Probing the Mantle Past

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Science  02 Mar 2012:
Vol. 335, Issue 6072, pp. 1051-1052
DOI: 10.1126/science.1219126

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Until recently, geologists generally believed that all evidence of the first few hundred million years of Earth's 4.56-billion-year history had been erased by mantle convection and plate tectonics. Although planetary analogs with Mars and the Moon suggest that Earth likely experienced large-scale melting soon after accretion, vigorous convection in the early mantle due to higher heat production from radioactive decay, core formation, and large impacts followed by 4 billion years of crustal recycling at plate boundaries was thought to have efficiently mixed and homogenized the mantle, obliterating all signs of Earth's youthful exuberance. That all changed with the discovery of zircon grains more than 4.0 billion years old from the Jack Hills in western Australia in 1983 (1), followed by the demonstration in 2003 that geochemical signals linked to decay of short-lived radioactive isotopes are preserved in some ancient crustal terrains. The recognition of 142Nd isotopic variations, produced by decay of 146Sm with a half-life of 103 million years, in 3.8-billion-year-old terrestrial rocks (2) proved that traces of Earth's earliest history are still legible in the rock record. On page 1065 of this issue, Touboul et al. (3) report highly precise measurements of the isotopic composition of tungsten (W) in 2.8-billion-year-old “komatiites,” a type of basaltic rock known to be derived from source regions deep within the mantle. The results help to produce a clearer picture of the dynamic processes involved in the formation of Earth's early mantle.