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Published August 4, 1998 | Published
Journal Article Open

A model to explain the various paradoxes associated with mantle noble gas geochemistry


As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer (≈1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess "primordial"^3He is responsible for ^3He/^4He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of ^3He and excessive depletion of ^4He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the ^3He/^4He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO_2 and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High ^3He/^4He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.

Additional Information

© 1998 National Academy of Sciences. Contributed by Don L. Anderson, June 15, 1998. I thank David Graham, John Eiler, Des Patterson, Youxue Zhang, Pete Burnard, Masahiko Honda, and Don Porcelli for constructive comments. Conversations with Ken Farley, Marc Javoy, David Graham, and Barry Hanan have been helpful. Supported by National Science Foundation Grant EAR 97-26252 Contribution no. 8530, Division of Geological and Planetary Sciences, California Institute of Technology.

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August 19, 2023
October 19, 2023