Deformation of silicate garnets; brittle-ductile transition and its geological implications

To understand the deformation behaviour of garnets, a series of experiments were performed on six representative garnets at 1173-1673 K, strain-rates of 10 (super -7) to 10 (super -4) /s and well-confined thermodynamic conditions. It was found that the critical T (T (sub c) ) for the brittle-ductile transition of garnet deformation, in terms of melting T (T (sub m) ) and strain-rate (<$>epsilon <$>). can be described by an empirical equation T (sub c) = T (sub m) (1.043 + or - 0.032) + (0.030 + or - 0.001) log (<$>epsilon <$>)]. It is suggested that crystal plasticity, enhanced by the activation of dislocation glide, is responsible for the brittle-ductile transition. Extrapolation to geological strain-rates (10 (super -16) to 10 (super -14) /s) suggests that the brittle-ductile transition of garnets in nature occurs at T > 0.65-0.70 T (sub m) . This indicates that almandine, pyralspite and grossular can be deformed in ductile fashion under extremely high T (> 1123 K). It is concluded that garnets are mostly rigid and brittle in the crust, but ductile as long as the conditions of high T and low strain-rate are satisfied.