Journal of Petrology Advance Access published online on November 30, 2006
Journal of Petrology, doi:10.1093/petrology/egl067
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Ferric Iron in CaTiO3 Perovskite as an Oxygen Barometer for Kimberlite Magmas II: Applications
School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8W 3P6, Canada
Received October 27, 2005; Revised typescript accepted October 16, 2006
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We apply an oxygen barometer based on the Fe content of CaTiO3 perovskite to estimate the oxygen fugacity (fO2) during the crystallization and emplacement of kimberlites in different eruptive phases of a single pipe, or between different pipes, clusters or provinces. Mineral chemical data for perovskite were compiled from the literature and obtained in our detailed study of perovskites from 11 kimberlites at Somerset Island and Lac de Gras, Canada. Perovskite compositions in kimberlites record a range in fO2 of many orders of magnitude from NNO5 to NNO+6 [where log fO2 is given relative to the nickelnickel oxide (NNO) buffer]. The range of fO2 recorded by different parageneses of perovskite within a single pipe can vary up to three orders of magnitude with trends toward both oxidation and reduction during crystallization. Kimberlites record some of the greatest ranges, and the highest known fO2 conditions for any terrestrial magma. This is attributed to the presence of deep and oxidized source regions and the variable interplay of ferricferrous vs carbonfluid equilibria during ascent of kimberlite magmas. Three kimberlite pipes from the Lac de Gras field show that higher fO2 values correlate with higher proportions of more resorbed diamonds, suggesting that this variable has a measurable effect on the physical properties of diamonds in a pipe.
KEY WORDS: kimberlites; oxygen fugacity; perovskite; diamond; redox; mantle
*Corresponding author. E-mail: dcanil{at}uvic.ca
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