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1423112 
Journal Article 
238U/235U Systematics in terrestrial uranium-bearing minerals 
Hiess, J; Condon, DJ; Mclean, N; Noble, , SR 
2012 
1 
Science
ISSN: 0036-8075
EISSN: 1095-9203 
335 
6076 
1610-1614 
English 
22461608 
WOS:000302062600041 
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. 
DECAY CONSTANTS; ISOTOPIC COMPOSITION; FRACTIONATION; CM-247; GEOCHRONOLOGY; ABUNDANCE; ANOXIA