Sound velocities and elasticity of cordierite and implications for deep crustal seismic anisotropy

Abstract

The elastic properties of cordierite, a common volatile-bearing metamorphic mineral, were measured using Brillouin spectroscopy under ambient conditions. We obtain a bulk modulus of KS =129(1) GPa, and a shear modulus of G=54.0(4) GPa. The bulk modulus of cordierite is much larger than those of other crustal framework silicates (e.g., quartz and feldspars), but is similar to KS for denser upper mantle phases such as olivine. This is likely a result of the cordierite crystal structure, as suggested by a similarly high value of KS for minerals with closely related structures. Cordierite has an unusually high K/G ratio of about 2.4, and a Poisson’s ratio of 0.31,which may be a diagnostic seismic properties of areas in which cordierite-rich metamorphic rocks occur. The overall velocity anisotropy of cordierite is relatively low (<14%) in comparison with many other metamorphic minerals. Calculated velocities for a representative lower crustal rock suggest that cordierite is not likely to explain the high seismic anisotropy observed in some lower crustal sections. Cordierite would have a strong influence on the bulk seismic anisotropy only in rocks where it is present in large concentrations and has a strong preferred orientation. Although such rocks are known to occur, they are uncommon.