A Study of Peridotitic Garnet Xenocryst Compositions from Selected Ultramafic Bodies in the Northern Alberta Kimberlite Province: Implications for Mantle Stratigraphy and Garnet Classification

Electron microprobe (EMPA) major-element analyses of peridotitic garnet xenocrysts from the northern Alberta kimberlite province typically have well-defined lherzolitic paragenesis with geochemical affinities that are uncharacteristic of garnet in diamondiferous kimberlite. Yet approximately 67% of the Buffalo Head Hills kimberlite field bodies contain diamonds with at least three kimberlite occurrences having estimated diamond contents of between 13 and 55 carats per hundred tonnes. This conundrum is important because the major element composition of periodotitic garnet has been used extensively to establish criteria for target evaluation in diamond exploration. A comprehensive set of garnet xenocrysts from the three separate ultramafic rock fields in northern Alberta were analyzed by LA-ICP-MS. These trace element data provide information additional to EMPA data that quantify parameters indicative of diamond potential and provide new information on the chemical nature of the lower crust-subcontinental lithosphere beneath northern Alberta.

This report shows that distinct compositional changes in garnet xenocryst Ti, REE, Y and Zr provide a means of separating garnets into distinct geochemical groups that disclose evidence for varying degrees of depletion or re-enrichment of the protolith. Based on garnet compositions—and using TNi as a proxy of depth-at least five lithological transitions are inferred for the lower crustal-sublithospheric mantle underlying northern Alberta. From low to high-T, these regions include the following: fertile lherzolite, chromite-clinopyroxene-garnet equilibrium trend garnet and wehrlite (<870°C), low-T lherzolite (870°C to 950°C), melt metasomatized wehrlite (950°—1000°C), depleted lherzolite and melt metasomatized lherzolite (1000°—1130°C), and moderately fertile lherzolite and high-Ti melt metasomatized lherzolite (>1130°C).

These compositional groups can serve as a proxy for future evaluation of garnet compositions in Alberta because they also distinguish inter- and intra-field mantle variations. In terms of diamond prospects in northern Alberta, one transitional mantle layer associated with the Buffalo Head Hills field includes a predominance of 1000° to 1130°C, low-Ti, Y and Zr-depleted lherzolite that implies a diamond window in the mantle underlying the Buffalo Head Hills of between 160 km and 180 km. In contrast, both the Mountain Lake and Birch Mountain areas seem to be characterized by either a hot, less depleted asthenospheric-type mantle, or by mantle regions characterized by relatively cool geotherms.

These findings have significant implication for the documentation and evaluation of known occurrences of kimberlite and in the evaluation of surficial kimberlite-indictor mineral surveys critical to target selection in Alberta and other areas of the western North America.