Vp/Vs Ratio of Hornblende Gabbro Xenolith from Ichinomegata Volcano, NE Japan

In order to interpret petrologically the inhomogeneous Vp, Vs and Vp/Vs structure beneath the NE Honshu arc (Nakajima et al., 2001), we measured P-wave velocity (Vp) and S-wave velocity (Vs) of Ichinomegata hornblende gabbro xenolith simultaneously within the solidus P-T conditions (up to 700° C and 0.8 GPa) in this study. The rock sample was loaded in the talc-pyrophyllite high-pressure cell. Travel times of elastic-wave through the rock sample were determined with the pulse reflection technique. A sample length was cut into about 6mm diameter core and doubly polished to a length of about 6 mm. Before velocity measurements, the rock sample was oven-dried at 120° C for 24 hours and the H₂O was not added to the rock sample. The measured rock includes the hornblende gabbro of Ichinomegata xenolith, which has ultrabasic composition (41.3 wt% SiO₂O) and is mainly composed of plagioclase (>An₈₆, 80.6 vol% in mode ratio), aluminous hornblende (13.9 wt.% Al₂0₃, 12.5 vol%) and magnetite (6.9 vol%). At constant pressure of 0.6 GPa, the Vp of the hornblende gabbro gradually decreased from 6.83 km/s at 25° C to 6.59 km/s at 600° C. Similarly the Vs values reduce from 3.87 km/s at 25° C to 3.74 km/s at 600° C. The Vp/Vs ratio of the hornblende gabbro is 1.76 at 600° C. The Vp/Vs ratio of the hornblende gabbro is comparable to that of the lower crustal layer beneath the western side of Ou back-born range to back-arc side (Vp/Vs = 1.75-1.79), except near active volcanic areas (Vp/Vs > 1.83). Nakajima et al. (2001) indicated that the low Vp, low Vs and high Vp/Vs anomalies in the lower crust to uppermost mantle beneath the active volcanic areas were perhaps caused by inclusions containing melt. These high Vp/Vs anomalies do not consist with the Vp/Vs ratio of the hornblende gabbro in this study. It is suggested that the high Vp/Vs anomalies in the lower crust to uppermost mantle is comparable to the high temperature conditions beyond the solidus with partial melting and/or other materials.