In order to better interpret results from experiments conducted at high temperatures, we have first made measurements at room temperature on samples that have previously been heat-treated to temperatures up to a maximum of 900C in order to induce thermal crack damage. Thermal cracking can occur due both to thermal expansion mismatch between different mineral phases, and to thermal expansion anisotropy within a single mineral phase. In a shallow crustal environment where the geothermal gradient is anomalously high, such as in Iceland, thermal stresses may well be large enough to induce such fracturing. Furthermore, where enough fractures propagate and link up to provide an interconnected network, they can provide permeable pathways for fluid flow which can in turn lead to embrittlement and weakening of the rock. All samples were heated in a tube furnace to the desired temperature at a controlled rate of 1C/min, then held at that temperature for one hour, before cooling to ambient temperature at the same rate. This rate results in a temperature gradient across the sample of less than 1C/cm, which is too low to cause any cracking due to thermal gradient stresses.
Thermal cracking in the basalt was monitored by measuring the compressional (P) and shear (S) wave velocities through the samples both prior to and following heat-treatment, and the results are shown in Figure 20.
Note that both P-wave and S-wave velocities remain essentially constant up to 400C, with values of about 5.3 km/s and 3.0 km/s respectively. For higher temperatures the velocities decrease rapidly, so that by 800C they have decreased to about 3.4 km/s and 2.2 km/s repectively.