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Kimberlites originate from the deepest-derived magmas on Earth and are characterised by ultrabasic, H2O and CO2-rich, and silica-poor compositions. These magmas entrain and transport mantle material (sometimes including diamonds) during their ascent to the Earth’s surface, before intruding the upper crust, or erupting explosively to form deep (2-3 km) conical diatremes. With the exception of one Quaternary occurrence, active kimberlite magmatism has not occurred since ~30 Ma, and surface deposits are often eroded. Assimilation of mantle material, crustal contamination and post-emplacement hydrothermal alteration modify the compositions of kimberlites during emplacement, hindering attempts to constrain original melt compositions. There is also uncertainty about the factors that control the emplacement style of these magmas and whether melt compositions have any influence. To improve constraints on the composition and evolution of kimberlite melts and their mode of emplacement, 30 coherent intrusive and extrusive kimberlites (CK), and two volcaniclastic kimberlites (VK) from the Lac de Gras (LDG) field, Northwest Territories, Canada were studied using petrographic and geochemical methods. Olivine rim and chromite compositions show that kimberlites at LDG derive from a range of primitive melt compositions. Increasing age-corrected Nd-Hf isotope ratios with time correlate directly with olivine rim Mg# [100xMg/(Mg+Fe2+)] compositions and inversely with chromite Ti# [100xTi/(Ti+Al+Cr)] compositions for central LDG kimberlites. These correlations indicate that melt compositional variations stem from partial melting of an evolving kimberlite source due to progressive assimilation of less refractory, deeply-subducted crustal material. These relationships are not observed when considering all the LDG kimberlites. This is attributed to decoupling of the kimberlite source and primitive melt compositions for all the LDG kimberlites by assimilation of laterally heterogenous mantl more...