Experimental determination of elastic properties of talc to 800 degrees C,0.5 GPa; calculations of the effect on hydrated peridotite, and implications for cold subduction zones

We have measured compressional wave velocity (V-p) in talc as a function of temperature at 0.5 GPa. V-p falls from 5.27 +/- 0.23 km s(-1) at 25 degree sC to 4.35 +/- 0.15 km s(-1) at 800 degreesC. Combining these results with a previously published bulk modulus (K) and deltaK/deltaP, gives a Poisson' s ratio (nu) of 0.268 and a shear modulus (G) of 22.6 GPa under atmospheric conditions. Assuming that nu is independent of temperature gives a deltaK/ deltaT 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 elast ic moduli of a peridotite hydrated with talc just above cold subducting lit hosphere. We find that an assemblage hydrated by only 0.9 wt% H2O has elast ic wave velocities (calculated from the Voigt-Reuss-Hill average) lower tha n the anhydrous rock by an average of 7.6% for V-p, and 9.7% for shear wave s (V-s) at 700 degreesC. These results are consistent with observations of converted phase behavior in the vicinity of the surface of a cold subductin g slab, and may place the low-velocity layer observed in the lowermost mant le wedge, not the uppermost slab as has previously been suggested. (C) 2000 Elsevier Science B.V. All rights reserved.