Texture and elastic anisotropy of a mylonitic anorthosite from the Morin Shear Zone (Quebec, Canada)

Abstract

a b s t r a c tA sample of anorthosite from the granulite facies Morin Shear Zone (Quebec, Canada) was investigated for crystal preferred orientation and elastic anisotropy. Time-of-flight neutron diffraction data obtained with the HIPPO diffractometer at LANSCE were analyzed with the Rietveld method to obtain orientation distribution functions of the principal phases (plagioclase, clinopyroxene and orthopyroxene). Texture and microstructures are compatible with the plastic deformation of the aggregate under high-T conditions. All mineral phases depict a significant preferred orientation that could be related to the general top-to-the north shearing history of the Morin Shear Zone. Texture patterns suggest that (010)[001] in plagioclase and (110)[001] in clinopyroxene are likely dominant slip systems. Using preferred orientation data P-and S-waves velocities and elastic anisotropy were calculated and compared with previous studies to explore elastic properties of rocks with different pyroxene-plagioclase mixtures. P-wave velocity, S-wave splitting and anisotropy increase with clinopyroxene content. Seismic anisotropy is linked to the texture symmetry which can lead to large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data, particularly for monoclinic or triclinic texture symmetries.

\baselineskip =13pt Orientation distribution function of biotite platelets based on optical, thin sections and $\mu $-CT image analysis in an Outokumpu (Finland) biotite gneiss: Comparison with neutron diffraction texture analysis

“…In the first case the neutron diffraction gave the best results [Ivankina et al, 2005;Lokajicek et al, 2014;Barreiro et al, 2015] while in the second case the image analysis of thin section or -computed tomography ( -CT) are preferred [Jerram and Higgins, 2007]. The information provided by these methods are rather complementary as the neutron diffraction data refer to the mineral lattices CPO while the optical methods evidence only Figure 2.…”

Introduction to Journal of Structural Geology special issue on “Deformation of the lithosphere. How small structures tell a big story”

“…Gomez Barreiro et al (2015) apply TOF neutron diffraction on an anorthosite sample of a granulite-facies shear zone (Grenville Province, Quebec, Canada) to determine the texture and elastic anisotropy of the composing pyroxenes and plagioclase. Based on these results, they reveal the sensitivity of seismic anisotropy calculations of gabbroic rocks to texture symmetry, which should definitively be taken into account when interpreting geophysical data and building models of the lower crust.…”

A Survey and Comparison of Methods for Quantitative Texture Analysis

“…Texture data has been collected on HIPPO for a wide variety of materials (Gómez Barreiro et al, 2015;Matthies et al, 2005;Vogel et al, 2004;Wenk et al, 2003). The software MAUD is used to perform Rietveld texture analysis (RITA), which fits the diffraction spectra recorded at each detector (bin) based on structural parameters for the sample, and then uses the intensity variation as a function of orientation (i.e.…”

“…A survey of some interesting problems in geology is given in Ivankina and Matthies (2015). Gómez Barreiro, Wenk, and Vogel (2015) measured and analyzed mylonitic anorthosite minerals from the lower crust near Quebec, Canada, to better understand geothermal processes in that region. Understanding the additive manufacturing of metals is a problem of particular importance and popularity in materials engineering, and strong textures quite commonly develop as a result of such processing.…”

“…This is commonly called Rietveld texture analysis (RITA) and one of its principal advantages over traditional methods is that it works even for diffraction patterns with severe peak overlap, such as in low crystal symmetry and/or multi-phase materials. For this reason, it is of special interest in the study of geological materials (Gómez Barreiro et al, 2015;Lutterotti et al, 1997;Wenk et al, 2001). The Materials Analysis Using Diffraction (MAUD) software implements the Rietveld method to analyze diffraction experiments for structure, strain, and/or texture (Kockelmann et al, 2006b;Lutterotti, 2010a;Wenk et al, 2010).…”