238U/235U Systematics in Terrestrial Uranium-Bearing Minerals

The present-day U-238/U-235 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 U-238/U-235 measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in U-238/U-235 values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional trends. Variation between comagmatic minerals suggests that U-238/U-235 fractionation processes operate at magmatic temperatures. A mean U-238/U-235 value of 137.818 +/- 0.045 (2 sigma) 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 U-238/U-235 composition.