Petrology and mineralogy of the Coolidge meteorite (CV4)
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概要
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The Coolidge meteorite is important because it is the only equilibrated carbonaceous chondrite which does not belong to the CK group. The petrology and mineralogy of the Coolidge meteorite were investigated by SEM and EPMA. Coolidge has some similarities to the "reduced group" of CV3 chondrites. First, abundant Fe-Ni metal and sulfide are kamacite, taenite, and troilite. Second, AOIs, CAIs, and chondrules are depleted in Na and K. Third, the main constituent the mineral of matrix is olivine. In addition to these similarities, Coolidge also has some unique properties. First of all, matrix as well as AOIs, CAIs, and chondrules is depleted in Na. This corresponds to the depletions of Na abundances in bulk compositions of Coolidge relative to CV3 chondrites. The depletion was probably established before agglomeration of this meteorite. Second, there are some plagioclase pyroxene chondrules in this meteorite. Different from type IV chondrules, they include various amounts of silica mineral. Third, bulk compositions of CAIs are rich in FeO and Cr_2O_3. Fourth, compositional variations of spinel in Coolidge are very wide. The Cr/(Cr+Al) ratio varies from 0.01 to 0.7,and the Mg/(Mg+Fe) ratio varies from 0.45 to 0.77. Fifth, correlations between some elements in spinel and those in pyroxenes in Coolidge are not similar to the "reduced group" of CV3 chondrites but similar to EOCs. The last three differences were probably established during thermal metamorphism. Olivine-spinel geothermometry suggests that Coolidge was heated to temperatures as high as ∿900℃. The duration of thermal metamorphism was short enough to preserve many petrographic features such as Fe-Mg compositional zoning in low-Ca pyroxene. Distribution of Co between kamacite and taenite suggest fast colling around ∿500℃.
- 国立極地研究所の論文
著者
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Noguchi Takaaki
Department Of Earth Sciences Ibaraki University
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Noguchi Takaaki
Department of Earth Science, Faculty of Science, Ibaraki University
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