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Chemical zoning and homogenization of olivines in ordinary chondrites and implications for thermal histories of chondrulesThe extent and degree of homogenization of chemical zoning of olivines in type 3 ordinary chondrites is studied in order to obtain some constraints on cooling histories of chondrites. Based on Mg-Fe and CaO zoning, olivines in type 3 chondrites are classified into four types. A single chondrule usually contains olivines with the same type of zoning. Microporphyritic olivines show all four zoning types. Barred olivines usually show almost homogenized chemical zoning. The cooling rates or burial depths needed to homogenize the chemical zoning are calculated by solving the diffusion equation, using the zoning profiles as an initial condition. Mg-Fe zoning of olivine may be altered during initial cooling, whereas CaO zoning is hardly changed. Barred olivines may be homogenized during initial cooling because their size is relatively small. To simulated microporphyritic olivine chondrules, cooling from just below the liquidus at moderately high rates is preferable to cooling from above the liquidus at low rates. For postaccumulation metamorphism of type 3 chondrites to keep Mg-Fe zoning unaltered, the maximum metamorphic temperature must be less than about 400 C if cooling rates based on Fe-Ni data are assumed. Calculated cooling rates for both Fa and CaO homogenization are consistent with those by Fe-Ni data for type 4 chondrites. A hot ejecta blanket several tens of meters thick on the surface of a parent body is sufficient to homogenize Mg-Fe zoning if the temperature of the blanket is 600-700 C. Burial depths for petrologic types of ordinary chondrites in a parent body heated by Al-26 are broadly consistent with those previously proposed.
Document ID
19870035701
Document Type
Reprint (Version printed in journal)
Authors
Miyamoto, Masamichi (Tokyo, University Japan)
Mckay, David S. (Tokyo Univ. Japan)
Mckay, Gordon A. (Tokyo Univ. Japan)
Duke, Michael B. (NASA Johnson Space Center Houston, TX, United States)
Date Acquired
August 13, 2013
Publication Date
November 10, 1986
Publication Information
Publication: Journal of Geophysical Research
Volume: 91
ISSN: 0148-0227
Subject Category
LUNAR AND PLANETARY EXPLORATION
Distribution Limits
Public
Copyright
Other