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 conditio ns. We obtain a bulk modulus of K-S = 129(1) GPa, and a shear modulus or 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), bur is simil ar 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 hig h 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,wh ich may be a diagnostic seismic properties of areas in which cordierite-ric h metamorphic rocks occur. The overall velocity anisotropy of cordierite is relatively law (<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 t he bulk seismic anisotropy only in rocks where it is present in large conce ntrations and has a strong preferred orientation. Although such rocks are k nown to occur, they are uncommon.