Thermal runaway and frictional melting in MORB-composition garnetite at high pressure: Implications for remote triggering of earthquakes in the transition zone

• Shear heating is very efficient in MORB garnetite compared with surrounding mantle in the transition zone. • Combined with the low solidus temperature thermal runaway to melting is more likely in MORB garnetite. • Thermal run away and shear localisation in MORB provides a viable mechanism for remote triggering of deep earthquakes. The origin of deep earthquakes remains enigmatic, but some seismic studies imply that in the deep transition zone transformation faulting cannot be the only (or even the major) mechanism. Here we present samples of co-existing basaltic-composition garnetite and San-Carlos-composition wadsleyite which were simultaneously deformed, resulting in a shear failure. Runaway frictional heating along the shear plane resulted in melting within the garnetite sample but not the wadsleyite sample. This fundamental difference in rupture evolution is also seen in the failure angles in the two samples, with the failure angle in the garnetite sample consistent with a low coefficient of friction, unlike the wadsleyite-hosted fault. Numerical shear-heating models confirm that the difference in behaviour is caused by the difference in thermal diffusivity of garnet and wadsleyite. We suggest therefore that thermal runaway is a viable mechanism for producing seismicity in crustal portions of subducted slabs in the deep transition zone.