The boundary at 660 km that separates the mantle transition zone from the lower mantle has been a focus of controversy, in particular regarding the question whether convection involves the whole mantle or is layered with a boundary at 660 km. In her Perspective, McCammon explains how tiny inclusions in diamonds from the top of the lower mantle are helping to resolve these controversies. The deep diamonds favor a hybrid model where limited mixing occurs between the upper and lower mantle, but chemical inhomogeneities are preserved.
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Catherine McCammon (2001) studied this question.
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