The effects of alteration and porosity on seismic velocities in oceanic basalts and diabases

Abstract

Seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases, chiefly to clays. Porosity accounts for 75-80% of the velocity variation in sonic log velocities in the lava sections of Holes 504B and 1256D, but the effect of alteration on the properties of the basalts has not been assessed. In this analysis, the grain velocities in basalt and diabase samples are estimated from an empirical linear relationship between grain density and the P wave modulus. The theoretical velocity in fresh, zero-porosity basalt, or diabase is 6.96±0.07 km/s. Grain velocities in the diabase samples are statistically indistinguishable from the theoretical velocity, and show no variation with depth; alteration does not significantly affect the velocities in the diabase samples from Hole 504B. This result is consistent with previous analyses, which demonstrated that velocities in the dikes are controlled by crack porosity. In basalt lab samples, alteration reduces the average sample grain velocity to 6.74±0.02 km/s; cracks at the sample scale further reduce the velocity to 5.86±0.03 km/s, and large-scale cracks in the lavas reduce the average in situ velocity to 5.2±0.3 km/s. Cracks account for nearly 90% of the difference between seismic (in situ) velocities and the theoretical velocity in the unaltered solid material. Basalt grain velocities show a small, but significant systematic increase with depth; the influence of alteration decreases with depth in the lavas, reaching near zero at the base of the lavas in Holes 504B and 1256D. Key Points: P wave velocities in unaltered, zero-porosity basalt are near 7 km/s Alteration has a only small affect, reducing velocities to about 6.7 km/s Cracks reduce velocities from 6.7 to in situ velocities near 5.2 km/s