Abstract
Abstract. This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks. The CPO characteristics as a function of the deformation regime (magmatic or crystal-plastic) are outlined and discussed. The studied samples are dominantly from slow- and fast-spread present-day ocean crust, as well as from the Oman ophiolite. Plagioclase is the dominant mineral phase in the studied samples. Plagioclase CPOs are grouped into three main categories: Axial-B, a strong point alignment of (010) with a girdle distribution of [100]; Axial-A, a strong point maximum concentration of [100] with parallel girdle distributions of (010) and (001); and P-type, point maxima of [100], (010), and (001). A majority of CPO patterns are Axial-B and P-type, in samples showing either magmatic or crystal-plastic deformation textures. Axial-A CPOs are less common; they represent 21% of the samples deformed by crystal-plastic flow. Although fabric strength (ODF J index) does not show any consistent variation as a function of the CPO patterns, there is a significant difference in the relationship between the ODF and pole figures J indices; the magmatic type microstructures have high (010) pole figures J indices, which increase linearly with ODF J index, whereas the high [100] pole figures J indices of plastically deformed samples vary in a more scattered manner with ODF J index. The multistage nature of plastic deformation superposed on a magmatic structure compared with magmatic flow, and the large number of possible slip-systems in plagioclase probably account for these differences. Calculated seismic properties (P wave and S wave velocities and anisotropies) of plagioclase aggregates show that anisotropy (up to 12% for P wave and 14% for S wave) tends to increase as a function of ODF J index. In comparison with the olivine 1998 CPO database, the magnitude of P wave anisotropy for a given J index is much less than olivine, whereas it is similar for S wave anisotropy. Despite a large variation of fabric patterns and geodynamic setting, seismic properties of plagioclase-rich rocks have similar magnitudes of anisotropy. There is a small difference in the aggregate elastic symmetry, with magmatic microstructures having higher orthorhombic and hexagonal components, whereas plastic deformation microstructures have a slightly higher monoclinic component, possibly correlated with predominant monoclinic simple shear flow in plastically deformed samples. Overall, plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, although our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry (triclinic) structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs.
Highlights
The study of the crystallographic preferred orientations (CPO) of naturally and experimentally deformed materials provides crucial insight into geodynamic processes (e.g., Nicolas and Christensen, 1987; Karato, 2008)
This study presents a unique database of 172 plagioclase Crystallographic Preferred Orientations (CPO) of variously deformed gabbroic rocks
Plots for CPO strength (ODF J index), pole figure strength, CPO symmetry and seismic anisotropy show significant scattering. This could be related to sampling statistics, our database is a factor of ten higher than the olivine database of 1998, or it could be related to the low symmetry structure of plagioclase resulting in the addition of degrees of freedom in the processes creating the CPOs
Summary
The study of the crystallographic preferred orientations (CPO) of naturally and experimentally deformed materials provides crucial insight into geodynamic processes (e.g., Nicolas and Christensen, 1987; Karato, 2008). They are a key component of our understanding of the material processes, mechanical behavior and deformation strain history, from the crustal levels to the deep mantle. Similar to a number of typically “crustal” phases (e.g. quartz, K-feldspars, phyllosilicates), the CPOs vary from mineral fabrics typically formed by magmatic processes to crystallographic orientations resulting from crystal-plastic processes These two processes are commonly superposed in the same aggregate, with crystal-plastic deformation overprinting magmatic fabrics, leading to complex fabrics that cannot be attributed to a dominant process. Dissolution processes during metamorphic reactions can be attributed to plagioclase deformation (e.g. Imon et al, 2002), and might result in the development of crystallographic preferred orientations
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