School of Earth Sciences - Theses
Permanent URI for this collection
Search Results
1 - 1 of 1
-
ItemPetrological and geochemical constraints on the source to surface evolution and emplacement style of the Lac de Gras kimberlites, Canada( 2021)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 mantle material, as indicated by a strong correlation between olivine rim and olivine core compositions, which are considered to be proxies for the compositions of primitive melt and entrained lithospheric mantle material, respectively. Different initial epsilon Nd and Hf, and olivine rim and chromite compositions for extrusive pipe-filling CK and intrusive kimberlite dykes from different LDG localities indicate derivation from different primary melt compositions. However, at some localities (e.g., Diavik), intrusive and extrusive kimberlites feature indistinguishable olivine and chromite compositions, indicating similar primitive melt compositions. These results indicate that primitive melt compositions may control the emplacement style of some, but not all, kimberlite magmas at LDG. Greater modal abundances of groundmass phlogopite and monticellite and lower groundmass abundances of carbonate for the extrusive versus intrusive kimberlites are attributed to greater volatile exsolution during the ascent and higher energy emplacement of the extrusive kimberlites. Greater SiO2, MgO and NiO, and lower incompatible element (i.e., TiO2, Nb, Ta, REE) whole-rock compositions for the extrusive versus the intrusive kimberlites cannot be explained by mixing lithospheric mantle or crustal compositions with reconstructed primitive kimberlite melt compositions, suggesting that these processes were not responsible for the different degrees of volatile exsolution evident in these kimberlites. Explosive emplacement of gas-rich magma excavated pipes at LDG prior to the emplacement of pipe-filling CK, which suggests that pipe-filling CK might reflect the waning stages of volcanic eruptions initiated by the explosive emplacement of a gas-rich dyke tip (VK) followed by the emplacement of melt-rich tails (pipe-filling CK). Further work is required to test the potential genetic relationship between CK and VK at LDG. Primitive melt composition, geological setting, the availability of water to trigger phreatomagmatic eruptions and/or magma segregation during ascent are suggested to influence the emplacement style of kimberlite magmas.