Microcrack studies of basalts from the Iceland Research Drilling Project

Microcavities in basaltic core from the IRDP borehole in Reydarfjördur, Iceland, can be characterized by using direct observations with the scanning electron microscope (SEM) and inversion of velocity data. In the Iceland basalts, neither method provides an accurate, complete picture of the crack aspect ratio spectrum, but by using both SEM and inversion data a complete spectrum may be obtained. SEM observations provide information on high aspect ratio pores and cracks (>0.005) that cannot be separated by velocity inversion theory, while velocity inversion theory provides information on low aspect ratio cracks (< 0.005) that are too narrow to be seen with the SEM. Alteration rims along cracks, blunt ends, and aspect ratios which imply closure pressures greater than the vertical stress all provide evidence that cracks are open in situ. Cracks coincident with grain boundaries are abundant in the basalt flows and suggest an origin early in the rock's history, probably during cooling. Crack healing and alteration are observed to increase with depth, causing a shift in crack shape distribution toward higher aspect ratios.