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Published August 6, 1993 | public
Journal Article

Thorium-Uranium Fractionation by Garnet: Evidence for a Deep Source and Rapid Rise of Oceanic Basalts

Abstract

Mid-ocean ridge basalts (MORBs) and ocean island basalts (QIBs) are derived by partial melting of the upper mantle and are marked by systematic excesses of thorium-230 activity relative to the activity of its parent, uranium-238. Experimental measurements of the distribution of thorium and uranium between the melt and solid residue show that, of the major phases in the upper mantle, only garnet will retain uranium over thorium. This sense of fractionation, which is opposite to that caused by clinopyroxene-melt partitioning, is consistent with the thorium-230 excesses observed in young oceanic basalts. Thus, both MORBs and QIBs must begin partial melting in the garnet stability field or below about 70 kilometers. A calculation shows that the thorium-230—uranium-238 disequilibrium in MORBs can be attributed to dynamic partial melting beginning at 80 kilometers with a melt porosity of 0.2 percent or more. This result requires that melting beneath ridges occurs in a wide region and that the magma rises to the surface at a velocity of at least 0.9 meter per year.

Additional Information

© 1993 American Association for the Advancement of Science. Received 18 March 1993; accepted 2 June 1993. We thank M. Baker and I. Hutcheon for assistance in the laboratory and D. Burnett, E. Stolper, M. Spiegelman, and P. Beattie for helpful discussions. This work was supported by a fellowship from the National Aeronautics and Space Administration (NASA) Graduate Student Researchers Program (T.Z.L.) and NASA grant NAGW-3297 (G.J.W.). Division contribution 5253 (805).

Additional details

Created:
August 20, 2023
Modified:
October 25, 2023