A general test of the hypothesis that transformation-induced faulting cannot occur in the lower mantle

Transformation-induced faulting, which has been proposed as a mechanism for deep-focus (>300 km) earthquakes, requires a thermal runaway associated with an exothermic chemical reaction. This shearing instability has been demonstrated for several polymorphic phase transformations including the olivine → wadsleyite and olivine → ringwoodite reactions. However, it has not been experimentally tested whether a strongly exothermic decomposition reaction can also lead to this instability, as would be the case if olivine were to be carried metastably into the lower mantle and break down to magnesium silicate perovskite + magnesiowustite. Here we report experiments designed to test this possibility using the proxy reaction albite → jadeite + coesite, a highly exothermic reaction with a large ΔV. We find no mechanical or microstructural evidence for transformation-induced faulting during this reaction. Applying these results to the reaction olivine (ol) → perovskite (pv) + mw potentially occurring in subducting oceanic lithosphere at the top of the lower mantle suggests that this breakdown reaction cannot trigger earthquakes. Thus, transformation-induced faulting as a mechanism of deep earthquakes is consistent with the abrupt cessation of earthquakes at the base of the mantle transition zone, but only if subduction zones are colder than currently proposed. We discuss other reasons for termination of earthquakes at ∼700 km and find them also difficult to reconcile with experimental evidence or natural observations.