Simultaneous high P-T measurements of ultrasonic compressional and shear wave velocities in Ichino-megata mafic xenoliths: Their bearings on seismic velocity perturbations in lower crust of northeast Japan arc

Simultaneous measurements of ultrasonic compressional (V-p) and shear wave velocities (V-s) were conducted under pressures and temperatures representative of the lower crust on five mafic xenoliths derived from the northeast Japan arc crust. For comparison with the observed seismic heterogeneity of the lower crust of the northeast Japan arc, the velocity deviations (dV(p) and dV(s)) of the xenoliths were calculated using the average reference V-p (6.61 +/- 0.1 km/s) and V-s (3.76 +/- 0.1 km/s). The dV(p) and dV(s) values of the xenoliths at 700 degrees C and 0.8 GPa are 0.8% and -1.2% for hornblende gabbro, 4.8% and 4.0% for hornblende-pyroxene gabbro, 0.9% and -1.5% for amphibolite, 2.5% and 1.7% for pyroxene amphibolite, and 0.9% and -5.6% for hornblendite, respectively. We compared the velocity perturbations in the lower crust determined by seismic tomography with dV(p) and dV(s) of the xenoliths using V-p - V-s - V-p/V-s deviation diagrams.'' On the basis of our measurements, we suggest that (1) the seismically high-V-p and V-s regions beneath the Tobishima Basin consist of hornblende-pyroxene gabbro, (2) hornblende gabbro is a predominant rock type beneath the Dewa Hills and Ou Backbone Range, (3) the low-velocity anomalies beneath the active volcano areas may be caused by the existence of partial melts of hornblende gabbro, and (4) the low-V-p and high-V-s regions beneath the Kitakami Mountains consist of quartz-plagioclase-bearing rocks. Our data demonstrate that the seismic heterogeneity in the lower crust of the northeast Japan arc reflects variations in rock composition and temperature that are related to the regional geological history.