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The effect of initial230Th disequilibrium on young U[sbnd]Pb ages: the Makalu case, Himalaya

Authors
Journal
Earth and Planetary Science Letters
0012-821X
Publisher
Elsevier
Publication Date
Volume
67
Issue
2
Identifiers
DOI: 10.1016/0012-821x(84)90114-6

Abstract

Abstract Comparative U Pb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic 206Pb which originate from an excess or deficit of 230Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by U Th fractionation between the crystallising mineral and the magma. Therefore, the U Pb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess 206Pb due to excess 230Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of 206Pb due to deficiency of 230Th. The extent of this correction depends on the degree of Th U fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of 206Pb were, with reference to the amount of radiogenic 206Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the Th U partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic 207Pb originating from the decay of initial excess 231Pa, also enriched during crystal growth. The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for Th U fractionation in continental crust. The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected 238U 206Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.

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