GEOLOGY: Akilia in the Old Days

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Science  30 Jun 2006:
Vol. 312, Issue 5782, pp. 1848c
DOI: 10.1126/science.312.5782.1848c

Deciphering the early history of Earth's surface and the clues to the evolution of life it provides has been complicated greatly by the fact that few sedimentary rocks older than ∼3.5 billion years are exposed. Moreover, these samples have been subjected to cycles of high-temperature metamorphosis and strong deformation, often to the point that simply recognizing the rocks as sediments can be problematic. One of the most discussed sequences, thought to be among the oldest, is exposed in an area of roughly 0.01 km2 on the island of Akilia, West Greenland. Trace minerals in some of these rocks have been suggested to harbor carbon isotopic evidence of Earth's earliest biosphere.

Manning et al. have sought to reconstruct the history and assess the origin of these rocks by marshaling detailed mapping studies, dating of complexly zoned zircons in magmas cutting the sequence, and a wide variety of other chemical data. The weight of their analysis argues for an origin through chemical precipitation from a body of water fed by hydrothermal vents, and the oldest zircon dates support an age of more than 3.8 billion years. Thus, these rocks may offer a long-sought glimpse into Earth's early surficial history; stay tuned. — BH

Am. J. Sci. 306, 303 (2006).

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