1. A Petrological and Microstructural Study of Ten Brachinites: A Cumulate Texture?
- Author
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Meunier‐Mili, N., Kaczmarek, M.‐A., and Bystricky, M.
- Subjects
OLIVINE ,ELECTRON probe microanalysis ,MATERIAL plasticity ,CRYSTAL growth ,CRYSTAL orientation ,SCANNING electron microscopy ,ROCK deformation - Abstract
Brachinites 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. Plain Language Summary: Brachinite meteorites, rich in olivine and pyroxene, are classified as ultramafic and unbrecciated primitive achondrites. However, the precise way in which they formed is still a subject of debate. This study conducted a thorough petrological and microstructural examination of 10 brachinites using scanning electron microscopy, microprobe, and electron backscatter diffraction. The findings reveal that the closure temperatures of the brachinites are between 814 ± 27 and 909 ± 20°C. Olivine crystal preferred orientations (CPO) indicate a B‐type fabric [001](010), characterized by a strong to moderate concentration of [001] axes (slip direction) parallel to the lineation and [010] axes (slip plane) perpendicular to the foliation. Grain misorientation distributions resemble those found in untextured rocks (e.g., undeformed granular peridotite), implying that the B‐type fabrics in olivine were a result of rigid crystal rotation or preferential crystal growth during cumulate formation or melt flow alignment and weak compaction. Olivine subgrain boundary misorientation axes suggest limited activation of [001](100) C‐type and [100](001) E‐type slip systems, which could be the result of compaction and primary plastic deformation during or after crystal settling. These new results indicate that brachinites developed a cumulate texture and accommodated only weak plastic deformation during early differentiation processes in the parent body. Key Points: Petrology and microstructural analyses allow classification of the studied brachinites into two groups suggesting different igneous processesThe brachinites display cumulate textures with magma flow and variable weak compaction intensitiesOlivine underwent early plastic deformation, and moderate to weak intensities of [010] axes normal to compaction point to relative brachinite locations [ABSTRACT FROM AUTHOR]
- Published
- 2024
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