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


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 studies1,2 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 indicated3,4,5,6, 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 transformation7. 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.

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Figure 1: The variation of the transformed volume fraction with duration at the desired temperature.
Figure 2: Transmission electron micrographs showing microstructures of the reaction zone in the post-garnet transformation of pure pyrope at 30.8 GPa and 1,273 K.
Figure 3: Comparison between the rates of the post-spinel and post-garnet transformations under subduction-zone conditions.
Figure 4: Effects of transformation kinetics on the density of oceanic crust (mid-ocean-ridge basalt, MORB) descending into the lower mantle.


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We thank D. H. Green and B. Hibberson for discussions and information on the larger ADC anvil, and K. Fujino for providing natural pyrope crystal and comments. This work was partially supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Correspondence to Tomoaki Kubo.

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Kubo, T., Ohtani, E., Kondo, T. et al. Metastable garnet in oceanic crust at the top of the lower mantle. Nature 420, 803–806 (2002).

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