Abstract
During the transformation of olivine (Fo90) to ringwoodite and wadsleyite at 18-20 GPa three transformation microstructures develop: (i) Reaction rims form around olivine by incoherent grain boundary nucleation; the growth rates of these rims decrease strongly with reaction time because the volume decrease causes elastic strain to develop. (ii) Intracrystalline, coherent ringwoodite lamellae nucleate on previously formed (100) α stacking faults. (iii) Polycrystalline ringwoodite and/or wadsleyite lenses develop in the interior of relict olivine crystals by nucleation of incoherent grains on the coherent ringwoodite lamellae. The observed intracrystalline transformation mechanism and the time-dependence of growth have important implications for predicting the depth of the olivine-spinel transformation in subducting lithosphere.
