COESITE AS AN INDICATOR OF ULTRAHIGH PRESSURES IN CONTINENTAL LITHOSPHERE
N.V. Sobolev
Institute of Mineralogy and Petrography, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Diamond, coesite, kimberlites, xenoliths, ultrahigh pressures, inclusions, eclogites, continental lithosphere, subduction
Pages: 94-104
Abstract
Data on compositions and parageneses of coexisting garnets and pyroxenes included in coesite-bearing diamonds from different deposits of the world and in xenoliths of coesite eclogites have been summarized. Remarkably, the diamonds with coesite inclusions (over 250 samples) have been detected in all deposits in operation. Diamonds with coesite as well as xenoliths of coesite eclogites contain all the parageneses represented in eclogite (E-type) diamonds in kimberlites and mantle-derived eclogites. These parageneses include a wide continuous series of compositions from websterites (pyroxenites) to kyanite eclogites, grospydites, and calc-silicate compositions. Data on exclusively wide variations of composition of mantle-derived coesite-bearing rocks and the available data on oxygen isotopy of coesite as well as carbon and nitrogen isotopes of diamonds with coesite inclusions suggest that coesite-bearing eclogite parageneses of kimberlites and lamproites are the result of subduction of ancient oceanic crust. Similar features of diamonds have been recorded in coesite-bearing in diamondiferous UHP metamorphic rocks.
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