Low-Frequency Measurements of Seismic Moduli and Attenuation in Antigorite Serpentinite

Laboratory measurements of seismic moduli and attenuation in antigorite serpentinite at a confining pressure of 200 MPa and temperatures up to 550 degrees C provide new results relevant to the interpretation of geophysical data in subduction zones. A polycrystalline antigorite specimen was tested via forced oscillations at small strain amplitudes and seismic frequencies (millihertz to hertz). The shear modulus has a temperature sensitivity, partial derivative G/partial derivative T, averaging -0.017 GPa/K. Increasing temperature above 500 degrees C results in more intensive shear attenuation (Q(G)(-1)) and associated modulus dispersion, with Q(G)(-1) increasing monotonically with increasing oscillation period and temperature. This "background" relaxation is adequately captured by a Burgers model for viscoelasticity and possibly results from intergranular mechanisms. Attenuation is higher in antigorite (log(10)Q(G)(-1) approximate to -1.5 at 550 degrees C and 0.01 Hz) than in olivine (log(10)Q(G)(-1) << -2.0 below 800 degrees C), but such contrast does not appear to be strong enough to allow robust identification of antigorite from seismic models of attenuation only.