How old are lherzolitic diamonds? Mesoproterozoic diamond formation in the root of the Sask craton

Abstract

Diamondiferous kimberlites of the Cretaceous Fort á la Corne (FalC) field erupted through the Sask craton. The Palaeoproterozoic age of its lithospheric mantle root provides an unconventional setting for a major diamond deposit. We report the first diamond formation ages for the Sask craton, using diamonds from the Star kimberlite. Sm-Nd dating of garnet and clinopyroxene inclusions of lherzolitic paragenesis yields an isochron of 1262 ± 37 Ma and an ɛNd_i value of -10.8 ± 1.2. The average initial ⁸⁷Sr/⁸⁶Sr at 1262 Ma is 0.70459 ± 0.00001. A single diamond-forming event is supported by the overall similar inclusion compositions (major and trace elements), host diamond carbon isotopic compositions, N-abundance and low N-aggregation states. A Monte Carlo mixing model to generate the initial Sr-Nd isotope compositions of the FalC diamond inclusion suite supports a scenario in which the diamond substrates acquired their geochemical characteristics through earlier infiltration of lithospheric lherzolite by variable amounts (8 to 10 wt%) of an incompatible element-enriched melt with isotopic characteristics resembling cratonic lamproite. We propose a model in which asthenosphere-derived melts produced during rifting or the Trans Hudson Orogeny formed a metasome in the deep Sask craton lithospheric root. This metasome evolved isotopically for ~ 0.6 to 0.8 Gyr, before being remobilized and refertilizing lherzolitic substrates, resulting also in diamond formation. Diamond formation was associated with minimal thermal disturbance, during mobilization of fluids triggered by either far-field effects from the Mackenzie dyke swarm (~ 1270 Ma) or the Grenville orogeny (1.3–0.9 Ga).