Geochemistry

Any Clues Left of the Early Crust?

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Science  10 Aug 2001:
Vol. 293, Issue 5532, pp. 1015
DOI: 10.1126/science.293.5532.1015d

The Earth's crust and mantle have different chemical compositions. Hence it might be expected that the timing of the extraction of the crust from the mantle of the early Earth would be recorded in the isotopic signatures of radioactive elements and their daughter products in these two reservoirs. This simple notion, however, is complicated by several factors. For example, some of the crust is recycled at subduction zones, and the mantle cannot be considered as a single or simple reservoir.

Albarède suggests that many commonly used isotopic systems, including Sm-Nd, Rb-Sr, Lu-Hf, and several of the U-Pb isotopes, have attained secular equilibrium in the Earth and do not record a memory of the early differentiation of the crust, and also that their systematics require the presence at depth in the mantle of an additional reservoir containing early recycled crust. One Pb isotope with a shorter half-life (207Pb) and 4He, which is largely produced by its decay, however, are not at secular equilibrium and may yet provide information on the early differentiation and recycling of the crust, a possibility addressed by Moriera and Kurz. — BH

Earth Planet. Sci. Lett.189, 59; 47 (2001).

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