Search

In the Lac de Gras (LDG) kimberlite field, Northwest Territories, Canada, coherent kimberlites (CKs) occur as tabular dykes, pipe-shaped diatremes, and irregular bodies without well-defined geometries. Combining the morphology of CK bodies with the occurrence of fragmented olivine microcrysts allows distinction of four CK types at LDG: (1) dykes with no broken olivine; (2) CK without well-defined but probable sheet geometry and no broken olivine; (3) pipe-filling CK (pfCK) with abundant broken olivine and (4) pfCK with no broken olivine. These features suggest an intrusive origin for type 1 and, probably, type 2 CK; a high-energy extrusive emplacement for CK type 3 and a low-energy intrusive or extrusive emplacement for the CK type 4. Here, we compare petrographic and whole-rock, olivine and spinel compositional data for high-energy extrusive pfCK, low-energy pfCK and intrusive CK units to understand the factors controlling their variable emplacement styles. Extrusive CK contain more abundant groundmass phlogopite and monticellite, lower carbonate/silicate mineral abundance ratios and significantly lower dolomite and pleonaste-spinel abundances compared to intrusive CK. This indicates greater CO2 loss and higher H2O/CO2 in the melt phase for the extrusive CK during emplacement. Lower incompatible element concentrations in the extrusive CKs and different chromite Ti# and olivine rim Mg# indicate derivation from distinct primitive melt compositions. The extrusive CK feature higher ɛNdi and marginally higher ɛHfi compositions than the intrusive CK, pointing to derivation from distinct sources. These findings strongly imply that distinct primary melt compositions were largely responsible for the differences in emplacement styles of CK at LDG. Low-energy pfCKs have similar olivine rim Mg#, chromite Ti# and, hence, primitive melt compositions to the high-energy extrusive CK samples. Their marginally different emplacement styles may depend on local factors, such as changing stress regimes, or slightly different volatile concentrations. Both types of pfCK might reflect the waning stages of volcanic sequences resulting from the eruption of a segregated magma column that started with pipe excavation and the explosive emplacement of gas-rich magma (volcaniclastic kimberlite), followed by the less energetic emplacement of melt-rich magma (pfCK). This hypothesis underscores different primary melt compositions for dyke vs pipe-forming (and filling) kimberlites and hence a fundamental primary melt control on the explosivity of kimberlites. [ABSTRACT FROM AUTHOR]

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 TiO 2 -rich and TiO 2 -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. • Isotopic evolution of the Lac de Gras kimberlite source with time. • The Lac de Gras kimberlites feature a range of primitive melt compositions. • Primitive melt compositions vary both laterally and temporally. • Lateral variants due to assimilation of compositionally diverse lithospheric mantle. • Temporal variations due to consumption of subducted material in the source. [ABSTRACT FROM AUTHOR]

Kimberlites are mantle-derived magmas that either crystallise as hypabyssal intrusions, erupt explosively after rapid ascent to the surface, or less commonly form lava lakes and flows, thereby creating texturally distinct kimberlite units. Efforts to fully understand the processes responsible for the explosive eruption of kimberlite magmas have been hindered by the widespread alteration and crustal contamination of most volcaniclastic kimberlites. To address this issue, we have undertaken a detailed petrographic and mineral chemical study of fresh (i.e. minimally altered) pyroclastic and hypabyssal kimberlites (HK) from the ca. 55-56 Ma A154 North and South kimberlite pipes in the Diavik Mine (Lac de Gras, Canada). These localities host exceptionally fresh kimberlites and are therefore ideally suited to this study. Kimberlite emplacement at A154 North and South initiated with the intrusion of hypabyssal kimberlite (external dykes), and was followed by the explosive formation of kimberlite pipes and volcaniclastic kimberlite infill. Subsequent kimberlite magmas intruded the volcaniclastic kimberlite units forming multiple cross-cutting, internal dykes. The studied volcaniclastic units feature abundant rounded magmaclasts and massive textures, suggestive of primary deposits. These units are classified as pyroclastic kimberlites (PK). Pyroclastic and hypabyssal kimberlite units at Diavik exhibit subtle mineral compositional differences. Samples from both internal HK units and PK units feature identical compositions for liquidus olivine rims (Mg# = 90.5 ± 0.1 and 90.7 ± 0.2, respectively), with a marginally lower Mg# of 90.2 ± 0.2 in olivine rims from the external HK dykes. Similarly, early-formed chromite compositions are the same for internal HK and PK units (Cr# = 79.1 ± 3.4 and 78.3 ± 5.7; Mg# = 60.0 ± 1.3 and 60.0 ± 2.2), but, differ in the external HK units (Cr# = 86.9 ± 2.7; Mg# = 52.8 ± 1.9). The internal HK and PK units also exhibit lower carbonate contents than the internal HK units. These compositional differences indicate that the external dykes were probably derived from slightly different primitive melt compositions to those parental to the internal HK and PK units. Spinel evolutionary trends from chromite to magnesian ulvӧspinel-magnetite (MUM) compositions are indistinguishable in internal HK and PK samples (Fe3+# = 47.2 ± 5.8 and 49.7 ± 9.3; Cr# = 25.7 ± 11.0 and 17.0 ± 14.0). These results demonstrate that the primitive melt compositions and early magmatic evolution processes are identical for the internal kimberlite units, regardless of whether the kimberlite melts erupted explosively or were emplaced as shallow intrusions. However, magmaclasts in the PK units contain higher abundances of phlogopite (<52 vol. %) and lower quantities of carbonate (<4 vol. %) than the groundmass of the hypabyssal kimberlite samples (<2 vol. % and 25–65 vol. %, respectively). This indicates that the explosively erupted magmas featured higher H 2 O/CO 2 ratios. In contrast, abundant carbonates, including dolomite, in the internal HK samples indicate that CO 2 , and therefore low H 2 O/CO 2 ratios, were retained during the emplacement of this magma, which likely prevented phlogopite crystallisation. Lower K and Rb whole-rock compositions for internal HK samples compared to PK samples, are attributed to the removal of these components in late-stage kimberlitic fluids, as indicated by hydrothermal alteration of the adjacent volcaniclastic kimberlite units. The above results clearly rule out variations in primitive melt composition and melt evolution trajectories as a primary control on the explosive behaviour of the kimberlite magmas at Diavik. Controls on kimberlite explosivity at Diavik are likely due to external factors, such as local stress regimes, the availability of groundwater (i.e. phreatomagmatism) and differing magma supply rates. • Melts parental to the Diavik kimberlites had at least two primitive compositions. • Hypabyssal and pyroclastic kimberlites have the same early melt evolutions. • Kimberlite emplacement style is independent of primary melt compositions. • The late evolution of kimberlite magmas is controlled by the emplacement mechanism. • Emplacement style is likely controlled by external factors, not melt composition. [ABSTRACT FROM AUTHOR]