Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)

Trace-element and Sr-Nd-Hf isotopic compositions of garnet and clinopyroxene in mantle xenoliths from the diamondiferous Lac de Gras kimberlites, central Slave craton, constrain the nature, effects and timing of metasomatic modification of the underlying subcontinental lithospheric mantle (SCLM). Hypothetical carbonatite melts in equilibrium with peridotitic garnet from the >3.3 Ga ultra-depleted shallow SCLM (<145 km) have strongly sinusoidal REE patterns and negative Ti-Zr-Hf anomalies similar to those in natural carbonatites. Patterns of calculated kimberlitic melts in equilibrium with garnet from the less-depleted similar to 33 Ga deep SCLM (>145 km), which has normal LREE-depleted to mildly sinusoidal REE patterns and weak (or absent) negative Ti anomalies, match natural kimberlite compositions. These systematics translate to distinctly higher Ce/Yb, Sm/Er and Lu/Hf, and lower Zr/Sm and Sm/Nd in shallow peridotites, compared to deep peridotites. The latter show a continuum of trace-element ratios compatible with progressive interaction between a kimberlite-like melt and garnet-bearing mantle, leading to loss of Y and HREE and enrichment in LREE and Sr in the melt, increasing sinuosity of the REE, and lower Sm/Nd and Lu/Hf in the garnet, as the melts evolve to more volatile-rich compositions. Minerals in the majority of deep peridotites display a small range of Nd-143/Nd-144 (0.5123-0.5129) over a large range of Sm-147/Nd-144 (0.08 to 0.43) at the time of entrainment in the kimberlite, which indicates isotopic homogenisation less than 350 Ma ago. Combined with their variable Hf-176/Hf-177 (0.2825-0.2881), this can be modelled by mixing an ancient fluid-metasomatised cratonic mantle residue with similar to 1 to 10% kimberlite melt, which obliterated the original Nd-isotope compositions, but was less efficient at overprinting Hf isotopes. While samples from the shallow SCLM are few, this layer appears to have largely escaped pervasive kimberlite-related overprint: it retained radiogenic Hf (Hf-176/Hf-177 = 0.2879) similar to that of undisturbed ancient depleted mantle, and highly unradiogenic Nd (Nd-143/Nd-144 = 0.5099). This may reflect Proterozoic or earlier metasomatism by HFSE-poor, strongly LREE-enriched metasomatic fluids. (C) 2013 Elsevier B.V. All rights reserved.