Crystallochemistry and origin of pyroxenes in komatiites |
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Authors: | Sebastien Bouquain N T Arndt E Hellebrand F Faure |
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Institution: | 1.LGCA, UMR 5025 CNRS,Université Joseph Fourier à Grenoble,Grenoble Cedex,France;2.Department of Geology and Geophysics,University of Hawaii,Honolulu,USA;3.CRPG, CNRS/INSU,Vandoeuvre-les-Nancy,France |
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Abstract: | We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo,
Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite.
Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation
that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization
sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite:
deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene
or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium
phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions
of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained
growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite.
The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities
in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites
from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high
CaO/Al2O3 of these Al-depleted komatiites. |
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