A redox profile of the Slave mantle and oxygen fugacity control in the cratonic mantle

The authors report a redox profile based on Mossbauer data of spinet and garnet to a depth of 210 km from mantle xenoliths of the northern (N) and southeastern (SE) Slave craton (northern Canada). The profile transects three depth facies of peridotites that form segments of different bulk composition, represented by spinet peridotite, spinet-garnet peridotite, low-temperature garnet peridotite, high-temperature garnet peridotite, and pyroxenite. The shallow, more depleted N Slave spinet peridotite records lower oxygen fugacities compared to the deeper, less depleted N Slave spinel-garnet peridotite, consistent with their different spinet Fe3+ concentrations. Garnet peridotites show a general reduction in Deltalog fO(2) (FMQ)s with depth, where values for garnet peridotites are lower than those for spinel-garnet peridotites. There is a strong correlation between depletion and oxygen fugacity in the spinet peridotite facies, but little correlation in the garnet peridotite facies. The strong decrease in Deltalog fO(2) (FMQ) with depth that arises from the smaller partial molar volume of Fe3+ in garnet, and the observation of distinct slopes of Deltalog fO(2) (FMQ) with depth for spinet peridotite compared to spinet-garnet peridotite strongly suggest that oxygen fugacity in the cratonic peridotitic mantle is intrinsically controlled by iron equilibria involving garnet and spinet.