Ti-rich hydroandradites from the Sanbagawa metamorphic rocks of the Shibukawa area,central Japan |
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Authors: | Hitoshi Onuki Masahide Akasaka Takeyoshi Yoshida Munetomo Nedachi |
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Institution: | (1) Department of Earth Sciences, Hirosaki University, 036 Hirosaki, Japan;(2) Department of Geology and Mineralogy, Hokkaido University, 060 Sapporo, Japan;(3) Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, 980 Sendai, Japan;(4) Department of Earth Sciences, College of General Education, Kagoshima University, 890 Kagoshima, Japan |
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Abstract: | Ti-rich hydroandradite often occurs, though usually as a minor constituent, in serpentinized ultramafic rocks and associated gabbroic rocks of the Sanbagawa metamorphic belt. The chemistry of the host rocks is commonly characterized by undersaturation with SiO2. Two Ti-rich hydroandradites from metasomatized gabbroic rocks of the Shibukawa area have been chemically analysed by the wet method. Mössbauer experiments indicate the presence of Fe3+ and Fe2+ in the octahedral sites of these Ti-rich hydroandradites. Wet chemical analyses for total reducing capacity of the present garnets by two different methods together with Mössbauer data imply the presence of octahedral Ti3+ in their structure. Ti-rich hydroandratites, apart from their hydrous property, have a similar crystal chemistry to natural Ti-rich andradites. Ti-rich hydroandradites were probably formed in rocks with unusual chemical compositions within a P-T region of 300–400° C and 4–7 kb under limited conditions of relatively low oxygen fugacity and low μCO2. |
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