  Laboratory measurements of seismic velocity and attenuation in antigorite
serpentinite at a confining pressure of $2$ kbar and temperatures up to
$550^\circ$C (i.e., in the antigorite stability field) 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 (mHZ--Hz). The shear modulus has a
temperature sensitivity, $\partial G/ \partial T$, averaging $-0.017$ GPa
K$^{-1}$. Increasing temperature above $500^\circ$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 ($\log_{10} Q_G^{-1} \approx -1.5$ at
$550^\circ$C and $0.01$ Hz than in olivine ($\log_{10} Q_G^{-1} \ll -2.0$ below
$800^\circ$C), but such contrast does not appear to be strong enough to allow
robust identification of antigorite from seismic models of attenuation only.
