Seismological evidence for metastable olivine inside a subducting slab

Abstract

THE nature and location of the olivine–spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle, due to the different temperature distribution inside the slabs. Two models have been proposed for this phase transition: one in which the location of the phase boundary between olivine and modified (β-phase) spinel is determined by equilibrium thermodynamics¹, and the other including a metastable olivine phase which persists to a depth of ∼550 km (ref. 2). The location of the olivine–spinel transition in the slab may be relevant to the generation of deep earthquakes^3–5, and to the buoyancy forces driving subduction⁶. Here we use travel-time residuals from deep earthquakes recorded by the dense seismograph network in Japan to investigate the configuration of the olivine–spinel phase boundary inside the subducting Pacific plate. Theoretical travel-time residuals for the equilibrium model do not fit the observed residuals, whereas those for the metastable model do, implying the presence of metastable olivine inside the subducting slab.