The lithospheric mantle of the Archean Superior Province as imaged by garnet xenocryst geochemistry

The major and trace element chemistry of over 500 garnet xenocrysts from 12 kimberlite and alkaline rock bodies are used to examine the mantle lithosphere below the Archean Superior Province. All of the Superior Province kimberlites contain garnets that were in equilibrium with clinopyroxene. The majority of the Superior province garnets have normal rare earth element (REE) patterns but LREE-enriched sinuous patterns are encountered in garnets from almost all the pipes. Ni-in-garnet geothermometry shows the garnets sampled mantle lithosphere over a temperature interval of 850 to 1225 °C, corresponding to a depth range of ∼110 to 210 km when projected to a xenolith-derived geotherm for the southeastern Superior Province. uous increases in Y, and decreases in Sc/V with temperature are typical of mantle sections sampled by garnet in Paleozoic and Mesozoic kimberlites. In almost all regions sampled in the Superior Province, depleted peridotite with less than 1.5 wt.% Al2O3 and high Sc/V constitutes the shallowest sections (<120 km depth) of the lithospheric mantle at TNi<950 °C. These shallow, infertile parts of the lithosphere support the compositional buoyancy inferred to support the Superior cratonic root against convective removal. A different trend with depth is observed for garnet suites from 1.1-Ga kimberlites and alkaline rocks, perhaps due to a different mantle structure at this time. Differences in the chemical signatures between garnet suites in kimberlites within only kilometers of one another across major fault grabens in the Lake Timiskaming region imply chemically modified mantle lithosphere along extremely narrow zones that are still observed there today by seismic tomography.