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Don L. Anderson
Don L. Anderson
California Institute of Technology, Seismological Laboratory 252-21, Pasadena, California 91125, USA
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Published:
January 2005

Models for noble gases in the Earth's mantle are evaluated against a number of observational constraints: (1) high 3He/4He ratios do not correlate with high (initial) 3He concentrations; (2) the 3He/4He data for mid-ocean ridge basalts and ocean island basalts do not represent two different distributions (Anderson 2001); (3) globally robust correlations between 3He/4He ratios and lithophile isotopic systems are not observed; (4) diverse local correlations exist that are broadly linear; (5) large, local geographical 3He/4He variations are observed, which are inconsistent with a strongly localized (i.e., plume-stem) flux of high-3He/4He material; and (6) dramatic temporal 3He/4He variations are observed on very short time scales (102 years). Layered (reservoir) models for noble gases, in which a deep and radially constrained region of the Earth's mantle preserves unradiogenic He and Ne isotopic compositions because of a high noble gas concentration, do not seem consistent with these observations. Heterogeneous (nonlayered) mantle models for noble gases, in which the carrier of unradiogenic He is a relatively noble gas–poor component scattered in the (upper) mantle, appear more consistent with the constraints.

We propose that the carrier of unradiogenic noble gases is primarily olivine. 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 of chemical composition, fertility, age, and isotopic signatures (Meibom and Anderson, 2004). The isotopic signatures of oceanic basalts, including noble gases, are obtained by partial melting of the SUMA under slightly different pressure and temperature (P-T) conditions, i.e., different degrees of partial melting and different degrees of homogenization prior to eruption (Morgan and Morgan, 1999; Meibom and Anderson, 2004; Rudge et al., 2005; Ito and Mahoney, 2005). Unradiogenic noble gas isotopic compositions are not tracers of deep-mantle components in the source materials of oceanic basalts. Noble gas isotopic compositions may, however, indirectly indicate potential temperature, because the order in which different upper-mantle lithologies melt depends on the P-T conditions.

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Contents

GSA Special Papers

Plates, plumes and paradigms

Edited by
Gillian R. Foulger
Gillian R. Foulger
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James H. Natland
James H. Natland
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Dean C. Presnall
Dean C. Presnall
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Don L. Anderson
Don L. Anderson
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Geological Society of America
Volume
388
ISBN print:
9780813723884
Publication date:
2005

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