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Mineral inclusions in diamonds from Wawa metaconglomerate : implications for thermal evolution of the lithospheric mantle Miller, Christine


Mineral inclusions in non-fibrous and fibrous diamonds from an Archean metaconglomerate deposit in Wawa, Ontario, Southern Superior craton were studied to characterize the compositional and thermal state of the lithospheric mantle from the Archean to present day. Electron microprobe analysis of Wawa non-fibrous diamonds shows large inclusions of Cr-pyrope, Mg-chromite, olivine, and enstatite indicating harzburgitic parent rock. Wawa fibrous diamonds host microinclusions of pyrope and olivine of predominantly lherzolitic assemblage. Thermobarometry calculations for non-fibrous diamonds yield temperatures and pressures consistent with formation in a cool, cratonic root reaching to a minimum depth of 190 km with a geotherm between 39-41 mW/m², located beneath the Southern Superior province during the Archean. Comparison to results from xenoliths in nearby post-Archean kimberlites, and to modern geophysics, indicates heating and thinning of the cratonic root. This effectively destroyed the diamondiferous portion of the lithospheric mantle, as early as 1.1 Ga in some areas of the Southern Superior, through tectonic erosion during amalgamation of terranes to the protocraton. Diamond inclusion analysis for Wawa fibrous diamonds and datasets for non-fibrous and fibrous diamonds from Diavik, Ekati (Panda kimberlite), and Koffiefontein (South Africa) reveal metasomatic trends of mantle rock evolution due to the influx of K-rich hydrous carbonatitic fluid related to fibrous diamond precipitation. Thermometry for fibrous diamond inclusions yields temperatures of 580-1030°C. Low formation temperatures, paired with the alkali-rich and hydrous nature of the metasomatic agent, result in subsolidus diamond growth in the absence of melting or thermal disturbance of the mantle. Fibrous diamond growth, previously linked to kimberlite generation, may be a temporally distinct and genetically independent event, as suggested by long mantle residence times for fibrous diamonds and contrasting chemistry of fibrous diamond fluid and kimberlites. This would make metasomatism associated with formation of fibrous diamonds a “cratonic root-friendly” process that would not have played any part in the destruction of the Southern Superior lithospheric root.

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