Comparison of fluid inclusion decrepitation and acoustic emission profiles of Westerly granite and Sioux quartzite

Abstract

Acoustic emissions recorded during slow heating of rocks at 1 atm have generally been attributed to intergranular stresses generated by differences in thermal expansion coefficients and elastic moduli of neighboring grains. These experiments have considered only the solid phases in analysis of the results, and little attention has been given to the role of intragranular or grain boundary fluid inclusions in initiating or facilitating fracturing. In principle, fluid inclusions may be thought of as a special case in which the “ neighboring grains” are fluids rather than the usual solid mineral phases. The major differences are that (1) the number of inclusions per unit volume is usually 10 2 to 10 6 times greater than the number of mineral grains in that same volume and that (2) the volume increase with temperature for most fluids is 1–2 orders of magnitude greater than for minerals. Thus, when fluid inclusion “grains” are considered, the absolute number of grain-to-grain contacts in a given sample volume is several orders of magnitude greater than if the inclusions are not considered. Further, the magnitude of the stresses between any two adjacent grains may be much higher if one of the “grains” is a fluid inclusion, owing to the larger coefficient of thermal expansion of the fluid relative to a solid phase. As a result, substantial overpressures may be generated at the fluid inclusion “grain”-mineral grain contact, during heating experiments. These overpressures result in brittle failure when the stress around the inclusion exceeds the local strength of the host crystal. In fluid inclusion terminology, this fracturing event is referred to as decrepitation. Samples of Westerly granite and Sioux quartzite were heated at 2° C/min in a gas-flow stage in order to determine the decrepitation profiles (number of decrepitations as a function of temperature) of these rocks. Decrepitation was monitored visually during heating experiments, and was evidenced by the explosive loss of fluids from the inclusion during a fracturing event. Decrepitation produces microfractures which originate at decrepitated fluid inclusions and propagate outward into the surrounding mineral. Four different types of inclusions, based on room temperature phase ratios and distribution within the host phase, were noted in Westerly granite. Type (1) inclusions contain a single aqueous liquid, type (2) are two-phase, liquid-rich inclusions, type (3) are mostly one-phase fluid inclusions that occur in dense clusters in plagioclase and type (4) are two-phase, moderate-density, H 2O-CO 2-“salt” inclusions. Type (5) inclusions are one-phase, aqueous liquid inclusions observed in Sioux quartzite. Fluid inclusions in Westerly granite decrepitate between 75° and 573° C with peaks at 275–300°C and 400–450°C. Type (4) inclusions in Westerly granite tend to decrepitate at lower temperatures (75–300°C) than other inclusion types. Initial fracturing of the quartz host surrounding type (1) inclusions (primary decrepitation) generally occurs over the range 100–400°C and the fractures continue to propogate through the quartz (secondary decrepitation) during further heating from 400° to 573°C. Fluid inclusions in Sioux quartzite decrepitate between 188° and 573°C with a broad maximum at 250–450°C. Primary decrepitation generally occurs at 200–450°C while secondary decrepitation occurs at 350°–573°C in Sioux quartzite. Some small inclusions in both samples do not decrepitate until the quartz α-β transition temperature (573°C) is reached. Decrepitation profiles determined in this study display many similarities and differences when compared to previously published acoustic emission profiles of Westerly granite and Sioux quartzite. Owing to fundamental differences in the techniques used to measure acoustic emission and decrepitation, and the potentially different energies associated with the two events, it is unclear whether decrepitating fluid inclusions are recorded during acoustic emission studies. However, if decrepitating fluid inclusions are being recorded during acoustic emission measurements, then our results suggest that: (1) decrepitation contributes more to low temperature portions of acoustic emission profiles than to high temperature portions for both Sioux quartzite and Westerly granite; (2) compared to Westerly granite, a larger percentage of acoustic emissions in the low temperature decrepitation profile of Sioux quartzite is due to fluid inclusions; (3) decrepitating fluid inclusions, not stresses generated by solid-solid interactions, cause the majority of intragranular fracturing in quartz below the temperature of the α-β transition in both these rocks.