Effect of temperature on elastic properties of biotite and biotite-rich rocks: estimation from experiment and molecular dynamics simulation

We measured the P-wave velocities (V-P) of mafic high-grade metamorphic rocks at 1.0 GPa pressure and a range of temperatures from 25 to 400 degrees C, along orthogonal directions corresponding to macroscopic fabric elements in the specimen. Our results showed that biotite-rich rocks (over 15 vol per cent biotite) displayed a strong decrease in V-P with increasing temperature whereas biotite-free rocks did not. The anisotropy of V-P (AV(P)) in biotite-rich rocks also increased with increasing temperature because of differential V-P reduction, indicating a thermal dependence of the elastic properties of biotite. We conducted molecular dynamics simulations to investigate the effect of temperature on the elastic stiffness constants (C-ij) of phlogopite, an end member of the biotite solid-solution series. The simulations indicated that C-ij of phlogopite have a strong anisotropy and temperature dependence. We then estimated C-ij of the bulk rock by using the Voigt-Reuss-Hill average procedure based on the estimated C-ij of phlogopite, reference values for C-ij of hornblende and plagioclase and measured crystal preferred orientations. From the resulting values of bulk-rock C-ij, we calculated the quasi-V-P and anisotropy parameter epsilon, under the assumption of transverse isotropy, and confirmed that temperature had a clear effect on velocity whereas AV(P) remained stable. This result suggests that the thermal effect on V-P of biotite-rich rocks can be explained by the thermal dependence of C-ij of phlogopite, which we attribute to a lattice rearrangement in phlogopite. This finding implies that the elastic properties of micas may change and affect the elastic properties of bulk rocks in the absence of mechanical breakage, dehydration reactions or phase changes. The thermal effect on AV(P) of biotite-rich rocks cannot be explained by a thermal dependency of the elastic constants of phlogopite. We speculate that the local concentration of thermal stress on biotite may account for the thermal dependency of AV(P) of biotite-rich rocks.

Automated summary

The P-wave velocities (V-P) of mafic high-grade metamorphic rocks were measured, and it was found that the rocks rich in biotite showed a significant decrease in V-P with increasing temperature. Molecular dynamics simulations revealed that the elastic stiffness constants (C-ij) of phlogopite, a type of biotite, have strong anisotropy and temperature dependence. The thermal effect on V-P of biotite-rich rocks can be explained by the thermal dependence of C-ij of phlogopite, suggesting a rearrangement of the phlogopite lattice.