PerspectiveGeochemistry

Keeping Time with Earth's Heaviest Element

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

You are currently viewing the summary.

View Full Text

Via your Institution

Log in through your institution

Log in through your institution


Summary

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.