Metastable gamet in oceanic crust at the top of the lower mantle

Tomoaki Kubo, Eiji Otani, Tadashi Kondo, Takumi Kato, Motomasa Toma, Tomofumi Hosoya, Asami Sano, Takumi Kikegawa, Toshiro Nagase

Research output: Contribution to journalArticlepeer-review

38 Citations (Scopus)


As oceanic tectonic plates descend into the Earth's lower mantle, garnet (in the basaltic crust) and silicate spinel (in the underlying peridotite layer) each decompose to form silicate perovskite - the 'post-garnet' and 'post-spinel' transformations, respectively. Recent phase equilibrium studies, have shown that the post-garnet transformation occurs in the shallow lower mantle in a cold slab, rather than at ∼800 km depth as earlier studies indicated, with the implication that the subducted basaltic crust is unlikely to become buoyant enough to delaminate as it enters the lower mantle. But here we report results of a kinetic study of the post-garnet transformation, obtained from in situ X-ray observations using sintered diamond anvils, which show that the kinetics of the post-garnet transformation are significantly slower than for the post-spinel transformation. Although metastable spinel quickly breaks down at a temperature of 1,000 K, we estimate that metastable garnet should survive of the order of 10 Myr even at 1,600 K. Accordingly, the expectation of where the subducted oceanic crust would be buoyant spans a much wider depth range at the top of the lower mantle, when transformation kinetics are taken into account.

Original languageEnglish
Pages (from-to)803-806
Number of pages4
Issue number6917
Publication statusPublished - 2002 Dec 26

ASJC Scopus subject areas

  • General

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