Mineralogical alteration of a type A CAI from Allende CV3 chondrite: Formation of secondary dmisteinbergite and its phase transition to anorthite

1Yuma Enokido,1Tomoki Nakamura,1Megumi Matsumoto,2Akira Miyake,3Takazo Shibuya,4Changkun Park,5Mike Zolensky
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13961]
1Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Japan
2Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto, Japan
3Super-cutting-edge Grand and Advanced Research (SUGAR) Program, Institute for extra-cutting-edge Science and Technology Avant-garde Research (X-Star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan
4Division of Earth-System Sciences, Korea Polar Research Institute, Incheon, Korea
5National Aeronautics and Space Administration, Lyndon B. Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Dmisteinbergite, a hexagonal form of CaAl2Si2O8, was found in a compact type A Ca-Al-rich inclusion (CAI) in the Allende CV3 chondrite. Scanning and transmission electron microscopic observations show that dmisteinbergite was always in contact with grossular and grossular was in contact with melilite. In addition, there is a crystallographic relationship between dmisteinbergite and anorthite. Based on the textural and crystallographic evidence, the following mineralogical alteration processes are proposed to have occurred in the CAI. (1) Melilite was replaced by grossular. High densities of vesicles in the grossular indicate that hydrogrossular might have been the primary alteration phase and dehydrated by later metamorphism. (2) Dmisteinbergite formed from (hydro)grossular through a reaction with Si-rich fluid. (3) Nano-sized minerals are formed within dmisteinbergite. (4) Dmisteinbergite was transformed to anorthite. (5) Both anorthite and dmisteinbergite were altered to nepheline. (6) Hydrogrossular was dehydrated to grossular. (Hydro)grossular, dmisteinbergite, anorthite, and nepheline in the CAI seem to have formed in the course of metasomatism that occurred in the Allende parent body. Except for the hydrogrossular dehydration, these reactions could have occurred at moderate temperature (200–250°C) in high pH fluids (pH 13–14) according to past experimental studies. Episodic changes in fluid composition seem to have occurred before reactions (2), (4), and (5), because these reactions were not completed before the next reaction started. Higher temperature is required for reactions (5) and (6) to occur. Our observation of the CAI suggests that it experienced multiple episodes of metasomatism as temperatures were rising in the Allende parent asteroid.


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