GEOCHEMISTRY: Plumbing the Depth

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Science  21 Jul 2000:
Vol. 289, Issue 5478, pp. 361b
DOI: 10.1126/science.289.5478.361b

Recently, several minerals that are thought to come from Earth's lower mantle (which lies at depths below about 670 kilometers) have been identified. These minerals, preserved as minute inclusions in diamonds, represent Earth's deepest sampled minerals and thus provide direct information on the nature of Earth's deep interior. They also demonstrate that there has been at least some material transfer across the boundary between the upper and lower mantle, which bears on a longstanding question regarding Earth's dynamics.

Now, Kerschhofer et al. have examined large (up to several millimeters across) baddeleyite crystals (ZrO2) from a kimberlite in the Congo. Transmission electron microscopy analysis of twin microstructures suggests that these grains likely formed with cubic symmetry, which is the stable phase at temperatures above 2370°C. Such high temperature would require an origin in the deep lower mantle (perhaps 2000 kilometers or deeper). Because baddeleyite also readily incorporates uranium and other elements useful for dating and geochemical analyses, these inclusions may provide important clues about the composition and history of the lower mantle. — BH

Earth Planet. Sci. Lett. 179, 219 (2000).

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