title: Metastable garnet in oceanic crust at the top of the lower mantle.
abstract: 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 approximately 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.
authors.affiliation: Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. tkubo@mail.cc.tohoku.ac.jp
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Kubo
authors.forename: Eiji
authors.initials: E
authors.lastname: Ohtani
authors.forename: Tadashi
authors.initials: T
authors.lastname: Kondo
authors.forename: Takumi
authors.initials: T
authors.lastname: Kato
authors.forename: Motomasa
authors.initials: M
authors.lastname: Toma
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Hosoya
authors.forename: Asami
authors.initials: A
authors.lastname: Sano
authors.forename: Takumi
authors.initials: T
authors.lastname: Kikegawa
authors.forename: Toshiro
authors.initials: T
authors.lastname: Nagase