Abstract
The simplest interpretation of uranium–lead mineral ages in igneous rocks embodies the assumption that melting of pre-existing rocks leads to homogenization of the lead isotopes, thus resetting the radiometric 'clock'. Zircons often violate this assumption by only partially dissolving and preserving a component of lead 'inherited' from before the melting episode, but until now such behaviour has not been observed in monazite. Here we report U–Pb data for zircons and monazites from a Tertiary granite from the Himalayas, in which zircons and monazites contain inherited radio-genic lead. An analysis of a single entirely igneous zircon crystal plots on concordia at 19.5±0.4 Myr. Eight monazite fractions lie on a line from an upper intercept of 471±10 Myr to the youngest fraction, which plots slightly above concordia at 21 Myr. The presence of excess 206Pb in the youngest monazite fractions indi-cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component, rather than a resetting of 471 Myr monazites at ∼20 Myr. These data suggest a closure temperature for Pb in monazite significantly higher than previously published estimates.
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Copeland, P., Parrish, R. & Harrison, T. Identification of inherited radiogenic Pb in monazite and its implications for U–Pb systematics. Nature 333, 760–763 (1988). https://doi.org/10.1038/333760a0
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DOI: https://doi.org/10.1038/333760a0
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