• Brantut, N.
• Hansen, L. N.
• Jackson, I.
• Mitchell, Thomas
• David, E.

Abstract

Emmanuel C. David, N. Brantut, L. N. Hansen, T. M. Mitchell and I. Jackson Laboratory measurements of seismic velocity and attenuation in antigorite serpentinite at confining pressure of 2kb and temperatures to 550°C provide new results relevant to the interpretation of geophysical data in subduction zones. A copper-jacketed polycrystalline antigorite specimen was tested via torsional and flexural forced-oscillations at small strain amplitudes (&lt;10<SUP>-5</SUP>) and low frequencies representative of the seismic band (mHz-Hz). The mechanical behaviour is independent of increasing annealing duration at 500°C; no evidence of dehydration is found in post-mortem microstructures. Both such observations indicate that the mechanical measurements were taken in the antigorite stability field. The shear modulus (G) has a temperature sensitivity ∂G/∂T averaging -0.015GPa.K<SUP>-1</SUP> for the 1s oscillation period. Increasing temperature above 500°C results in more intensive shear attenuation (Q<SUP>-1</SUP>) and associated modulus dispersion, with Q<SUP>-1</SUP> increasing monotonically with increasing oscillation period and temperature (Q<SUP>-1</SUP>≈0.03 at 550°C and 0.01Hz). This "background" relaxation is adequately captured by a Burgers viscoelastic model and arises from solid-state, diffusional processes possibly operative at grain boundaries. This new dataset reveals contrasting seismic properties between antigorite and olivine at comparable temperatures, with noticeably higher levels of shear dissipation in antigorite, and a shear modulus in antigorite that is approximately half of that in olivine....

Topics

• impedance spectroscopy
• dispersion
• grain
• copper
• annealing