Chemical zoning and homogenization of olivines in ordinary chondrites and implications for thermal histories of chondrules

Authors:
  • MMMasamichi Miyamoto
    Masamichi Miyamoto
  • DMDavid S. McKay
    David S. McKay
  • GMGordon A. McKay
    Gordon A. McKay
  • MDMichael B. Duke
    Michael B. Duke
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Abstract
We studied the extent and degree of homogenization of chemical zoning of olivines in type 3 ordinary chondrites 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. We have calculated cooling rates or burial depths needed to homogenize the chemical zoning 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 in order to keep Mg‐Fe zoning unaltered, the maximum metamorphic temperature must be less than about 400°C if we assume cooling rates based on Fe‐Ni data. 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 26Al are broadly consistent with those previously proposed.

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