The Nd of the Innocence

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Science  07 Jun 2013:
Vol. 340, Issue 6137, pp. 1142
DOI: 10.1126/science.340.6137.1142-c

There are few remaining geologic clues about the structure and geodynamics operating on Earth from its initial differentiation around 4.5 billion years ago until modern plate tectonics began. Debaille et al. analyzed the geochemistry of an ancient layered lava flow from Ontario, Canada, that serves as a window into a partially melted mantle plume from around 2.7 billion years ago. Excess 142Nd contents, which are daughter products of now extinct radiogenic 146Sm, imply that the mantle contained heterogeneities generated by relatively sluggish mixing—especially when considering that the temperatures and convection rates at that time would have been much higher than they are in the modern mantle. Using numerical models, the authors show that the preservation of this signature could occur in a stagnant regime of plate tectonics independently of high convection rates, meaning that early subduction events occurred only in brief and sporadic episodes. Combined with other geochemical data, the ancient lava flow data are consistent with the onset of modern (i.e. continuous) plate tectonics around 2.7 to 3.0 billion years ago.

Earth Planet Sci. Lett. 10.1016/j.epsl.2013.04.016 (2013).

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