Thermal history of ureilite, Pecora Escarpment 82506 deduced from cation distribution and diffusion profile of minerals

• Main Authors: Toyoda,Hiromi, Haga,Nobuhiko, Tachikawa,Osamu, Takeda,Hiroshi, Ishii,Teruaki
• Format: Report
• Language: English
• Published: Mineralogical Institute, Faculty of Sciences, University of Tokyo/Mineralogical Institute, Faculty of Sciences, University of Tokyo/Mineralogical Institute, Faculty of Sciences, University of Tokyo/Mineralogical Institute, Faculty of Sciences, University of Tokyo/Ocean Research Institute, University of Tokyo 1986
• Subjects: Mull; Pecora Escarpment; Memoirs of National Institute of Polar Research; Polar Research
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=1965; http://id.nii.ac.jp/1291/00001965/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=1965&item_no=1&attribute_id=18&file_no=1

PCA82506 is relatively unshocked compared with most ureilites. Large unshocked pigeonite and olivine crystals were studied by means of single crystal X-ray diffraction, analytical TEM, optical microscope and an electronprobe. The chemical compositions of the PCA82506 pigeonites are (Ca)_6(Mg)_<76>(Fe)_<18>, while the average olivine core composition is (Fa)_<20>. A region about 50 micrometer apart from the olivine rim is reduced by the carbonaceous (C) matrix. From the diffusion profiles of olivine crystals, a cooling rate of 10-15℃/h is estimated by the computer simulation method of MIYAMOTO et al. (Proc. Lunar Planet. Sci. Conf., 16th, Pt. 1,D116,1985). The cell dimensions of the PCA82506 pigeonite determined from 23 reflections are : a, 9.6649(9); b, 8.8719(6); c, 5.2134(5)A; and β, 108.51(6)°. The site occupancy factors are M1 (Mg 0.064,Fe 0.936) and M2 (Ca 0.116,Mg 0.568,Fe 0.316). This cation distribution gives an equilibrium temperature of 800℃ by plotting the K value of PCA82506 on the distribution isotherm of SAXENA et al. (Earth Planet. Sci. Lett., 21,194,1974). The PCA82506 pigeonites may have cooled more rapidly than terrestrial pigeonites from the Isle of Mull and almost as rapid as the most rapidly cooled mare basalts.