We have developed a comprehensive experimental capability to study thermally induced microfractures in rock. We count acoustic emissions and measure compressional wave velocities while a sample is heated to a maximum temperature of 300°C under confining pressures up to 55 MPa. After a heating cycle we measure crack porosity and crack compressibility. We count the cracks under an SEM, sorting them by grain boundary and intragranular types and by mineral pair occurrence. Our observations reveal that (1) significant thermal cracking occurs in granites heated above 200° or 250°C under 28 and 55 MPa and above 100°C in granites heated under 7 MPa, (2) most newly created thermal cracks close at pressures below 40 MPa (1.5 km depth), (3) grain boundaries between quartz grains or between quartz and another mineral are preferentially cracked, (4) the threshold temperature for acoustic emissions is a positive, linear function of pressure, (5) total acoustic emission counts in Westerly granite heated to 300°C are slightly lower under 55 than 28 MPa confining pressure and are substantially lower under 28 than 7 MPa, (6) the compressional velocity decreases 22% as Westerly granite is heated to 300°C at 7 MPa, 15% at 28 MPa, and 10% at 55 MPa, and (7) greater hysteresis in compressional velocities occurs in samples heated at lower pressures. The acoustic emission counts in Westerly granite during heating show one or more peaks of activity as a function of temperature, with more peaks at lower pressures. We interpret this result to mean that additional crack populations are able to open at lower confining pressures. The curves of acoustic emission count rates can be fit by summing one to three Gaussian density functions.

Thermal stress cracking in granite

Progress in seismic methodology and ambitious large-scale seismic projects are enabling high-resolution imaging of the continental crust. The ability to constrain interpretations of crustal seismic data is based on laboratory measurements on rock samples and calculations of seismic properties. Seismic velocity calculations and their directional dependence are based on the rock microfabric, which consists of mineral aggregate properties including crystallographic preferred orientation (CPO), grain shape and distribution, grain boundary distribution, and misorientation within grains. Single-mineral elastic constants and density are crucial for predicting seismic velocities, preferably at conditions that span the crust. However, high-temperature and high-pressure elastic constant data are not as common as those determined at standard temperature and pressure (STP; atmospheric conditions). Continental crust has a very diverse mineral composition; however, a select number of minerals appear to dominate seismic properties because of their high-volume fraction contribution. Calculations of microfabric-based seismic properties and anisotropy are performed with averaging methods that in their simplest form takes into account the CPO and modal mineral composition, and corresponding single crystal elastic constants. More complex methods can take into account other microstructural characteristics, including the grain shape and distribution of mineral grains and cracks and pores. Dynamic or active wave propagation schemes have recently been developed, which offer a complementary method to existing static averaging methods generally based on the use of the Christoffel equation. A challenge for the geophysics and rock physics communities is the separation of intrinsic factors affecting seismic anisotropy, due to properties of crystals within a rock and apparent sources due to extrinsic factors like cracks, fractures, and alteration. This is of particular importance when trying to deduce crustal composition and the state of deformation from seismic parameters.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016RG000552

Seismic properties and anisotropy of the continental crust: Predictions based on mineral texture and rock microstructure

The effects of grain size on the elastic properties of quartz through the α–β phase transition have been investigated by resonant ultrasound spectroscopy. It is found that there are three regimes, dependent on grain size, within which elastic properties show different evolutions with temperature. In the large grain size regime, as represented by a quartzite sample with ~100–300 µm grains, microcracking is believed to occur in the vicinity of the transition point, allowing grains to pull apart. In the intermediate grain size regime, as represented by novaculite (1–5 µm grain size) and Ethiebeaton agate (~120 nm grain size), bulk and shear moduli through the transition follow closely the values expected from averages of single crystal data. The novaculite sample, however, has a transition temperature ~7 °C higher than that of single crystal quartz. This is assumed to be due to the development of internal pressure arising from anisotropic thermal expansion. In the small grain size region, agates from Mexico (~65 nm) and Brazil (~50 nm) show significant reductions in the amount of softening of the bulk modulus as the transition point is approached from below. This is consistent with a tendency for the transition to become more second order in character. The apparent changes towards second order character do not match quantitative predictions for samples with homogeneous strain across elastically clamped nanocrystals, however. Some of the elastic variations are also due to the presence of moganite in these samples. True 'nanobehaviour' for quartz in ceramic samples thus appears to be restricted to grain sizes of less than ~50 nm.

http://iopscience.iop.org/article/10.1088/0953-8984/20/7/075229/pdf

Grain size dependence of elastic anomalies accompanying the α–β phase transition in polycrystalline quartz

To interpret seismic images, rock seismic velocities need to be calculated at elevated pressure and temperature for arbitrary compositions. This technical report describes an algorithm, software, and data to make such calculations from the physical properties of minerals. It updates a previous compilation and Excel® spreadsheet and includes new MATLAB® tools for the calculations. The database of 60 mineral end-members includes all parameters needed to estimate density and elastic moduli for many crustal and mantle rocks at conditions relevant to the upper few hundreds of kilometers of Earth. The behavior of α and β quartz is treated as a special case, owing to its unusual Poisson's ratio and thermal expansion that vary rapidly near the α-β transition. The MATLAB tools allow integration of these calculations into a variety of modeling and data analysis projects.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GC006171

A MATLAB toolbox and Excel workbook for calculating the densities, seismic wave speeds, and major element composition of minerals and rocks at pressure and temperature

Composition and tectonic evolution of the Chinese continental crust constrained by Poisson's ratio

The compressional and shear wave velocities in quarzite, granite, and granulite are determined at a fixed confining pressure of 2 kb as a function of temperature up to 720° C. The high-low quartz transition of the constituent quartz minerals is associated with a pronounced decrease in velocity of the compressional waves when approaching the transition and with a significant velocity increase after the transition. In contrast, the effect of the α-β transition on shear wave velocities is small. The drop of V
P is explained by the elastic softening of structure of the constituent quartz minerals near the α-β transition and the opening of grain-boundary cracks, caused by the very high volumetric thermal expansion of the quartz relative to the other component minerals. The velocity increase in the β-field may be attributed to an elastic hardening of the quartz structure. Poisson ratios computed from the velocity data are anomalous for a solid: they become negative within the transition regime. The transition temperature, as indicated by the minimum velocities, is higher in the polycristalline rocks than is expected on grounds of single crystal behavior, and the discrepancy is more marked in granite than in quartzite. The shift of the transition temperature to higher values is explained by internal stresses that arise from the anisotropy of the thermal expansion and compressibility of individual grains and the differences in thermal expansion and compressibility between different component minerals. The role of the α-β quartz transition as a possible cause of low-velocity layers is discussed.

H. Kern

Papers

Effect of high-low quartz transition on compressional and shear wave velocities in rocks under high pressure