A new mineralogical approach for CO3 chondrite characterization by X-ray diffraction: Identification of primordial phases and thermal history

1,2Naoya Imae,3Yoshihiro Nakamuta
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12996]
1Antarctic Meteorite Research Center, National Institute of Polar Research, Tachikawa, Tokyo, Japan
2Department of Polar Science, School of Multidisciplinary Sciences, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo, Japan
3Kyushu University Museum, Kyushu University, Fukuoka, Japan
Published by arrangement with John Wiley & Sons

Using the in-plane rotation of polished thin section, the X-ray diffraction patterns exhibiting a high degree of randomness similar to powder pattern were obtained for 10 CO3 chondrites, which distinguished 130 reflections of olivine in the chondrules from that in the matrix, and showed systematic differences among subtypes based on the full width at half maximum intensity of two olivine 130 peaks. A lower petrologic subtype is characterized by sharp and strong peaks for forsteritic olivines in type I chondrules and by a weak and broad peak for ferroan matrices, and the higher petrologic subtypes are characterized by sharp and strong peaks for recrystallized matrices and a weakened or absent peak of magnesian olivines. The systematic change in the split peak of olivine 130 was linked with the mean diffusion length of Mg-Fe in olivine phenocrysts in type I chondrules. Fe-Ni diffusion in metals was considered to estimate the peak temperature of CO3.0, near the surface on the parent body. The peak metamorphic temperatures were estimated to be ~600–910 K using the onion-shell model when the cooling time was 106–108 yr on the parent body. A weak peak for ferroan olivine of CO3.0 suggests the amorphous silicate in matrices. The modal abundance of the amorphous Fe-silicate for subtype 3.0 (15% for Allan Hills [ALH] 77307 and 9% for Yamato [Y]-81020) was also evaluated from the deviation in trend of the relative peak ratios of the Fe-rich (≥Fa25) and Mg-rich (<Fa25) olivines for subtypes. The existence of martensites was suggested for ALH 77307. Amorphous silicate in matrices is a more resistant primordial component that produced the CO3 chondrites than martensite.

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