Shear wave attenuation and melting beneath the Mid-Atlantic Ridge

Because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting, the mapping of seismic Q beneath the mid-ocean ridge systems is a useful tool to outline boundaries between lithosphere and asthenosphere and to constrain the mechanics of the intrusion process. Our approach is to measure the differential attenuation of long-period shear waves, using a spectral ratio technique, from earthquakes on the ridge and to look for variations in attenuation with propagation direction. We correct for propagation distance and, where known, the upper mantle attenuation beneath the receiving stations. The azimuthal dependence of attenuation of S waves from an earthquake offset from the ridge axis on a transform fault indicates the existence of a low-Q zone, no wider than 100 km and confined to depths shallower than about 50 to 150 km, beneath the crest of the mid-Atlantic ridge. The absence of appreciable azimuthal variation in shear wave attenuation for an earthquake on the ridge crest suggests that the low-Q zone is at least 50 km wide. Q within such a zone must be 10 or less for long-period S waves. The most likely explanation of such a low-Q zone of limited spatial extent and with sharply defined boundaries is that the zone is a region of extensive partial melting, probably at temperatures in excess of the anhydrous solidus of mantle material. Such a region of large melt concentration is consistent with the chemistry of rocks from the mid-ocean ridges and with models of the temperature field derived from numerical calculations of flow beneath spreading centers.