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Exsolution of garnet within clinopyroxene of mantle eclogites: major- and trace-element chemistry

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Abstract

Eclogite xenoliths from the mantle have experienced a wide variety of processes and P-T conditions, many of which are recorded in the mineral compositions and textures. Exsolution of garnet from clinopyroxene is one such texture, occurring in a minority of mantle eclogites. New analyses of clinopyroxene and garnet of eclogite xenoliths from kimberlites at Bellsbank (South Africa) and Obnazhënnaya (Yakutia, Russia) are presented here, and these are combined with data from the literature. Exsolution of garnet from clinopyroxene is generally lamellar, although lens-shaped garnets are also present. Major- and trace-element characteristics show a wide range of compositions and include eclogite Groups A, B, and C. Rare-earth element (REE) concentrations of garnet and pyroxene were determined by SIMS, and the REE patterns are subtly different from those in “ordinary” eclogites. Differences include the absence of prominent Eu anomalies in samples of this study and differences in the slopes of chondrite-normalized REE patterns. It is possible that these “signatures” are unique to exsolved eclogites, a result of subsolidus elemental partitioning during exsolution. Some reconstructed whole-rock compositions are aluminuous; comparison with ordinary eclogites shows only minor differences, implying a similar origin. If the immediate precursor to the exsolved eclogites was a monomineralic pyroxenite, the excess aluminium was tied up in Tschermak's molecule, although the occasional presence of kyanite exsolution lamellae is indicative of a Ca-Eskola component. Reconstructed “pyroxenes” from kyanite- and corundum-rich samples contain unrealistic amounts of aluminium for mantle pyroxenes. A protolith (or parental pyroxene) “threshold” of ∼24% Al2O3 may exist, above which (as in a plagioclase cumulate) the final assemblage is kyanite- and/or corundum-bearing.

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Author notes

  1. Ghislaine Crozaz

    Present address: McDonnell Center for the Space Sciences, Washington University, St. Louis, 63130, MO, USA

Authors and Affiliations

  1. Department of Geological Sciences, University of Tennessee, 37996, Knoxville, TN, USA

    Eric A. Jerde & Lawrence A. Taylor

  2. Earth and Planetary Sciences Department, Washington University, St. Louis, 63130, MO, USA

    Ghislaine Crozaz

  3. Institute of Mineralogy and Petrography, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia

    Nikolai V. Sobolev

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  1. Eric A. Jerde
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  2. Lawrence A. Taylor
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  3. Ghislaine Crozaz
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  4. Nikolai V. Sobolev
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Jerde, E.A., Taylor, L.A., Crozaz, G. et al. Exsolution of garnet within clinopyroxene of mantle eclogites: major- and trace-element chemistry. Contr. Mineral. and Petrol. 114, 148–159 (1993). https://doi.org/10.1007/BF00307752

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  • DOI: https://doi.org/10.1007/BF00307752

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