The spatial and temporal evolution of primitive melt compositions within the Lac de Gras kimberlite field, Canada: Source evolution vs lithospheric mantle assimilation

Abstract

Kimberlites are the deepest-derived magmas and provide unique insights into the composition and evolution of the deep Earth. However, this is hindered by contamination of kimberlite melts by mantle and crustal material during ascent. Previous work has shown that the Nd and Hf isotope compositions of the 47 – 75 Ma Lac de Gras (LDG) kimberlites (western Canada) evolve with time from geochemically enriched compositions towards more radiogenic values, which could result from either radiogenic ingrowth or a decreasing contribution of deeply subducted crustal material to the kimberlite source. While previously published TiO2-rich and TiO2-poor whole-rock compositions indicate that at least two distinct primitive melt compositions occur in the LDG field, it is uncertain whether temporal evolution of the kimberlite source influenced primitive melt compositional variability. In this study, the compositions of early formed olivine (rims overgrowths on xenocrysts) and chromite, together with xenocrystic olivine (cores) in 28 LDG kimberlites are combined with new and existing Nd and Hf isotopic compositions and geochronological constraints to examine the temporal and spatial variability of primitive kimberlite melt compositions and their relation to source evolution.Olivine rim Mg# [=100 × Mg/(Mg + Fe2+); 89.6 ± 0.2 to 91.8 ± 0.2] and chromite Ti# [=100 × Ti/(Ti + Al + Cr); 2.5 ± 0.3 to 10.1 ± 1.5], used as proxies for primitive kimberlite melt compositions, indicate that a range of melt compositions were parental to the LDG kimberlites. A positive correlation between olivine rim Mg# and commonly resorbed xenocrystic olivine cores (Mg# = 89.4 ± 1.9 to 92.1 ± 1.3) indicate that assimilation of laterally heterogeneous lithospheric mantle contributed to primitive melt variability. Spatially proximal kimberlites in the Panda, Falcon and Arnie clusters in the centre of the field feature olivine rim Mg# and chromite Ti# with moderate to strong correlations with emplacement ages (47 to 75 Ma), and initial 176Hf/177Hf and 143Nd/144Nd ratios. These correlations indicate that once the effects of lithospheric mantle assimilation are screened out by considering proximal kimberlites, which traversed a similar lithosphere, the compositions of primitive melts at LDG are controlled by the temporal evolution of the sub-lithospheric source. The temporal variability of primitive melt compositions at LDG (i.e., increasingly radiogenic Nd and Hf isotopes and increasing melt Mg# with decreasing age) suggests that the source of these kimberlites evolved via progressive consumption of a deeply subducted, enriched component that was previously mixed with a geochemically more depleted mantle. This work highlights that the compositions of kimberlites are profoundly influenced by both their asthenospheric sources, including deeply subducted material, and assimilation of laterally heterogeneous lithospheric mantle during ascent.