Olivine evidence for an ultramafic silicate precursor melt for the Jericho kimberlite (Slave Craton, Canada)

Olivines are a major constituent of kimberlites and carry important information on the evolution of the kimberlite magma. In this study, eighty-six olivines from the Jericho kimberlite in the Slave Craton, Canada were analyzed for major and trace elements, and fourteen major element transects were acquired across magmatic rims. Three main olivine populations are identified based on chemical composition: peridotite xenocrysts, megacryst xenocrysts, and phenocrysts. Peridotite xenocrysts (72 vol%) have Al-in-olivine temperatures (950–1100 °C) indicating that they are derived from the 150 km interface between the shallow and deep lithospheric mantle layers. Megacryst xenocrysts (18 vol%) have positively-correlated Mg# (87–90) and NiO contents (0.15–0.45 wt%) which can be reproduced by crystal fractionation of an Fe-rich, reduced siliceous melt originating at ca 350 km depth as constrained by melting experiments. This modelling and geothermobarometric calculations on polymineralic megacryst xenoliths place emplacement depth at ca 150 km. Phenocrysts (10 vol%) have a wider range of Mg# (89.6–91) than is typical for other kimberlites and come in two varieties. High-NiO phenocrysts (0.25–0.40 wt%) are often angular and finely-veined, and have compositions similar to the transitional and internal zones of rimed xenocrysts. Low-NiO phenocrysts (0.10–0.25 wt%) are small and euhedral, have compositions similar to the outer rim, and are higher in Na, Ca, Al, Cr, V, Sc and lower in Y, Zr, Ti, Nb, P compared to high-NiO phenocrysts. These petrographic and compositional characteristics indicate that high-NiO phenocrysts represent olivine from peridotite metasomatized by a melt, while low-NiO phenocrysts represent crystallization products. Low-NiO phenocryst compositions can be reproduced by orthopyroxene digestion and olivine fractionation of a reduced, siliceous “protokimberlite” melt which had fractionated 12 to 20% olivine. These results suggest a major change in the properties and composition of the Jericho kimberlite parental melt at the 150 km interface between the shallow and deep lithospheric mantle layers, which is attributed to assimilation of carbonate at this depth.