Dry metastable olivine and slab deformation in a wet subducting slab

Abstract

A widely accepted explanation for deep-focus earthquakes is that they are caused by the delayed transformation kinetics of dry olivine, which would seem to require dry subducting slabs. However, many geochemical and geophysical observations and mineral physics data indicate that water is present within both hydrous and nominally anhydrous minerals, implying hydrated subducting slabs. The presence of metastable olivine in wet slabs is therefore paradoxical, and the hydration state of the slabs remains an open question. Here, we report results of water-partitioning experiments between olivine, wadsleyite and a major dense hydrous magnesium silicate in slabs, hydrous phase A, under water-undersaturated conditions. We show that olivine and wadsleyite coexisting with hydrous phase A are kinetically dry and contain less than 1 ppm and approximately 300 ppm water, respectively. Our results suggest that olivine and wadsleyite show dry transformation kinetics even in wet slabs. It is therefore possible that olivine transformation as a cause of deep-focus earthquakes and large slab deformation creating stagnant slabs could occur in the water-undersaturated wet slabs. These processes could be caused jointly by dehydration of hydrous minerals and the subsequent rapid phase transformation when the dehydration starts at lower temperatures than the phase transformation. Transformation kinetics of olivine may be a cause of deep-focus earthquakes even in wet slabs, according to water-partitioning experiments, which show that olivine remains relatively dry even under wet subducting slab conditions.