Abstract
Although diamonds from kimberlites in the Guaniamo region of Venezuela overwhelmingly belong to the eclogite suite, the mantle xenocryst suite is dominated by peridotitic garnets, with a significant harzburgite component. In the 0.5 to 1.0 mm size fraction of heavy mineral concentrate from the La Ceniza kimberlite, approximately 23% of the garnet xenocrysts are classified as G10 (harzburgitic), 75% are G9 (lherzolitic) and only 2% are eclogite-derived. In the peridotite group, both lherzolitic and harzburgitic garnet xenocrysts have oxygen isotope ratios typical of mantle peridotites (δ18O = +5.25 to +5.47‰ VSMOW). Eclogitic garnet xenocrysts overlap the major-element compositional range of eclogite-suite garnets included in diamond from Guaniamo. The oxygen isotope ratios of the eclogitic xenocrysts are in the range of +5.32 to +9.26‰. The higher values represent the most extreme oxygen isotope compositions yet reported for mantle eclogites. These oxygen isotope data support earlier suggestions, based on carbon isotope analyses of diamonds and oxygen isotope analysis of their mineral inclusions, that subduction of altered oceanic lithosphere beneath the Guyana craton played a major role in formation of Guaniamo eclogite-suite diamonds.
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