ソリダス温度における弾性波速度の見積りと低速度層の温度構造

Abstract

Laboratory data of seismic velocities in upper mantle peridotite are used to estimate the thermal structure of the low velocity zone. Here we calculate seismic velocities at mantle solidus temperature to 10GPa by using elasticity data of mantle minerals and employing a third-order finite strain theory. We compare the calculated velocities and laboratory velocity data with seismic observations, and estimate temperatures and partial melt fractions in the low velocity zone. The comparison, for example, yields up to 4 and 2vol.% partial melt in 0-23Ma and 23-53Ma asthenosphere, respectively, beneath the Pacific Ocean. The velocity structure of the reference earth model such as PREM or 1066A indicates that the low velocity zone in average is hot (compared with the high-velocity lithosphere) but subsolidus, and partial melt exists only in regions of anomalously low velocity. Beneath the Rio Grande rift, the Salton Trough, and the Kenya rift, for example, low velocity anomalies (Vp=7.3-7.4km/s) indicate the presence of 2, 3, and 3vol.% partial melt, and 1500°C at 100km depth, 1410°C at 75km depth, and 1310°C at 50km depth, respectively. Temperatures determined here from seismic velocity data are generally consistent with those from surface heat flow. This implies that the velocity drop in the low velocity zone is mostly due to the increase in temperature.