Geochemistry

Split When the Going Gets Hot

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Science  12 Mar 2010:
Vol. 327, Issue 5971, pp. 1303
DOI: 10.1126/science.327.5971.1303-b

When magma within Earth cools, minerals precipitate according to their crystallization temperatures. Because the isotopic distribution of elements within the resultant mineral grains is assumed to be relatively stable at high temperatures, this information has been used to estimate the composition of the parent melt and also to deduce the region in Earth's interior where the grains formed. However, Huang et al. observed mass-dependent calcium isotope fractionation between two different silicate mineral phases (orthopyroxene and clinopyroxene) in peridotite rocks from the upper mantle. This fractionation, which was previously observed only at low temperatures between seawater and calcite, probably depends on Ca-O bond strengths and not kinetic processes related to isotopic diffusion. These measurements provide a robust estimate for Ca isotope composition of the upper mantle, which can be used to make comparisons of isotopic abundances between other former melts in the solar system, such as the Moon and meteorites.

Earth Planet. Sci. Lett. 10.1016/j.epsl.2010.01.042 (2010).

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