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Diamonds from the deep: pipe DO-27, Slave craton, Canada

2019; Linguagem: Inglês

10.29173/ikc2671

R. Davies, Norman J. Pearson, Suzanne Y. O′Reilly, William L. Griffin, Anita S. Andrew, Buddy Doyle,

Maritime and Coastal Archaeology

This is the first report of an ongoing investigation of diamonds (mineral inclusions, diamonds' physical and chemical characteristics) from kimberlite pipe DO-27, near Lac de Gras in the Slave Craton, Canada.This study is a component of our Lithosphere Mapping project on the Slave Craton, which integrates petrological and geophysical data to understand the composition, structure and origin of the lithospheric mantle; this information is critical to diamond exploration models for the Slave Craton, which has a unique lithospheric structure (Griffin et al., this volume). Physical CharacteristicsDiamonds examined weigh between 0.01 and 0.42 carats; 75% were <0.10 carat.75% of the stones are coloured, from shades of brown (55%) to yellow/brown (5%), yellow (9%) and grey (6%).Morphology ranges from planar octahedra and composite octahedra with minor resorption (30% of stones) to heavily resorbed dodecahedra.All resorption categories (Robinson et al., 1989) are represented, and more than half of the diamonds have lost 25% to 65% of their original mass.Resorbed forms consist of equal proportions of dodecahedra, flattened dodecahedra, aggregates and fragments with resorbed external faces.12% of stones are cubes and cubo-octahedra, many of which are fibrous and/or have hopper faces.Octahedral diamonds have smooth finely stepped planar surfaces and ribbed edges.Negative etch trigons and hexagons are common on primary faces.Dodecahedral forms preserve dissolution laminae and large drop-shaped hillocks.Ruts are common and resorbed surfaces are often frosted.Slip plane dislocations resulting from plastic deformation are evident on about half the stones, as glide planes and shagreen texture on resorbed dodecahedral faces. Diamond InclusionsMineral inclusions were extracted by breaking diamonds in an enclosed cell, then placed in epoxy on glass slides and polished for electron microprobe analysis.Small inclusions exposed on cleavage surfaces were analysed in situ.Representative analyses are given in Table 1.Eclogitic paragenesis: =50% of the inclusions are eclogitic.Eclogitic garnets have 9-16% CaO and variable Na^ contents; no majorite component is present.Their composition suggests they are derived from kyanite-bearing eclogites, similar to observed xenoliths (Pearson et al., this vol.).One "omphacite" has a high level of opx solid solution, implying a high-T origin; another contains Jd=25%.Diopside inclusions also occur but may be epigenetic (Table 1).Low-Ni (<2.9% Ni) iron sulfides of the eclogite paragenesis were recovered from 5 diamonds . Peridotitic paragenesis:This paragenesis includes olivine, Cr-pyrope and pentlandite.Pyrope inclusion 27G has very high Cr203, but is only mildly subcalcic.It is extremely depleted in Y and Zr, but contains significant Sr, as is typical of diamond-inclusion Cr-pyrope garnets.Two lherzolitic garnets intergrown with diamonds give TNj = 1130 °C and 920 °C.All of the olivines have high mg# (92.8-94.0),suggesting a harzburgitic paragenesis.One pentlandite inclusion has been recovered.Super-deep paragenesis : At least 25% of the inclusion-bearing stones contain inclusions of ferropericlase ((Fe,Mg)0) or Mg-perovskite.The ferropericlase inclusions have mg#, Cr and Ni contents similar to inclusions reported from Orroroo, Koffiefontein and Sloan (Scott-Smith et al., 1984;Otter and Gurney, 1989).In diamond 14A the ferropericlase is accompanied by an inclusion with MgSi03 stoichiometry, interpreted as the corresponding Mg-perovskite phase, and by a tiny inclusion of essentially pure Ni.Phlogopite and other possible epigenetic phases.Diamond 26D contained a granular mass of phlogopite with irregular intergrowths of heterogeneous almandine garnet and diopside.This assemblage is believed to be due to infiltration of fluids along a crack between two parts of the stone.Phlogopite, calcite, sphene and phlogopite + diopside + calcite intergrowths have been found in other stones.