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Diamonds reveal subducted slab harzburgite in the lower mantle
Heamanite-(Ce), (K 0.5 Ce 0.5 )TiO 3 , a new perovskite supergroup mineral found in diamond from Gahcho Kué, Canada
Preface
Carbon and Nitrogen in Mantle-Derived Diamonds
Mineral Inclusions in Lithospheric Diamonds
Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle
Abstract 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 TiO 2 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 TiO 2 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 <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 143 Nd/ 144 Nd 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 <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.
Abstract The morphology and color of Diavik diamonds, their nitrogen concentrations and δ 13 C values, and the composition of their mineral inclusions provide insights into the formation of diamonds and the evolution of the mantle root beneath the central Slave craton. The minerals which make up inclusions reflect a largely peridotitic mantle source region (77% peridotitic, 18% eclogitic, 1% ultradeep, and 4% ferropericlase bearing) predominantly composed of garnet-harzburgite. The major element geochemistry of the inclusions indicates that the degree of primary melt depletion during formation of the central Slave cratonic mantle root was distinctly lower than for other cratons worldwide—for example, beneath Yakutia and the Kaapvaal craton. The formation of peridotitic diamonds in the Paleoarchean was likely followed by lithosphere-scale cooling by about 150°C, based on differences in equilibration temperatures for touching and nontouching inclusion pairs and nitrogen aggregation-based residence temperature estimates for their respective diamond hosts. The protolith of the eclogitic diamond substrate likely was basaltic-gabbroic oceanic crust, as shown by trace element patterns that include weak positive Eu N anomalies in one garnet and one clinopyroxene inclusion, and a normal mid-ocean ridge basalt (NMORB) signature for the calculated bulk-rock eclogite. The analysis of microinclusions in fibrous diamonds reveals the presence of high-density fluids that span a continuous compositional range between carbonatitic and saline end-members. The diamonds grew from continuously evolving and fractionating melts/fluids that moved through the mantle. Cathodoluminescence imaging of fibrous and clear monocrystalline diamonds indicates that they grew in pulses with intermittent periods of resorption.