- 著者
- Tomohiro TAKEBAYASHI Yui KOUKETSU Katsuyoshi MICHIBAYASHI
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- pp.221219d, (Released:2023-07-28)
Needle-shaped rutile inclusions occur in garnet within the quartz-eclogite at Mt. Gongen in the Sanbagawa belt, central Shikoku. They are approximately 5–25 µm along the long axis and are typically oriented along three directions, each intersecting at 120°. This indicates that the needle-shaped rutile is a lamella exsolved from the garnet. Garnet with needle-rutile inclusions is restricted to the quartz-poor domain of the quartz eclogite sample, which consist of quartz, garnet, omphacite, phengite, epidote, kyanite, and hornblende. Garnet grains with rutile lamellae show a composition of the almandine-pyrope series with 14–21 mol% grossular content. Rutile exsolution lamellae were concentrated in the range of 27–34 mol% pyrope of garnet crystals. The garnet host with rutile lamellae has a higher TiO2 content (TiO2 = 0.06–0.19 wt%) than those in rutile-free areas. These chemical compositional characteristics suggest that Ti was incorporated into the crystal structure during garnet growth and subsequently partially exsolved as rutile lamellae during the retrograde stage. Rutile lamellae in garnet have generally been regarded as indicators of ultrahigh-pressure metamorphism, but the present report from quartz-eclogite of the Sanbagawa belt, where no coesite has been found, provides evidence in a natural sample that the appearance of rutile exsolution lamellae is not necessarily under ultrahigh-pressure conditions.
- 著者
- Sayantani CHATTERJEE Debaditya BANDYOPADHYAY Eiichi TAKAZAWA Katsuyoshi MICHIBAYASHI
- 出版者
- Japan Association of Mineralogical Sciences
- 雑誌
- Journal of Mineralogical and Petrological Sciences (ISSN:13456296)
- 巻号頁・発行日
- vol.116, no.3, pp.170-175, 2021 (Released:2021-07-17)
- 参考文献数
- 27
- 被引用文献数
- 1
Oxidation states within the planetary interior are intrinsically linked with the broad scale tectonism; however, it is difficult to estimate the actual oxidation conditions. Orthopyroxene–magnetite symplectite formed by olivine oxidation may provide a significant clue into such oxidation events. Here we report detailed mineralogical and petrological synthesis of such orthopyroxene–magnetite symplectites from olivine gabbros of Oman Ophiolite (Hole GT2A, ICDP Oman Drilling Project). In order to understand how oxidation affects different olivine compositions, we employed a phase equilibria approach and computed several temperature–composition diagrams at a fixed pressure (1 kbar). Our experiments predict the coexistence of olivine with Fo75–76 and Fo71 with the orthopyroxene (En79 and En76), respectively, which is remarkably similar to the mineral chemistry obtained from the Oman lower crustal gabbros. From the magnetite content, we also infer that the symplectite formation may have taken place over a range of temperatures (600–1000 °C) via subsolidus olivine oxidation and/or melt (oxidizing)–olivine interaction. The latter is more probable, considering the partial occurrence of orthopyroxene and clinopyroxene rim adjacent to the symplectites.