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Coesite-Sanidine Eclogites from Kimberlite: Products of Mantle Fractionation or Subduction?

Daniel J. Schulze and Herwart Helmstaedt
The Journal of Geology
Vol. 96, No. 4 (Jul., 1988), pp. 435-443
Stable URL: http://www.jstor.org/stable/30062158
Page Count: 9
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Coesite-Sanidine Eclogites from Kimberlite: Products of Mantle Fractionation or Subduction?
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Abstract

Eclogite xenoliths from kimberlite have long been considered as products of crystallization of basaltic magmas within the upper mantle, and the Ca/(Ca + Mg) and Fe/(Fe + Mg) values of garnets in some suites have been interpreted as representing a fractionation sequence. Many recent studies, however, have concluded that some xenolithic eclogites have originated by subduction of oceanic crust. We show that coesite and/or sanidine occur throughout virtually the entire compositional range of mantle-derived eclogites, although in igneous crystallization models these minerals should be present only in the more "evolved" (i.e., Ca- and Fe-rich) compositions. The distribution of these two minerals constitutes petrographic and major element evidence consistent with an origin for these eclogites through subduction of altered oceanic crust, without subsequent partial melting.

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