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Mineral inclusions in diamonds from the Panda kimberlite, Slave Province, Canada

¹ Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada
² Division of Earth Sciences, Centre for Geosciences, University of Glasgow, Glasgow, G12 8QQ, UK
³ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543, USA
⁴ Institut für Mineralogie, Universität Frankfurt, 60054, Frankfurt/Main, Germany

^* E-mail: rtappert{at}ualberta.ca

Ninety diamonds from the Eocene Panda kimberlite (Ekati Mine, Northwest Territories, Canada) were analyzed for the major and trace element compositions of their mineral inclusions using electron microprobe techniques (EPMA) and secondary ion mass spectrometry (SIMS). Additionally, nitrogen aggregation characteristics of the host diamonds were measured using Fourier-transform infrared spectroscopy (FTIRS). Within the cratonic lithosphere, Panda diamonds are principally derived from peridotitic sources (85 %) with a minor content of eclogitic diamonds (10 %). Ferropericlase bearing diamonds (5 %) contain combinations of ferropericlase with either Mg-Al spinel plus olivine or with olivine or with a pure silica phase. The chemical characteristics of these inclusions are in accordance with a lithospheric origin from ferropericlase-bearing dunites. Ferropericlase coexisting with CaSiO₃ (most likely originally included as Ca-silicate perovskite), however, is regarded as evidence for a lower mantle origin of the host diamond.

Major element compositions show that the peridotitic diamonds formed in a moderately depleted environment, indicated by the presence of harzburgitic garnet inclusions with calcium contents generally > 2.5 wt% CaO and olivines with Mg numbers (100*Mg/(Mg+Fe)) of 92–93.5. Rare earth element (REE) concentrations in peridotitic garnets largely follow subdivisions based on major elements with lherzolitic garnets showing middle REE to heavy REE enriched, slightly sinusoidal patterns, whilst harzburgitic garnets have distinctly sinusoidal REE_N. Inclusion geothermobarometry indicates formation of peridotitic diamonds in the temperature range 1100–1250°C, following a geothermal gradient of 40–42 mW/m², in accordance with similar observations world-wide. Touching garnet-olivine and garnet-orthopyroxene inclusion pairs equilibrated at lower temperatures of 1000–1100°C, corresponding to a geothermal gradient around 37 mW/m². The higher temperatures are considered to be those prevailing during diamond formation.

Nitrogen contents in Panda diamonds vary strongly from below detection (< 10 ppm) to 2700 atomic ppm. Nitrogen aggregation ranges from poorly aggregated (Type IaA diamond) to highly aggregated (Type IaB diamond). If all diamonds that show signs of plastic deformation during mantle residence are excluded from the dataset, then a diamond subset becomes apparent with an overall low nitrogen aggregation state of < 30 % B-center. This result may indicate that plastic deformation increases the aggregation of nitrogen in Panda diamonds. Taking the Early Archean Re-Os isochron date for sulfide inclusions in Panda diamonds (Westerlund et al., 2003b) at face value, the low aggregation states of undeformed diamonds may indicate mantle residence at relatively low temperatures (< 1100°C). If this is the case, the decrease in temperature inferred from the comparison of touching and non-touching inclusion pairs must have occurred soon after diamond formation. Thus diamond formation beneath the central Slave may be restricted to short lived and localized thermal events. An apparent increase in geothermal gradient with depth in the lithospheric mantle beneath the Central Slave for the time of kimberlite eruptions (Upper Cretaceous to Eocene) may have a similar cause and reflect transient heating of the deep lithosphere during melt infiltration.

Key-words: inclusions in diamonds, lithospheric mantle, lower mantle, Lac de Gras, Slave craton.