Keeping Time with Earth's Heaviest Element

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Science  30 Mar 2012:
Vol. 335, Issue 6076, pp. 1585-1586
DOI: 10.1126/science.1220333

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Uranium is the heaviest naturally occurring element on Earth. It has three natural isotopes (238U, 235U, and 234U), of which 238U and 235U are the parent nuclides of the 238U- and 235U-decay series chains, which ultimately decay to stable isotopes of lead (Pb), thereby forming the basis of the U-Pb chronometer. Conventional theories of stable isotope fractionation have dictated that uranium is too heavy to display resolvable mass-dependent isotope effects. The expectation was that Earth would display homogeneous 238U/235U isotopic compositions. The convention has been to adopt an invariant present-day 238U/235U ratio equal to 137.88 throughout the solar system, on the basis of early studies of uranium ore deposits. This critical assumption, which underpinned the veracity of the U-Pb chronometer for the past 30 years, was overturned by the discovery of surprisingly large 238U/235U variations in Earth's surface environments (1, 2). On page 1610 of this issue, Hiess et al. (3) report the 238U/235U composition of a large suite of U-bearing accessory minerals to facilitate a more accurate U-Pb geochronometer. These new results also provide fundamental but unexpected insights into the mechanisms controlling 238U/235U fractionation.