Nd, Sr, Pb, Ar, and 0 isotopic systematics of Sturgeon Lake kimberlite, Saskatchewan, Canada: constraints on emplacement age, alteration, and source composition

Rb-Sr isotopic dating of phlogopite megacryst samples separated from Sturgeon Lake kimberlite, Saskatchewan, yields a crystallization age of 98±1 Ma (2 s, MSWD=1.2; ⁸⁷Sr/⁸⁶Sr(t)=0.7059). The ⁴⁰Ar/³⁹Ar analyses of a phlogopite megacryst sample indicate the presence of large amounts of excess ⁴⁰Ar and yield an excessively old age of ~410 Ma. Assessment of the Ar data using isotope correlation plots indicates clustering of the data points about a mixing line between the radiogenic ⁴⁰Ar component at 98 Ma and a trapped component with uniform ³⁶Ar/⁴⁰Ar and Cl/⁴⁰Ar. Values of {ie212-1} as high as +20%. (VSMOW) for calcite from the groundmass and a whole-rock sample indicate pervasive lowtemperature alteration. The {ie212-2} of matrix carbonate is-11.3%. (PDB), slightly lighter than typical values from the literature. The {ie212-3} values of about +5%. (VSMOW) for brown phlogopite megacrysts may be primary, green phlogopites are interpreted to be an alteration product of the brown variety and are 2%. heavier. Initial Nd-Sr-Pb isotopic ratios for a whole-rock sample {ie212-4}; ⁸⁷Sr/⁸⁶Sr=0.7063, ²⁰⁶Pb/²⁰⁴Pb=18.67, ²⁰⁷Pb/²⁰⁴Pb=15.54, ²⁰⁸Pb/²⁰⁴Pb=38.97) suggest an affinity with group I kimberlites. Initial {ie212-5} values of +1.7 and +0.5 (⁸⁷Sr/ ⁸⁶Sr(t)=0.7053 and 0.7050) for eclogitic and lherzolitic garnet megacryst samples, and values of 0.0 for two phlogopite megacryst samples reflect an origin from an isotopically evolving melt due to assimilation of heterogeneous mantle. Lilac high-Cr lherzolitic garnet megacrysts give an unusually high {ie212-6} of +28.6 (⁸⁷Sr/⁸⁶Sr=0.7046) indicating a xenocrystic origin probably from the lithospheric mantle. The very radiogenic ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios of the kimberlite are consistent with melting of EM II (enriched) mantle components.