title: 238U/235U Systematics in terrestrial uranium-bearing minerals.
abstract: The present-day (238)U/(235)U ratio has fundamental implications for uranium-lead geochronology and cosmochronology. A value of 137.88 has previously been considered invariant and has been used without uncertainty to calculate terrestrial mineral ages. We report high-precision (238)U/(235)U measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in (238)U/(235)U values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional trends. Variation between comagmatic minerals suggests that (238)U/(235)U fractionation processes operate at magmatic temperatures. A mean (238)U/(235)U value of 137.818 ± 0.045 (2σ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and "bulk Earth" (238)U/(235)U composition.
authors.affiliation: NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, Nottinghamshire, UK. jies@bgs.ac.uk
authors.forename: Joe
authors.initials: J
authors.lastname: Hiess
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Condon
authors.forename: Noah
authors.initials: N
authors.lastname: McLean
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Noble