Diamond Window into the Lower Mantle

Tiny inclusions in diamonds reveal subduction of oceanic crust to depths of at least 700 km. Nature's secrets are often well hidden, but painstaking investigation of minute quantities of material may unravel complex histories of mineral formation and provide major insights into Earth's evolution. On page 54 of this issue, Walter et al. (1) illustrate this point by revealing the extent of the subduction of oceanic crust into Earth's interior. They found that natural diamonds, carried to the surface by kimberlite volcanoes 92 to 95 million years ago in Juina (Brazil), contained minute (0.015 to 0.040 mm long) inclusions composed of several minerals such as nepheline, Nakalsilite, and MgFe-spinel. These minerals are expected to form at depths of less than 200 km. However, careful investigation showed that these minerals had formed by the breakdown of other minerals known to form only at very high pressures and depths in excess of 700 km.

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