Geotherms in the Pacific Ocean from laboratory and seismic attenuation studies

Abstract

Seismic methods have been used extensively to measure attenuation coefficients in various regions of the Earth. Attenuation is inversely related to the quality factor Q, which may be measured directly. A low-velocity and high-attenuation zone in the upper mantle is commonly reported^1–3. Until now, there has been no direct means to derive thermal structures from such seismic investigations. Recently, Q values have been determined in the laboratory for a dry peridotite, an upper mantle rock, for pressures and temperatures of up to 0.73 GPa and 1,280 °C respectively⁴. The results indicate a quantifiable relation between Q and temperature. Here we use these results to derive geotherms from Q measurements for a region beneath the Pacific Ocean. The geotherms are consistent with temperatures obtained from heat flow observations^5,6, and imply that the oceanic upper mantle is rather dry. In asthenos-phere older than 40 Myr, geotherms for this region are sub-solidus and no partial melt is expected.