USING CRYSTAL PREFERRED ORIENTATION (CPO) TO BETTER INTERPRET IGNEOUS ROCK TEXTURE

Electron Backscatter Diffraction (EBSD) is a well-established analytical technique that can be utilized to identify crystalline phase and phase orientation. While EBSD has gained popularity amongst metamorphic petrologists, it has yet to be fully embraced by igneous petrologists. Here, we present how the addition of phase orientation can be leveraged to gain further insight alongside traditional textural analyses (i.e. crystal shape and crystal size distributions). Samples used in this study derive from a well-exposed, relatively thick (14 meters) feeder-dike of the Columbia River Flood Basalts. Samples were collected at 140 cm spacing with orientation preserved. In our case, the resulting thin-sections are taken along the horizon, with the long-axis aligned with the strike of the dike. EBSD analyses were used to generate phase maps, crystal maps, and crystal preferred orientation (CPO) figures. From crystal maps, 2D crystal intersections (i.e. between the crystal and the thin-section) are measured, which are then input into adopted crystal shape and crystal size distribution calculators. This follows the majority of textural analyses, and at this stage, the results are frequently non-unique, leading to ambiguous interpretations across the literature. By including CPO, additional constraints can be imposed on interpretations. Combining CPO with the estimate of crystal shape allows for tying the crystal long-axis to its respective crystallographic axis, and can be used to determine whether the crystal fabric is representative of a preferential alignment. In our analyzed samples, the plagioclase shape is A>C>B. For samples collected near the wall-rock, the CPO indicates an aligned fabric, with the long-axis parallel to the contact and lineated along the horizon, indicating lateral flow. Within the interior of the dike, the fabric rapidly degrades. CSDs are generally kinked, revealing two crystal populations. Combining CPO with CSDs allow these populations to be divided into allochthonous, flow-aligned crystals, and autochthonous, grown in situ crystals.