Seismic properties of layer 2 basalts

Abstract

In this paper the physical properties of oceanic layer 2 basalts are examined with the emphasis on microcrack porosity and alteration-related changes in physical properties. Over 160 core samples from some of the more significant Deep Sea Drilling Project/Ocean Drilling Program holes, including 417D, 418A, 504B and 801C are included in the present investigation. Overall physical property relationships are discussed in the first half of this study. At 200 MPa confining pressure, compressional-and shear-wave velocities of the sample suite have modal values of 6.1 kms-1 and 3.5 kms-1, respectively. A Poisson's ratio of 0.28 characterizes the majority of the basalts. Samples that have experienced extensive low-temperature alteration, identified through elevated K2O and decreased CaO and MgO contents, exhibit low velocities (and densities) and high Poisson's ratios. The microcrack porosity in the basalt suite is examined using velocity-pressure data, a pore-aspect-ratio inversion scheme, and scanning electron microscope (SEM) imaging. Microcracks sealed by alteration minerals such as smectite and calcite or healed by late crystallization are common throughout the sample set. Rocks from the transition zone and sheeted dyke section of Hole 504B are marked by numerous smectite- and chlorite-filled cracks. More significantly, the data indicate an increase of low-aspect-ratio open microcracks with increasing depth in samples from several DSDP holes, particularly Hole 504B. These microcracks, also observed using the SEM, are attributed to stress relief as the original drill cores were removed from in situ pressure conditions. Stress-relief microcracking is apparently more severe in the holocrystalline, coarser-grained samples from massive flows than in the phyric basalts with fine-grained groundmasses characteristic of pillow units. This difference in microcracking is explained through basic fracture theory.