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Intrinsic Elastic Anisotropy of Westerly Granite Observed by Ultrasound Measurements, Microstructural Investigations, and Neutron Diffraction

Journal Article · · Journal of Geophysical Research. Solid Earth
DOI:https://doi.org/10.1029/2020jb020878· OSTI ID:1849572
 [1];  [2];  [3];  [3];  [4];  [4];  [5];  [6]
  1. Czech Academy of Sciences, Prague (Czech Republic). Inst. of Geology. Dept. of Physical Properties of Rocks; Czech Academy of Sciences, Prague (Czech Republic)
  2. Joint Inst. for Nuclear Research (JINR), Dubna (Russian Federation). Frank Lab. of Neutron Physics
  3. Czech Academy of Sciences, Prague (Czech Republic). Inst. of Geology. Dept. of Physical Properties of Rocks
  4. Inst. of Criminalistics Praha, Prague (Czech Republic)
  5. Inst. of Criminalistics Praha, Prague (Czech Republic); SG Geotechnika Praha, Prague (Czech Republic)
  6. Univ. of California, Berkeley, CA (United States). Dept. of Earth and Planetary Science
+ Show Author Affiliations
Westerly granite (WG) has been generally accepted as an isotropic homogeneous rock. Here, we return to WG and observe significant elastic anisotropy using multidirectional ultrasonic sounding on spherical samples at pressures up to 400 MPa. Thermal treatment of WG leads to formation of microcracks that reduce elastic wave velocities and increase its elastic anisotropy. The 3D distribution of P-wave velocities at low pressure is close to orthorhombic symmetry. Application of hydrostatic pressure closes most of thermally induced microcracks and decreases elastic anisotropy of WG, but at high pressure the anisotropy is practically reversed compared to low pressure: maximum P-wave velocity direction at low pressures is near minimum velocity direction at high pressure and vice versa. To understand this effect, microstructures of the rock were investigated by optical and scanning electron microscopy. Preferred orientations of four major rock-forming minerals—quartz, orthoclase, plagioclase, and biotite—were measured by time-of-flight neutron diffraction, which confirms significant crystal alignment. All these data were used to numerically model anisotropic elastic properties of WG. It is shown that WG possesses weak intrinsic elastic anisotropy related mainly to the preferred orientation of feldspars formed during igneous crystallization. Observed microcracks are mostly related to the cleavage planes of feldspars and biotite, and thus also demonstrate preferred orientation. Higher preheating temperatures produce larger quantity of longer microcracks. A numerical model shows that these microcracks act against the weak intrinsic elastic anisotropy of WG, and define the elastic anisotropy at low pressures.
Research Organization:
Univ. of California, Oakland, CA (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
FG02-05ER15637
OSTI ID:
1849572
Journal Information:
Journal of Geophysical Research. Solid Earth, Journal Name: Journal of Geophysical Research. Solid Earth Journal Issue: 1 Vol. 126; ISSN 2169-9313
Publisher:
American Geophysical UnionCopyright Statement
Country of Publication:
United States
Language:
English

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58 GEOSCIENCES
Geochemistry & Geophysics