Simultaneous Measurements of Compressional and Shear wave Velocities of Ichino-megata Ultrabasic Xenoliths: Implication for Petrology of the Northeastern Japan Arc

Compressional and shear wave velocities (Vp and Vs) of Ichino-megata xenoliths were measured simultaneously with the pulse reflection technique under the subsolidus conditions (up to 800°C and 1.0 GPa). The measured xenoliths are ultrabasic rocks (38.7-48.5 wt.% SiO2O) incorporated in the calc-alkaline andesite erupted at the Ichino-megata volcano, northeastern (NE) Japan. The measured xenoliths are hornblende gabbro, hornblende-pyroxene gabbro, and amphibolite. At 1.0 GPa, Vp of the hornblende gabbro decreases from 7.13 km/s at 25°C to 6.79 km/s at 800°C and Vs changes from 3.95 km/s at 25°C to 3.74 km/s at 800°C. Similarly, Vp and Vs of the hornblende-pyroxene gabbro decrease from 7.34 km/s and 4.20 km/s at 25°C to 7.12 km/s and 4.02 km/s at 800°C, respectively. The Vp and Vs values of the amphibolites are 7.03 ~ 7.23 km/s and 3.90 ~ 4.08km/s at 25°C and 6.76 ~ 6.92 km/s and 3.72 ~ 3.85 km/s at 800°C, respectively. Combining the measured velocities with the seismic refraction/reflection profile (Vp) and tomographic model (dVp and dVs) of the NE Japan arc, we interpret petrological characteristics of the lower crustal layer of the NE Japan arc. The measured velocities of the hornblende gabbro and amphibolite xenoliths at subsolidus conditions (600 ~ 800°C) are consistent with those estimated in the seismic profile of lower crustal layer (6.6 ~ 7.0 km/s) except for the upper part of the lower crust beneath the Kitakami Mt. region. In the tomographic model of 20 ~ 25 km depth, intermediate dVp and dVs values (-2 ~ 2 %) beneath the back arc region are also comparable to the measured velocities of the hornblende gabbro and amphibolite xenoliths at subsolidus conditions (dVp = -1.2 ~ 0.1%, dVs = -2.4 ~ 1.1 % and 600 ~ 700°C). For the active volcanic areas, the velocity values of the hornblende gabbro and amphibolite estimated at suprasolidus conditions (dVp < -2.5%, dVs < -4.1 % and > 800°C) are comparable to the low velocity anomalies in the seismic tomographic model (dVp < -3%, dVs < -4%). These low velocity anomalies in the seismic tomographic model for the lower crust of the NE Japan arc are probably caused by the difference of the temperature in the velocity values of the hornblende gabbro and/or amphibolite, except that the upper part of the lower crust beneath the Kitakami Mt. region (dVp = -6 ~ -3% and dVs = 0 ~ 4%) is probably composed of quartz and/or albite-rich rocks.