Kimberlite and related rocks from Garnet Lake, West Greenland, including their mantle constituents, diamond occurrence, age and provenance

Abstract Observations of thickness, orientation and morphology and mineral chemistry of the principal diamondiferous intrusive sheet and associated bodies in the vicinity of Garnet Lake, Sarfartoq, West Greenland are reported. The principal body dips to the east on a NE/SW (true) trend and reaches a maximum thickness of 4.25 m. Multiple intrusive events are identified within the main sheet including sub-parallel bands occasionally exhibiting grain size sorting, cross-cutting layers and late-stage carbonate-rich emplacement, particularly at the contacts with country rock. Phenocrystic mineral assemblages and compositional measurements reveal two principal petrological types. The dominant type is an aillikite and the second rock type is a kimberlite. The kimberlite exhibits thin Ba-rich rims (towards kinoshitalite) on Al-rich phlogopite crysts, and an abundance of perovskite. Compositional zonation in groundmass spinels suggest a later transition towards an aillikite component. The aillikite is characterised by abundant phlogopite, heavily zoned with tetraferriphlogopite rims, transitional Type 1-Type 2 spinel compositions, rare Al,Ti-rich groundmass clinopyroxene and occasional exotic Sr-carbonate phases such as olekminskite. The Garnet Lake main sheet is characterised by mantle phases occurring as individual grains, most strikingly as garnet xenocrysts up to 5 mm and disaggregated mantle olivine crysts. Xenoliths occur rarely and are typically garnet dunites and garnet lherzolites. Heavy mineral separation reveals an abundance of G10D garnets and, whilst peridotitic garnets dominate, eclogitic G3D and G4D garnets also occur. Trace element compositions of garnet crysts reveal sinusoidal REE patterns in harzburgitic garnets however a component of flat and REE-enriched G11 garnets is apparent, reflecting significant mantle refertilisation. Thermorbarometric calculations on assemblages in Garnet Lake main sheet garnet lherzolites reveal equilibrium conditions clustering closely around 1258 °C and 6.20 GPa, equivalent to a depth of 195 km along a 41 mWm − 2 mantle geotherm. Garnet Lake xenoliths thus lie at the deepest extent of mantle samples recovered from western Greenland lamprophyric rocks and from well within the diamond stability field. Diamonds are typically colourless and occur as fragments or with a dominant octahedral morphology. Crysts exhibit very high quantities of nitrogen (up to 2133 ppm N) and yet are not strongly aggregated to IaB centres (typically about 33% aggregation). This suggests either a short residence time, low residence temperature or likely a combination of both. Employing the restricted range of equilibrium conditions exhibited by mantle xenoliths, calculated diamond formation ages peaking around 583–593 Ma are favoured. This is a short time before the 568 Ma Garnet Lake main sheet emplacement age which coincides with the age of the nearby Sarfartoq carbonatite complex. A model of mantle refertilisation and diamond formation culminating in conditions favourable for creating carbonate-rich melts acting as carriers of mantle constituents to the surface is presented. Data suggest close temporal, chemical and geographic associations between kimberlite, aillikite and carbonatite.