Cooling rates of olivine xenocrysts in the EET79001 shergottite

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The chemical zoning profiles of olivine xenocrysts in lithology A of the Elephant Moraine (EET) 79001 basaltic shergottite have been measured in order to investigate its cooling history. Olivines are zoned from Fa_<23> in the cores to Fa_<44> in the rims. The major and minor element compositions of olivine xenocryst cores in EET79001 correspond to those of homogeneous olivines in the lherzolitic shergottites, reconfirming the previous studies. We assume that the initial profile of olivine was uniform and it was modified by atomic diffusion due to contact with the Fe-rich surrounding melt when olivine was incorporated as a xenocryst. Comparing the observed Fe zoning profiles with the calculated ones, we have estimated the minimum cooling rates of the xenocrystic olivines by numerically solving the diffusion equation. The inferred minimum cooling rates (0.07-0.5℃) are faster than that for Zagami estimated from the width of pyroxene lamella, consistent with a hypothesis that EET79001 experienced significant undercooling unlike Shergotty and Zagami. We also performed the calculations by applying two different Fe-Mg interdiffusion coefficients to evaluate them. There is no significant difference in the curvature of the calculated profiles between the two diffusion coefficients, whereas one gives roughly fivefold higher cooling rates than the other.
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