Résumé
The Oman ophiolite interpreted as ancient oceanic lithosphere has been drilled recently, and core samples of the lower crustal and mantle sections were collected (Oman Drilling Project). Seismic velocity of those samples is important to interpret marine seismic refraction data because lithological models of oceanic lithosphere are partly based on ophiolite stratigraphy. We measured compressional and shear wave velocity (V (sub P) , V (sub S) )of core samples under high confining pressure conditions consistent with lithostatic pressure, and the experimental results are compared with velocity profile based on outcrop samples of Oman ophiolite (Christensen and Smewing, 1981). Cores were collected from the lower crustal section of the Oman ophiolite (Hole GT1A: layered gabbro, GT2A: foliated gabbro), across the boundary between the sheeted dikes and gabbro (GT3A), and across the crust-mantle transition zone (CM1A&2B). We selected olivine gabbro, gabbro, oxide gabbro and diabase as experimental samples represented from each borehole. V (sub P) and V (sub S) were measured using the pulse transmission method. Intra-vessel deformation and fluid flow apparatus at Hiroshima University were used to conduct the measurements under high confining pressure of up to 200 MPa. Two loading cycles were applied to each specimen to measure velocity successively under both dry and wet conditions. In wet condition, pore pressure was set at 10 MPa using a syringe pump. Both V (sub P) and V (sub S) slightly increased with increasing confining pressure, suggesting closing cracks. At 200 MPa under dry condition, olivine gabbro and gabbro from GT1, GT2 and CM1 show V (sub P) = 6.65 to 7.21 km/s and V (sub S) = 3.70 to 4.09 km/s. Diabase and oxide gabbro from GT3 show V (sub P) = 6.20 to 6.60 km/s and V (sub S) = 3.57 to 3.83 km/s. Although velocities of samples from GT1, GT2 and CM1 are similar in dry and wet conditions, V (sub P) of samples from GT3 under wet conditions increased relative to those determined under dry condition, suggesting that pore fluid markedly increases the rock bulk modulus due to relatively high porosity. Although the experimental results are almost consistent with data from Christensen and Smewing (1981), olivine gabbro collected from outcrops shows wide ranges of velocity and density (V (sub P) = 6.87 to 7.68 km/s, V (sub S) = 3.52 to 4.20 km/s, rho = 2.85 to 3.12 g/cm (super 3) ), reflecting probably variable olivine content.