Petrological and geochemical study of the Yamato-74359 and Yamato-74360 achondrites

• Main Authors: Yukio Ikeda, Akira Tsuchiyama, Mitsuru Ebihara
• Format: Report
• Language: English
• Published: Department of Earth Sciences, Faculty of Science, Ibaraki University 1991
• Subjects: Antarctic; Yamato; Antarc*
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=4541; http://id.nii.ac.jp/1291/00004541/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=4541&item_no=1&attribute_id=18&file_no=1

Two Antarctic achondrites, Yamato (Y)-74359 and Y-74360,are very similar in bulk chemical compositions to the silicate portions of H chondrites, but the siderophile elements are extremely depleted in comparison to those of H chondrites. They consist of olivine, pyroxene and cryptocrystalline feldspar with minor amounts of chromite, kamacite, and troilite, and the chemical compositions of the constituent minerals are similar to those in H chondrites. Taking into consideration that the two achondrites have oxygen isotopic compositions typical of H chondrites, they were produced from H chondrites or the precursors of H chondrites. Chondrules are not observed, and the texture of the two achondrites is similar to one another, although Y-74360 is coarser-grained than Y-74359. Olivine occurs as euhedral or subhedral grains, mostly smaller than 100 microns in diameter, and they show slight chemical zoning, from magnesian cores (Fo_<82-83>) to ferroan rims (Fo_<80-78>). Orthopyroxene occurs as euhedral grains, larger than olivine grains, and also shows slight chemical zoning, from Ca-poorer magnesian cores to Ca-richer ferroan rims. Clinopyroxene occurs as rims of orthopyroxenes and shows remarkable chemical zoning continuously from pigeonitic interiors to augitic rims. Cryptocrystalline feldspar occurs in interstitial spaces between olivine and/or pyroxene, and seems to have crystallized in interstitial liquids in a rapid cooling condition. The cryptocrystalline feldspars in Y-74359 are chemically heterogeneous, classified into three groups, albite (An_<1-4>Ab_<94-96>Or_<2-4>), anorthoclase (An_<6-10>Ab_<79-86>Or_<4-15>), and intermediate alkali feldspar (An_<1-2>Ab_<51-56>Or_<42-47>), and occur in different portions of the thin section. The two achondrites seem to have formed by partial melting from heterogeneous silicate precursors of unequilibrated H chondrites.