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). Hy...

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Veröffentlicht in:Chemical geology 2013-08, Vol.352, p.153-169
Hauptverfasser: Aulbach, Sonja, Griffin, William L., Pearson, Norman J., O'Reilly, Suzanne Y.
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Sprache:eng
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Zusammenfassung: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 >3.3Ga ultra-depleted shallow SCLM (145km), 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 143Nd/144Nd (0.5123–0.5129) over a large range of 147Sm/144Nd (0.08 to 0.43) at the time of entrainment in the kimberlite, which indicates isotopic homogenisation less than 350Ma ago. Combined with their variable 176Hf/177Hf (0.2825–0.2881), this can be modelled by mixing an ancient fluid-metasomatised cratonic mantle residue with ~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 (176Hf/177Hf=0.2879) similar to that of undisturbed ancient depleted mantle, and highly unradiogenic Nd (143Nd/144Nd=0.5099). This may reflect Proterozoic or earlier metasomatism by HFSE-poor, strongly LREE-enriched metasomatic fluids. •Trace-elements and Sr–Nd–Hf isotopes constrain metasomatic agents, processes and timing.•Shallow mantle interacted with fractionated melt during Proterozoic or earlier metasomatism.•It preserved unradiogenic Nd and radiogenic Hf due to ancient melt extraction and LR
ISSN:0009-2541
1872-6836
DOI:10.1016/j.chemgeo.2013.05.037