Pyroxenes in the Shaw (L-7) chondrite |
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Authors: | RT Dodd JE Grover GE Brown |
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Institution: | 2. Max-Planck-Institut für Kernphysik, Heidelberg, West Germany |
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Abstract: | The Shaw L-group chondrite differs from orthodox type 6 ordinary chondrites in ways which suggest that it experienced unusually high metamorphic temperatures and anatexis. Electron microprobe and single crystal X-ray diffraction data indicate that Shaw contains three pyroxenes: the augite (Fs11.3Wo38.2) and calcic orthopyroxene (Fs19.4WO4·5) reported by other workers and a second, Ca- and Al-poor orthopyroxene (Fs16·8Wo1·2) which we interpret as inverted protobronzite. Comparison of the Shaw assemblage with experimental data suggests that a two-phase (augite-protobronzite) assemblage developed at peak metamorphic temperatures of ~1250–1300°C, that partial reaction of augite and protobronzite produced calcic orthopyroxene and by-product spinel at temperatures approximately 150°C lower and that protobronzite inverted to bronzite free of stacking disorder during subsequent slow cooling. The intracrystalline distribution of Fe and Mg in the Ca-poor bronzite (KE + 0·07; determined by crystal structure analysis) indicates an equilibration temperature of ~500°C.Shaw differs sufficiently in texture and mineralogy from type 6 ordinary chondrites to justify its assignment to a separate petrologic type: L-7. |
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