Composition of olivine,silica activity and oxygen fugacity in kimberlite |
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Authors: | Roger H Mitchell |
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Institution: | Mineralogisk-Geologisk Museum, Sars gt. 1, Oslo 5, Norway |
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Abstract: | Two generations of primary olivine are present in kimberlite, rounded phenocrysts (Fo94 Fo91) and euhedral groundmass olivines (Fo91 Fo88-5). Rounded phenocrysts less magnesian than Fo88 are considered to be mantle derived xenocrysts; such crystals comprise up to 40% of the phenocrysts material in kimberlite. Calculated silica activity ranges from 10?1,5 at 1200 °C, 50 kbs, to between 10?1,6 and 10?2.4 at 600 °C, 0.5–1.0 kb. Silica buffers involving olivines, pyroxenes and garnet are considered. Oxygen fugacities on the order of 10?20 bars indicate that kimberlite magmas are highly reduced at the time of groundmass formation. |
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