SHEAR-PROMOTED PHASE-TRANSITIONS IN FE2SIO4 AND MG2SIO4 AND THE MECHANISM OF DEEP EARTHQUAKES

Phase transformations in end-member olivines have been investigated in the temperature range comparable to the interior of subducting slabs. This work constitutes the experimental evidence that the kinetics of transformation of silicate olivine (alpha phase) to modified spinel (b eta) and spinel (gamma) phases is enhanced by shear deformation. Natur al fayalite (alpha-Fe2SiO4 ) subjected to a pressure gradient from 0 t o 25 GPa at 380-degrees-C in the diamond anvil cell (DAC) developed a ring of gamma phase where the pressure was in the stability field of t he gamma phase and shear stress was large enough to promote the alpha- ->gamma transition. The sample inside the ring, despite being at highe r pressure, remained dominantly as alpha phase. The outermost, lower-p ressure region of the sample also remained as alpha phase. In the Mg2S iO4 system, the transition from alpha to beta was observed at 575-degr ees-C in runs in which pressure covered the stability fields of beta p hase, gamma phase and mixed oxides. These results show that the charac teristic transformation temperature T(Ch) can be lowered as much as ap proximately 200-degrees-C by shear deformation. On the basis of these observations, we propose a nonhydrostatic kinetic boundary for the alp ha-->beta and alpha-->gamma transitions in mantle olivine. For tempera tures below this boundary, the transformations are kinetically inhibit ed, while above it, the transformations ran be promoted by shear defor mation. Therefore, olivine carried to a depth of several hundred kilom eters in a subducting slab can remain as metastable a phase until shea r deformation causes its transformation. We suggest that this region o f shear-promoted transformation in the cold interior of the subduction zone is responsible for the generation of deep earthquakes.