EXPERIMENTAL DETERMINATION OF ELASTIC PROPERTIES OF TALC TO 800°C, 0.5 GPA; CALCULATIONS OF THE EFFECT ON HYDRATED PERIDOTITE, AND IMPLICATIONS FOR COLD SUBDUCTION ZONES

Abstract

We have measured compressional wave velocity (VP) in talc as a function of temperature at 0.5 GPa. VP falls from 5.27+/-0.23 km s-1 at 25°C to 4.35+/-0.15 km s-1 at 800°C. Combining these results with a previously published bulk modulus (K) and δK/δP, gives a Poisson's ratio (ν) of 0.268 and a shear modulus (G) of 22.6 GPa under atmospheric conditions. Assuming that ν is independent of temperature gives a δK/δT value of -19.3+/-0.64 MPa K-1 at 0.5 GPa. We used the experimental data, in combination with published data, to calculate bounds on the elastic moduli of a peridotite hydrated with talc just above cold subducting lithosphere. We find that an assemblage hydrated by only 0.9 wt% H2O has elastic wave velocities (calculated from the Voigt-Reuss-Hill average) lower than the anhydrous rock by an average of 7.6% for VP, and 9.7% for shear waves (Vs) at 700°C. These results are consistent with observations of converted phase behavior in the vicinity of the surface of a cold subducting slab, and may place the low-velocity layer observed in the lowermost mantle wedge, not the uppermost slab as has previously been suggested.