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Significance of Eclogitic and Related Parageneses of Natural Diamonds

NIKOLAI V. SOBOLEV, VLADIMIR N. SOBOLEV¹
Institute of Mineralogy and Petrography
Russian Academy of Sciences, Siberian Branch
670309 Novosibirsk, Russia

GREGORY A. SYNDER
Planetary Geosciences Institute
Department of Geological Sciences
University of Tennessee
Knoxville, Tennessee 37996-1410

EMILIYA S. YEFIMOVA
Institute of Mineralogy and Petrography
Russian Academy of Sciences, Siberian Branch
630309 Novosibirsk, Russia

AND

LAWRENCE A. TAYLOR
Planetary Geosciences Institute
Department of Geological Sciences
University of Tennessee
Knoxville, Tennessee 37996-1410

Abstract

    Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnet-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na₂O, intergrown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websterite-pyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents a transitional paragenesis between peridotitic or ultramafic (U-type) and E-type parageneses.
    Bimineralic eclogites, ilmenite eclogites, coesite + corundum + kyanite eclogites, and grospydites occur not only as sets of inclusions in diamonds but, with a few exceptions (ilmenite and coesite eclogites), also as diamondiferous eclogite xenoliths. The coesite eclogite paragenesis is a significant inclusion suite in diamonds, and was detected in about 15 diamond occurrences worldwide. It represents from 15% to 22% of all E-type diamonds in several occurrences, and thus should not be considered as rare.

¹Also at the Planetary Geosciences Institute, Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410.

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International Geology Review, Vol. 41, 1999, p. 129-140.