Seismic Velocities and Elastic Properties of Oceanic Gabbroic Rocks from Hole 735B

Abstract

The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, V p and V s were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. , reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in V p , both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among V p values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of V s of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.

Geochemical and Petrological Constraints on Velocity Behavior of Lower Crustal and Upper Mantle Rocks from the Fast-Spreading Ridge at Hess Deep

“…As illustrated in Figure 11, drilling at Hole 735B recovered a thick section (>500 m) of gabbroic rock that exhibited a vague but identifiable increase in velocity with depth (data from Iturrino et al, 1991). The samples recovered from Hess Deep also have a distinct increase in velocity with depth (Fig.…”

“…Since moderately dipping (average 75° relative to core axis) magmatic foliation was commonly recognized in the gabbroic rocks from Site 894 (Gillis, Mével, Allan, et al, 1993), and because minicores were taken perpendicular and parallel to the core axis, maximum velocities for these samples would be at some oblique angle to the minicore axis. This type of preferred mineral orientation coupled to velocity decrease has been described for a much more extensive data set from Hole 735B, Southwest Indian Ridge (Iturrino et al, 1991).…”

“…Gabbro compositions compiled by Birch (1961) have a MAW of 21.8. According to lines of MAW corrected to 2 kb (Iturrino et al, 1991), gabbroic rocks from the Hess Deep area have comparable values of 20.5 to 22, with an average of about 21.3. In contrast, dunites (excluding Fe-rich compositions) in Birch's compilation (1961) have a MAW of 21.…”

Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge

“…Furthermore, when the degree of serpentinization is between 20 -40 %, the P-and S-wave velocities and the density of partially serpentinized peridotite remain in the same range as those of fresh or altered mafic rocks (Iturrino et al, 1991).…”

Does Anisotropy of Thermal Contraction Control Hydrothermal Circulation at the Moho Level below Fast Spreading Oceanic Ridges?

“…All peridotites from the shallow suboceanic mantle are affected by serpentinization, whose rate, based on correlation with density measurements (Christensen, 1966), cluster between 30 and 60% for drilled samples from Hess Deep (Iturrino and Christensen, 1990) and for samples collected in Oman (Dewandel, 1996). Dewandel et al (2003) have explored possible relationships of the penetrative lizardite network with the primary olivine and orthopyroxene LPO, in FIG.…”