Possible mechanism for seismic attenuation in rocks containing small amounts of volatiles

This report presents measurements of the specific dissipation factor Q −1 s of rocks containing small amounts of volatiles. Q −1 s was measured for shear waves as a function of relative partial pressure P/P 0 for benzene, hexane, ethanol, methanol, and water. The measurements were carried out at about 10 kHz with the vibrating bar technique in a chamber in which the relative partial volatile pressure P/P 0 was varied between almost zero and about 0.9. The results revealed that in the regime of one‐ or two‐monolayer coverage of absorbed volatiles, Q −1 s increased dramatically with exposure to the alcohols and water but only negligibly with exposure to the hexane and benzene. The slopes of Q −1 versus monolayer coverage appeared to correlate with the dipole moment per unit volume of the volatiles. These data are presented in the context of previous measurements, which showed that the influence of volatiles on Q −1 persist to high levels of outgassing (1×10 −10 torr vacuum), to elevated hydrostatic confining pressures (at least 0.5 kbar), and for a variety of crystalline rocks including terrestrial analogs of lunar basalt. Both direct (ellipsometry data) and indirect evidence (absorption isotherm data) are presented for the presence of thin films of adsorbed volatiles at low partial vapor pressures. Finally, the measurements are discussed and interpreted in terms of a physical model relating Q −1 to the relative mass of the adsorbed volatiles, the surface area of the rock, the heat of desorption of the volatile, and the rock temperature.