Small mantle fragments from the renard kimberlites, Quebec: Powerful recorders of mantle lithosphere formation and modification beneath the eastern superior craton

The origin and evolution of diamondiferous lithospheric mantle sampled by the Neoproterozoic Renard kimberlites (eastern Superior Province, Quebec) is constrained based on mantle-derived microxenoliths and xenocrysts. The dataset illustrates the wealth of knowledge that can be gleaned from small samples (1·;2 mg-2·;2 g) through an integration of multiple, mainly single mineral based approaches. Our samples document the presence of an ∼200 km thick lithosphere with a 'cold' (38 mW m ^-2 surface heat flow) model geotherm at the time of kimberlite emplacement (c. 632 Ma), resulting in a large diamond window from 130 to 200 km (42-60 kbar). On the basis of the mantle xenolith and xenocryst record and excluding megacrysts, the lithospheric mantle beneath Renard was dominated by peridotite (91%), composed of lherzolite (72% of samples), harzburgite (24%) and wehrlite (5%), with minor eclogitic (3%) and websteritic (6%) portions. Comparatively abundant harzburgite probably establishes the principal diamond source, but elevated Na contents in eclogitic garnet suggest the additional presence of diamond-stable eclogites. A number of events have modified the lithospheric mantle underlying the eastern Superior Province, including the following: (1) evolving 'kimberlitic' melts pervasively re-fertilized the originally strongly depleted lithospheric mantle with respect to highly and moderately incompatible trace elements; (2) less pervasive fluid style metasomatism is indicated by selective re-enrichment of highly incompatible elements that occurred within a depth range of 125-170 km. In situ Pb isotope data obtained for clinopyroxenes suggest a model age of ∼2·;7 Ga for the protolith(s) of the cratonic lithospheric mantle beneath Renard. This age coincides with a major phase of continental crust generation within the Superior Province and throughout the Laurentia supercontinent (e.g. Greenland).