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Published April 28, 1998 | Published
Journal Article Open

The helium paradoxes


The ratio ^3He/^4He (R) plays a central role in models of mantle evolution that propose an undegassed lower mantle, rich in the primordial isotope ^3He. A large primordial volatile-rich reservoir, a feature of recent models, is inconsistent with high-temperature accretion and with estimates of crustal and bulk Earth chemistry. High R can alternatively reflect high integrated ^3He/(U+Th) ratios or low ^4He abundances, as expected in refractory portions of the upper mantle. I show that high R materials are gas-poor and are deficient in radiogenic ^4He compared with midocean ridge basalts. The seemingly primitive (i.e., high R) signatures in "hotspot" magmas may be secondary, derived from CO_2-rich gases, or residual peridotite, a result of differential partitioning of U and He into magmas. A shallow and low ^3He source explains the spatial variability and the temporal trends of R in ocean islands and is consistent with a volatile-poor planet. A shallow origin for the "primitive" He signature in ocean island basalts, such as at Loihi, reconciles the paradoxical juxtaposition of crustal, seawater, and atmospheric signatures with inferred "primitive" characteristics. High ^(238)U/^(204)Pb components in ocean island basalts are generally attributed to recycled altered oceanic crust. The low ^(238)U/^3He component may be in the associated depleted refractory mantle. High ^3He/^4He ratios are due to low ^4He, not excess ^3He, and do not imply or require a deep or primordial or undegassed reservoir. ^(40)Ar in the atmosphere also argues against such models.

Additional Information

© 1998 by The National Academy of Sciences. Contributed by Don L. Anderson, March 4, 1998. I thank David Graham, John Eiler, Des Patterson, Youxue Zhang, Pete Burnard, Masahiko Honda, Francis Albaréde, and Don Porcelli for extensive comments. Conversations with John Eiler, Ken Farley, Marc Javoy, David Graham, and Barry Hanan have been helpful. This work was supported by National Science Foundation Grant EAR 92–18390. This is contribution 5,742 from the Division of Geological and Planetary Sciences, California Institute of Technology.

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