The Diamondiferous Mantle Root Beneath the Central Slave Craton
Authors: Aulbach Sonja , Heaman Larry M. , Stachel Thomas 
Published: February 2020

The diamondiferous mantle root beneath the Lac de Gras area in the central Slave craton (northwestern Canada) is now one of the world’s best characterized lithospheric mantle sections with regard to geochemical and thermophysical information. Its most spectacular feature is its marked stratification. An ultradepleted, highly oxidized, shallow layer to ~150-km depth consists dominantly of granoblastic harzburgite with olivine Mg# (100Mg/(Mg + Fe)) of 92 to 94. Garnet in this layer has very low TiO2 and Zr contents (avg 0.05 wt % and 9.5 ppm, respectively), and strongly sinusoidal REE patterns. The shallow stratum, which exhibits enhanced conductivity, is separated from a less conductive, less depleted, and less oxidized, dominantly lherzolitic layer by a seismically and geochemically imaged sharp discontinuity. The deep stratum features an olivine Mg# of 91 to 92, average garnet TiO2 of 0.26 wt % and Zr of 33.4 ppm, and includes porphyroclastic varieties. It reaches the thermal and mechanical lithosphere-asthenosphere boundary at ~220 km.

The ultradepletion of the shallow subcontinental lithospheric mantle (SCLM) may require an origin by polybaric melting at excess mantle potential temperature, accompanied by shallow plate interactions at ca 3.5 Ga, while the mild depletion of the deep SCLM could be explained by ca 3.3 Ga subcretion of upwelling mantle after a short melting interval, or may alternatively have formed by accretionary processes. The formation of both strata produced peridotitic diamond populations and was followed by amalgamation of the ancient (4.0−2.8 Ga) cratonic core with juvenile (2.7 Ga) domains in the eastern Slave craton, which may have led to the incorporation of coeval lithospheric mantle portions. Ancient (Proterozoic or Archean) interaction with fractionated high field strength element (HFSE)-poor fluids is inferred from garnet with strongly sinusoidal REE patterns and peridotite minerals with radiogenic Sr and Hf, but unradiogenic Nd, and was accompanied by diamond formation. A &lt;350 Ma metasomatic event by an evolving and increasingly fractionated kimberlite liquid is indicated by a spectrum of garnet REE patterns from “normal” light rare earth element (LREE) depleted to increasingly sinusoidal, and by relatively constant 143Nd/144Nd at variable Sm/Nd. The unfractionated melt was either destructive to diamonds or at least not conducive to diamond growth, whereas the signature of fractionated melt is identified in diamondiferous peridotite xenoliths and may have produced some fibrous overgrowth on diamonds.

Short-lived accretionary processes at the western craton margin are reflected in ca 1.85 Ga eclogite xenoliths that make up &lt;5% of the lithosphere column beneath Lac de Gras and that have trace element systematics consistent with gabbroic-or boninite-like precursors. They are concentrated just below the intralithospheric discontinuity and their mode of emplacement into substantially older mantle lithosphere remains enigmatic. Some eclogitic diamonds were likely generated during the metasomatic episodes identified in peridotite samples. However, the accretion itself produced a disproportionately high (relative to the absolute eclogite/peridotite ratio) abundance of sulfide-included and perhaps also other diamonds and eventually helped to conserve the diamondiferous mantle root beneath the central Slave craton.