Identification of inherited radiogenic Pb in monazite and its implications for U–Pb systematics

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.