Electron microscope study of the reaction olivine plus H2O+TiO2 -> titanian clinohumite plus Titanian chondrodite synthesized at 8 GPa, 1300 K

The nucleation and growth processes of titanian clinohumite (Ti-Cl) and tit anian chondrodite (Ti-Ch) during hydrous alteration of olivine at 8 GPa, 13 00 K are described in detail. We observed two different nucleation sites an d orientation relationships for Ti-Cl: (1) heterogeneous nucleation at oliv ine grain or phase boundaries, yielding semicoherent (001) lamellae with (0 01)(ol)//001)(Ti-Cl) and (010)(ol)//010)(Ti-Cl); (2) formation of coherent nanometer-sized nuclei inside olivine with the orientation relationship (10 0)(ol)//001)(Ti-Cl) and (011)(ol)//(020)(Ti-Cl). Only the former, for which the interface with olivine facilitates the transport of OH- and Ti necessa ry for the formation of the humite-group minerals, survives to large sizes. These results suggest that Ti-Cl should form readily in a Ti- and H-rich e nvironment at mantle temperatures, independent of F content, from shallow e nvironments to those at depths exceeding 250 km. and could play a role in r ecycling water back into the deep mantle.