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

Abstract

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