Relationship between the physical properties and the lithological variation of gabbroic core samples from Hole U1309D, IODP Exp. 304/305

Hole U1309D at the Atlantis Massif at 30 degree N, Mid-Atlantic Ridge was penetrated 1415 mbsf during IODP Exp. 304 and 305 (Blackman et al., 2006). Variable types of gabbroic cores were sampled with high recovery (74.8%). They are mainly gabbro (Cpx + Pl), olivine gabbro, gabbronorite, oxide gabbro, troctolite, olivine-rich troctolite, felsic veins and diabase. The most samples include olivine even gabbro in a narrow sense. The modal composition of olivine varies from less than 5 vol% in gabbro, gabbronorite and oxide gabbro to 85 vol% in the olivine-rich troctolite (Fig. 1). On-board measurement of the compression wave velocity of those samples from Hole U1309D shows a slightly slower range (5.0 6.7 km/s) than that (6.0 7.0km/s) of the typical gabbroic rocks [1]. Physical properties of the samples with different modal compositions of olivine were measured and analyzed in order to determine its correlation with the mineral mode, chemistry, and degree of alteration. All measurements, including porosity, density, elastic wave (P & S) velocities, are conducted under atmospheric pressure. The total alteration intensity of the samples has a negative correlation with the P wave velocity, and a positive correlation with the porosity in general. Except for the olivine-rich troctolite, the olivine mode of the samples, however, more affects to the porosity and P wave velocity as same as the serpentinization of peridotite samples [2]. This is because that olivine is easier to be altered (serpentinized) compared to other phases in gabbroic rocks. The connectivity of the olivine grains in the samples also influence to reduce the velocity. The result of our study imply the possibility of the reversal velocity structure in the oceanic lower crust if it is olivine-rich and is sufficiently serpentinized, although the pressure effects should be considered. [1] Blackman et al.(2006) Proc. IODP Exp. 304/305; [2] Christensen (1996) JGR, 101; Escartine and Hirth (2001) Geology, 29.