The Relative Influence of Quartz and Mica on Crustal Seismic Anisotropy

Ward, Dustin Evans

Graduate Thesis Or Dissertation

The Relative Influence of Quartz and Mica on Crustal Seismic Anisotropy Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/2j62s5177

Abstract

Seismic anisotropy is becoming an important observation when characterizing deformation in the middle and lower continental crust. Key contributors to this phenomenon (under mid-crustal conditions) include mineralogy and the degree of alignment among anisotropic phases. Micas are the most anisotropic phases in continental crust, and until recently, little consideration was given to the contribution of other crustal minerals. As quartz is one of the most common crustal minerals and its contribution to seismic anisotropy is poorly constrained, two mylonitic, micaceous quartzites were examined to investigate the influence of quartz microstructure on seismic anisotropy. Electron backscatter diffraction (EBSD) was the primary method used for identifying sample mineralogy and textural characteristics. These data coupled with mineral elastic constants were used to calculate sample seismic attributes. Both quartzite samples exhibit calculated P-wave anisotropies between 6 and 8 percent. Additional calculations were performed varying the modal proportions of quartz and mica. These calculations suggest that the presence of aligned quartz decreases the overall anisotropy produced by aligned mica up to a threshold modal proportion (~70-80% quartz), and that quartz alters the symmetry of anisotropy, which may have important implications when interpreting crustal deformation. One of the challenges to this study was acquiring quality EBSD data from phyllosilicates. These phases are historically difficult to characterize with EBSD. The underlying cause of this problem is under debate, however, prior studies suggest poor sample surface preparation and problems inherent to the phyllosilicate structure are to blame. Ion milling is a technique ideally suited for EBSD sample preparation as it offers the ability to smooth sample surface topography and remove damage induced by mechanical polishing. The viability of this method was tested for preparing polyphase, mica-bearing geological materials for EBSD analysis. Results show minimal improvement in phyllosilicate EBSD data from samples prepared with an ion mill. This appears to be due primarily to preferential etching along grain boundaries and weak Van der Waals bonds in the phyllosilicate structure.

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