Permanent strain of thermal expansion and thermally induced microcracking in Inada granite

Abstract

I made measurements of the permanent strain of thermal expansion, compressional wave velocity, and density of microcracks of Inada granite, which was subjected to slow and homogeneous temperature change under atmospheric pressure. The permanent strain implying the cumulative amount of newly generated microcracks and the opening of preexisting microcracks appeared distinctly at a temperature between 100° and 125°C and increased almost exponentially in the range from 200°C to immediately before the temperature of α‐β transition in quartz (573°C) as the peak temperature of the thermal cycle was elevated. At 600°C, the strain increased further. The density of the microcracks was heightened as the peak temperature increased. Moreover, the said trend coincided with that of the crack density parameter estimated from the compressional wave velocity. At the same time, the maximum width of the microcracks, which was determined by microscopic observation, and the average width increment of the microcracks, calculated from the permanent strain and microcrack density, showed a similar trend. Using the mercury intrusion porosimetry method, the average width increment of the microcracks coincided with the equivalent width of the microcracks measured. There are two causes of thermally induced microcracking: the mismatching of the coefficients of thermal expansion between the adjacent mineral grains and the decrepitation of the fluid inclusions. The density of the fluid inclusion microcracks showed a remarkable increase starting from the peak temperature of 300°C. At temperatures equal to or higher than 400°C, the fluid inclusion microcracks accounted for approximately 35% of all the intragranular cracks.