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

Abstract

The composition of the lithospheric and sublithospheric mantle beneath the Slave craton was investigated, using 88 diamonds and their mineral inclusion. The inclusions were analyzed using electron (major elements) and ion micro probes (trace and ultra-trace elements.

Major element data show that the majority of inclusions belongs to the peridotitic suite and suggests formation in a moderately depleted environment, without indications for low Ca-harzburgitic and dunitic sources.

Garnet-olivine, garnet-opx and Zn in spinel geothermobarometry indicate diamond formation under P-T conditions and along a geothermal gradient (40-42 mW/m2) similar to other cratons worldwide. Touching inclusion pairs appear to have equilibrated at lower temperatures corresponding to a geothermal gradient of around 37 mW/m2 (surface heat flow). A similar Cretaceous to Tertiary palaeo-geotherm was found for garnet peridotite xenoliths from the Lac de Gras area. This apparent discrepancy possibly relates to partly low aggregation levels of nitrogen impurities within Panda diamonds which suggest a drop in temperature by at least 100°C soon after diamond formation.

Eclogitic mineral inclusions are rare in Panda diamonds, similar other kimberlites within the Ekati property and Snap Lake but in contrast to DO27.

Beside the lithospheric inclusions three diamonds contain phases that indicate a possible origin from the lower mantle. The characteristic inclusion phase in these diamonds is ferropericlase. In one case ferropericlase is associated with CaSi-perovskite, in a second sample with a SiO2-phase. Another diamond contained a touching inclusion pair of ferropericlase and MgAl-spinel (with minor amounts of Fe and Cr). This spinel probably originated as epsilon-spinel, a lower mantle phase experimentally predicted for corundum saturated bulk compositions. A characteristic feature of all assumed lower mantle diamonds is an irregular crystal shape in combination with nitrogen concentrations below detection (Type II diamonds).

REE concentrations in ten peridotitic garnets indicate that the trace element pattern is in general agreement with the major element composition. Lherzolitic garnets show MREEN-HREEN enriched or slightly sinusoidal REE pattern, whereas harzburgitic garnets derived from depleted lithologies show sinusoidal shapes. Only one harzburgitic garnet shows an overall enriched REE pattern and an additional enrichment in Zr and Y. Combined enrichment in MREE-HREE and other HFSE is taken as evidence that metasomatic overprint in this case was caused by a silicate melt.

A model for the evolution of the lithospheric and sublithospheric mantle beneath the Slave craton involves an early stage of melt depletion of a primitive source in the spinel stability field, causing the high Cr/Al ratios in garnets, typical for peridotitic garnets worldwide. In a second stage the depleted mantle is stacked beneath the early Slave craton and successively re-enriched in major and trace elements. The high geothermal regime for the separate inclusions and the inferred “rapid” cooling after diamond formation may relate diamond preciptitation and mantle metasomatism to short lived thermal perturbations (magmatic intrusions), which may also have introduced the sublithospheric diamond suite.