Equilibration conditions of upper-mantle eclogites: implications for kyanite-bearing and diamondiferous varieties
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Abstract New mineral data for kyanite-bearing eclogites supplement existing data for eclogite nodule suites from the Roberts Victor and Bellsbank kimberlite intrusions, South Africa. Calculation of equilibration temperatures using the refined K D calibration of Ellis and Green (1979) indicates that the majority of these eclogites equilibrated over a considerably narrower range (940–1185 °C) than implied by earlier estimates which did not take account of the influence of the Ca-component of garnet on K D . Most of the Roberts Victor diamondiferous eclogites appear to have equilibrated in the lower part of this range, whilst the equilibration temperatures for the Roberts Victor diamond-free eclogites extend to higher values than those from Bellsbank. The petrogenetic implications of the (calculated) temperature and pressure equilibration conditions for these eclogite suites are discussed with particular reference to possible uppermantle geotherms and available data on the graphitediamond transition boundary.In Haüy's original definition of eclogite, the presence of kyanite as an accessory constituent was recognized, and it is apparent that he was familiar with examples from the Sau Alps and Styria. The best known of eclogites—those of the Bavarian Fichtelgebirge (e.g. Silberbach, Weissenstein, and Eppenreuth)—contain significant kyanite interspersed among the omphacite grains or associated with accessory quartz and muscovite in the rocks. Of these rocks rather extended descriptions have been published. Analyses of the minerals and the rocks themselves belong to an early date and do not reach the standard of modern analyses. Later, Drill (1902) published a systematic account of these Bavarian eclogites and has added an analysis of an eelogite with kyanite from Unterpferdt near Silberbach and an analysis of a garnet from a Silberbach eclogite.
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Mineralogy of diamondiferous eclogite xenolites showing metasomatosis evidence from the Udachnaya kimberlite pipe is discussed. The paper also reviews features of diamonds they contain, compositions of primary garnets and omphacites as well as alteration of structural and species compositions of original garnets and clinopyroxenes during metasomatosis. Based on pyrope structure update, two-phase garnet composition is suggested, which is mostly represented by complex pyrope associated with Ca-pyrope. In all samples, primary omphacite is replaced by another clinopyroxene variety depleted in Na2O, which is typical of partial melting products. Geothermometry results suggested that the eclogites formed within a temperature range of 1,000–1,2000 °C. Based on diamond morphology, data on total N content in diamonds and its aggregation, multiple stages of diamond formation in eclogites and the most probable growth of later diamond generations impacted by metasomatizing mantle fluids containing carbon are postulated. It is suggested that certain diamond formation stages probably had a time gap of several hundred million years.
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Kyanite-bearing eclogitic assemblages occur in the highest-grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite-eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca-rich/Mg-poor inner and Ca-poor/Mg-rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg-rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower-pressure conditions of subsequent epidote-amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3-2.4 GPa/675-740 °C. The metamorphic P-T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg-rich composition was in equilibrium with other eclogite facies phases. Therefore, P-T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re-examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.
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