5:15 PM - 6:45 PM
[SCG44-P09] Effects of confining pressure on the steady-state recrystallization microstructures of wet quartz
Keywords:quartz, high-PT deformation experiment, dynamic recrystallization, dislocation creep
Recrystallization microstructures of quartz in high-pressure metamorphic belts and ductile shear zones in the continental crusts vary with metamorphic grades. These microstructures are believed to reflect the deformation conditions such as temperature and strain rates. In order to investigate the relationship between the microstructures and deformation conditions, high-pressure and temperature (high-PT) deformation experiments, along with microstructure observation, have been carried out.
Masuda and Fujimura (1981) (M&F) carried out high-PT deformation experiments using agate, which consists of fine-grained fibrous quartz and molecular water, at a confining pressure of 0.4 GPa, temperatures of 700–1000ºC and strain rates of 10-6-10-4/sec. They reported that at low temperature and high strain rate conditions, Type S microstructure with oblate grains and serrated grain boundaries developed, while at high temperature and low strain rate conditions, Type P microstructure with polygonal grains and straight grain boundaries developed. They interpreted these quartz microstructures as steady-state microstructures, which are independent of time and strain. In contrast, Hirth and Tullis (1992) (H&T) used natural coarse-grained quartzite as a starting material at a confining pressure of 1.5 GPa, a temperature of about 500–1200ºC and a strain rate of about 10-7–10-5/sec and classified microstructures into three groups corresponding to the main recrystallisation mechanism.
In order to extrapolate the S-P boundary to the natural conditions, it is necessary to consider the dependence of the steady-state microstructure on confining pressure, but in H&T, the steady state microstructures were not attained in all experimental runs because coarse-grained quartzite was used. In addition, the criteria for classifying the microstructure of H&T were different from those of M&F. Following M&F, we performed deformation experiments of agate at the temperature range of 800–1000ºC and strain rates of 10-6–10-4/sec, but at higher confining pressures of 1.5 GPa.
The cylindrical specimens of agate were cored parallel to the original fibrous structure. Talc was used as a pressure medium and a sleeve made of pyrophyllite or talc was placed between the sample specimen and graphite heater so that water released at high temperatures is supplied to the specimen to promote plastic deformation. To facilitate the addition of water to the sample, the sample was not covered with a metal jacket.
All experiments were performed using a Kumazawa-type solid pressure deformation apparatus. One of its characteristics is that the accurate differential stress can be obtained using load cells attached to the upper and lower pistons (Shimizu and Michibayashi, 2022) (S&M). The mechanical data were fairly in good agreement with the dislocation creep flow law for quartz derived semi-empirically by Fukuda and Shimizu (2017).
Optical microscopy of thin sections after experiments showed two types of microstructures similar to Type S and P of M&F. Type S was observed at temperatures of 900 ºC and strain rates of 10-5/sec, where Type P was previously observed by M&F at 0.4 GPa confining pressure. Hence, the higher confining pressure might cause the S-P boundary shifted to the high-temperature and low-strain rate side. EBSD analysis of type S of M&F by S&M shows that the c-axis orientation of the relatively large, flat recrystallized quartz grains is concentrated in the σ1 direction, suggesting that the basal slip of the quartz facilitated the development of the S-type structure. High-PT deformation experiments of quartz conducted by Ave'Lallemant and Carter (1971) indicated that the basal slip was dominant at high confining pressures. These experimental observations suggest that the shift of the S-P boundary might have been caused by a change in the dominant slip system of the quartz.
References
Fukuda, J. and Shimizu, I. (2017) J. Geophys. Res. Solid Earth, 122, 5956-5971.
Masuda, T. and Fujimura, A. (1981) Tectonophysics, 72, 105-128.
Shimizu, I. and Michibayashi, K. (2022) Minerals, 12, 329.
Ave'Lallemant, H. and Carter, N. (1971) American J. Science, 270, 218-235.
Masuda and Fujimura (1981) (M&F) carried out high-PT deformation experiments using agate, which consists of fine-grained fibrous quartz and molecular water, at a confining pressure of 0.4 GPa, temperatures of 700–1000ºC and strain rates of 10-6-10-4/sec. They reported that at low temperature and high strain rate conditions, Type S microstructure with oblate grains and serrated grain boundaries developed, while at high temperature and low strain rate conditions, Type P microstructure with polygonal grains and straight grain boundaries developed. They interpreted these quartz microstructures as steady-state microstructures, which are independent of time and strain. In contrast, Hirth and Tullis (1992) (H&T) used natural coarse-grained quartzite as a starting material at a confining pressure of 1.5 GPa, a temperature of about 500–1200ºC and a strain rate of about 10-7–10-5/sec and classified microstructures into three groups corresponding to the main recrystallisation mechanism.
In order to extrapolate the S-P boundary to the natural conditions, it is necessary to consider the dependence of the steady-state microstructure on confining pressure, but in H&T, the steady state microstructures were not attained in all experimental runs because coarse-grained quartzite was used. In addition, the criteria for classifying the microstructure of H&T were different from those of M&F. Following M&F, we performed deformation experiments of agate at the temperature range of 800–1000ºC and strain rates of 10-6–10-4/sec, but at higher confining pressures of 1.5 GPa.
The cylindrical specimens of agate were cored parallel to the original fibrous structure. Talc was used as a pressure medium and a sleeve made of pyrophyllite or talc was placed between the sample specimen and graphite heater so that water released at high temperatures is supplied to the specimen to promote plastic deformation. To facilitate the addition of water to the sample, the sample was not covered with a metal jacket.
All experiments were performed using a Kumazawa-type solid pressure deformation apparatus. One of its characteristics is that the accurate differential stress can be obtained using load cells attached to the upper and lower pistons (Shimizu and Michibayashi, 2022) (S&M). The mechanical data were fairly in good agreement with the dislocation creep flow law for quartz derived semi-empirically by Fukuda and Shimizu (2017).
Optical microscopy of thin sections after experiments showed two types of microstructures similar to Type S and P of M&F. Type S was observed at temperatures of 900 ºC and strain rates of 10-5/sec, where Type P was previously observed by M&F at 0.4 GPa confining pressure. Hence, the higher confining pressure might cause the S-P boundary shifted to the high-temperature and low-strain rate side. EBSD analysis of type S of M&F by S&M shows that the c-axis orientation of the relatively large, flat recrystallized quartz grains is concentrated in the σ1 direction, suggesting that the basal slip of the quartz facilitated the development of the S-type structure. High-PT deformation experiments of quartz conducted by Ave'Lallemant and Carter (1971) indicated that the basal slip was dominant at high confining pressures. These experimental observations suggest that the shift of the S-P boundary might have been caused by a change in the dominant slip system of the quartz.
References
Fukuda, J. and Shimizu, I. (2017) J. Geophys. Res. Solid Earth, 122, 5956-5971.
Masuda, T. and Fujimura, A. (1981) Tectonophysics, 72, 105-128.
Shimizu, I. and Michibayashi, K. (2022) Minerals, 12, 329.
Ave'Lallemant, H. and Carter, N. (1971) American J. Science, 270, 218-235.