Geochemistry

Fixing a Precise Date

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Science  15 Sep 2000:
Vol. 289, Issue 5486, pp. 1841
DOI: 10.1126/science.289.5486.1841a

Some of Earth's oldest rocks, in what is known as the Itsaq Gneiss Complex, are exposed in southern Greenland. These rocks are thought to harbor the primary evidence for Earth's earliest life, and their age bears on questions about the rate and origin of continental crust and the early evolution of Earth's mantle. Their exact age, however, is controversial; estimates have ranged from more than 3.85 billion years ago (Ga) to about 3.6 Ga. The obstacle is not the method—uranium-lead dating of zircons within these rocks—but that many of the zircons show intricate zoning and can yield multiple ages, and they can have older cores and younger rims. Also, the crosscutting relations of many of the rocks are complex. Nutman et al. present additional results for zircon age and field mapping, and summarize previous data on the age of these rocks. They conclude that some of the rocks, including those containing evidence for early life, date to about 3.85 Ga. — BH

Geochim. Cosmochim. Acta64, 3035 (2000).

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