A Petrological and Microstructural Study of Ten Brachinites: A Cumulate Texture?

Abstract

AbstractBrachinites are igneous, ultramafic and unbrecciated primitive achondrites mainly composed of olivine and pyroxene, with a partial melt residue or cumulate origin that is still debated. This study presents a petrological and microstructural study of 10 brachinite meteorites to identify igneous and deformation processes responsible for their formation. Detailed microstructural analyses were performed using secondary electron microscopy, electron probe microanalysis, and electron backscatter diffraction. The olivine‐spinel and two‐pyroxene closure temperatures of these brachinites ranged from 814 ± 27 to 909 ± 20°C. Olivine [001] axes coincide with the elongation of olivine grains and the crystallographic preferred orientations (CPO) of clinopyroxene [001] axes, which led us to infer lineation as olivine [001] axes parallel to X for all samples. Olivine CPO displays a strong or medium concentration of [001] axes parallel to X and [010] axes parallel to Z pointing to a B‐type fabric. Limited internal deformation of olivine grains and slip systems identified from low‐angle boundary misorientation analyses are inconsistent with the CPO and grain misorientation distributions are close to those found in untextured rocks. Olivine B‐type fabrics may therefore result not from plastic deformation but rather from rigid crystal rotation or preferential crystal growth during compaction. Olivine subgrain boundary misorientation axes imply limited activation of [001](100) and [100](001) slip systems, which could be due to primary plastic deformation during or after crystal settling. These new results reveal that brachinites developed a cumulate texture and accommodated only weak plastic deformation during early differentiation processes in the parent body.