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A new mineralogical approach for CO 3 chondrite characterization by X‐ray diffraction: Identification of primordial phases and thermal history
Author(s) -
Imae Naoya,
Nakamuta Yoshihiro
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12996
Subject(s) - olivine , chondrite , chondrule , parent body , silicate , analytical chemistry (journal) , mineralogy , enstatite , geology , amorphous solid , crystallography , chemistry , meteorite , physics , astrobiology , organic chemistry , chromatography
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 CO 3 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 CO 3.0, 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 10 6 –10 8 yr on the parent body. A weak peak for ferroan olivine of CO 3.0 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 (≥Fa 25 ) and Mg‐rich (<Fa 25 ) 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 CO 3 chondrites than martensite.