Mantle structure beneath the SW Slave craton, Canada: Constraints from garnet geochemistry in the Drybones Bay kimberlite

This study describes the petrography of peridotite xenoliths, and the major and trace element geochemistry of garnets in both xenoliths and coarse concentrate from the Drybones Bay kimberlite. The temperature and depth of equilibration of clinopyroxene and garnet show that the mantle lithosphere beneath the SW margin of the Slave Province was at least 160 km thick at the time of kimberlite emplacement (similar to450 Ma). The garnet population is dominated by grains with sinuous light rare earth element (LREE)-enriched trace element patterns many of which are ultra-depleted in Zr, Y and Ti. Normal heavy REE (HREE)-enriched garnets make up a small proportion of the garnet population. The spectrum in garnet geochemistry may be explained by heating and percolation of a metasomatic fluid from through an ultra-depleted mantle protolith before kimberlite eruption. A shallow ultra-depleted layer of mantle lithosphere recognized in garnets from the central Slave Province may exist beneath the SW margin of the craton, but the layer has been modified or overprinted by the metasomatic front, which has caused enrichment in Ca and incompatible elements in garnets. The changes in garnet chemistry in the Drybones Bay garnets are coincident with a unique seismic discontinuity exhibiting multiple layer anisotropy commencing at 110 km depth similar to50 km north of Drybones Bay; correlation of the two features is, however, tenuous.