Compositions and textures of relic forsterite in carbonaceous and unequilibrated ordinary chondrites |
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| Institution: | 1. TRIUMF, Accelerator Division, Vancouver, BC, V6T 2A3, Canada;2. Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada;3. Department of Physics, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada;4. Institute of Control and Information Engineering, Poznań University of Technology, Poznań 60-965, Poland;1. Vinogradov Institute of Geochemistry, Russian Academy of Sciences, Siberian Branch, Irkutsk 664033, Russia;2. Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Siberian Branch, Novosibirsk 630090, Russia;3. Novosibirsk State University, Novosibirsk 630090, Russia;4. ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC, Department of Earth & Planetary Science, Macquarie University, NSW 2109, Australia;5. Geosciences Institute, Goethe University, Frankfurt am Main, 60438, Germany |
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| Abstract: | Several percent of the olivine in the C2, C3 and unequilibrated ordinary chondrites (UOC) can be distinguished by blue cathodoluminescence (CL) and an unusual composition for forsterite. This olivine has the following textural features: - 1.(1) forms cores in single olivine grains;
- 2.(2) shows subhedral to euhedral boundaries against rim olivine;
- 3.(3) rarely contains inclusions;
- 4.(4) has embayments containing olivine like that of the rim;
- 5.(5) occurs within chondrules especially in UOC meteorites.
The blue olivine is always Fe-poor (0.25 < FeO < 1.0%) and shows the following average and maximum values (%): Al2O3 (0.25, 0.5), TiO2 (0.05, 0.09), CaO (0.5, 0.8), Cr2O3 (0.15, 0.5), and MnO (0.02, 0.15); vanadium is present. Within a single olivine and within all blue olivines Al, Ca and Ti are strongly positively correlated as are Mn, Fe, and Cr in olivine surrounding the blue. The blue cores are not zoned but each element shows a marked change at the boundary of the blue with Al showing the most rapid change. These are interpreted as diffusion profiles between rim and core olivine.Textures suggest initial free growth probably from a gas and later addition of olivine by liquid crystallization to form single crystals or chondrules. The unusual olivine composition indicates high temperature growth from a refractory-rich reservoir with Al entering olivine in tetrahedral coordination. Vapor growth is suggested as the process allowing the high minor element levels. The occurrence of blue olivine in all primitive meteorites indicates that it is relic material which was widespread prior to chondrule and hence meteorite formation. Similarities in composition exist between this relic olivine and olivine of cosmic dust and Deep Sea Particles pointing to this olivine being a common component in all primitive extraterrestrial material. |
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