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

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 measurem...

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Bibliographic Details
Published in:Science (American Association for the Advancement of Science) 2012-03, Vol.335 (6076), p.1610-1614
Main Authors: HIESS, Joe, CONDON, Daniel ], MCLEAN, Noah, NOBLE, Stephen R
Format: Article
Language:English
Subjects:
Earth sciences
Earth, ocean, space
Exact sciences and technology
Fractionation
Geochronology
Geological time
Invariants
Isotope geochemistry. Geochronology
Lithology
Mathematical analysis
Minerals
Uranium
Zircon
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Summary: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.
ISSN:0036-8075
1095-9203
DOI:10.1126/science.1215507