Mantle source and petrogenesis of kimberlites from the Foxtrot Kimberlite Field of northern Québec, Canada

Kimberlite whole rock geochemistry has been under-appreciated in deciphering Group I kimberlite petrogenesis. Although there is considerable debate on the definition of Group I kimberlite, there is agreement on certain characteristics. These include that Group I kimberlites are ultramafic rocks (MgO >15 wt%), have near primitive mantle nickel (~ 1400 ppm) and chromium (~1600 ppm) contents, have close to bulk silicate earth radiogenic isotopic signatures, are volatile rich (predominantly CO2) and have stable isotopic signatures typical of mantle sources. The debate, however, centers on petrogenetic models and parental magmatic compositions of Group I kimberlites. Petrogenetic models include both low degree (<1 %) partial melting of metasomatized asthenospheric mantle (Dalton and Presnall, 1998a; 1998b; Becker and Le Roex, 2006) and, conversely, high degree partial melting (10<20 %) of metasomatized veined asthenospheric mantle (Mitchell, 1995, 2004). Inferred kimberlite parental magma compositions have been suggested to be low silica (<5 wt%) and high CO2 (<44 wt%) melts, as suggested by high-pressure melt experiments conducted on synthetic carbonatized mantle (Dalton & Presnall, 1998a, 1998b), or much higher silica (<30 wt%) and lower CO2 (<12 wt%) melts as suggested by geochemical analysis of aphanitic kimberlite (Price et al., 2000; Kopylova et al., 2007). In an effort to contribute to the resolution of this debate on petrogenic models and parental magma compositions of Group I kimberlites, I have completed a systematic investigation of the whole rock geochemistry and petrology of three kimberlite occurrences in the Foxtrot Kimberlite Field of Northern Quebec. A clear correlation exists between whole rock chemistry, mantle source, and diamond grade of the three Group I kimberlites in the Foxtrot Kimberlite Field. Two phases, olivine and carbonate, dominant theserocks and thus control the kimberlites geochemistry. Electron microprobe analysis of over 800 olivines in the Foxtrot kimberlite indicate that it is predominantly xenocrystic (Mg# (Mg/(Mg+Fe)) of 91.5), largely derived from harzburgite mantle, but the absence of harzburgitic orthopyroxene in the Foxtrot kimberlites suggests that it has been assimilated. Whole rock geochemistry (major and trace elements) of the Foxtrot hypabyssal kimberlite dykes indicate they have chemical characteristics similar to Group I kimberlite from the Slave Province. The olivine content of the Foxtrot kimberlites, and thus their Mg#'s, are positively correlated with diamond abundance indicating that diamond grade is a reflection of the incorporation of harzburgite mantle. These results indicate that Group I kimberlite petrogenesis in the Foxtrot example is best modeled as a mixture of harzburgitic mantle xenocrysts and a carbonate rich fluid, and that the parental magma is silica poor "5 wt %) and CO 2 rich (<40 wt%).

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