Deformation of silicate garnets: Brittle-ductile transition and its geological implications

To understand the deformation behavior of silicate garnets, we performed experiments on six representative silicate garnets at temperatures (T) of 1173-1673 K, strain-rates ((epsilon) over dot) of 10(-7)-10(-4)/s and well-controlled thermodynamic conditions. On the basis of mechanical data, microstructures and the comparison between experimental results with deformation of natural garnets, this study yields three new insights into the deformation behavior of silicate garnets. (1)The critical temperature (T-c) for the brittle ductile transition of garnet deformation, in terms of melting temperature (T-m) and strain-rate ((epsilon) over dot), can be described by an empirical equation: T-c = T-m [(1.043 +/- 0.032) + (0.030 +/- 0.001)log((epsilon) over dot)]. In the ductile regime, where T > T-c = T-m [1.075 + 0.029log((epsilon) over dot)], steady-state creep of garnets follows a power law: (epsilon) over dot = A(sigma/mu)(n) exp(-g T-m/T), where n = 3.0 +/- 0.5, lnA(/s) = 40.1 +/- 5.6, g = 32 +/- 2, sigma is the flow stress at steady-state creep, and mu is sheer modulus of garnet. (2) A microstructural investigation suggests that crystal plasticity, enhanced by the activation of dislocation glide (slip systems 1/2 [111]{1 (1) over bar 0}), is responsible for the brittle-ductile transition. (3) Extrapolation of the experimental results to geological strain-rates (10(-16)-10(-14)/s) suggests that the brittle-ductile transition of silicate garnets in nature occurs at T> 0.65-0.70 T-m. This indicates that crustal garnets such as almandine, pyralspite and grossular can be deformed in ductile fashion under extremely high temperature (T > 1123 K). The extrapolation also shows that in the crust, garnet (e.g., "pyralspite") is much stronger than quartz and feldspar at temperature lower than 1123 K, but the rheological contrast of garnet with quartz and particularly with feldspar is minimal at temperature higher than 1123-1173 K. In the upper mantle, however, pyrope is invariably about two orders of magnitude stronger than olivine, and the rheological contrast between these two minerals is almost constant. We conclude that silicate garnets are mostly rigid and brittle in the crust, but ductile as long as the conditions of high temperature and low strain-rate are satisfied.