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

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 K_S =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 K_S 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 K_S 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. Received: 23 Deceber 1997/ Revised, accepted: 12 October 1998