Thermal state and redox conditions of the Kaapvaal mantle: A study of xenoliths from the Finsch mine, South Africa

Abstract

A total of 28 peridotite xenoliths from the Finsch mine, South Africa, have been studied to assess the geotherm and redox conditions at depth in the subcontinental lithospheric mantle. Most samples reveal internal mineral equilibrium i.e. they reflect the physical conditions of the mantle at the time of kimberlite eruption, 118 Ma ago. Pressures and temperatures range from 4.4 to 6.5 GPa and 1050 to 1250 °C and plot along a conductive continental geotherm between 40 and 41 mW/m 2. Shallow, coarse-grained peridotites from younger southern African localities (Kimberley pool, Jagersfontein, Lesotho) centre around a 42 mW/m 2 geotherm and their deep seated, sheared xenoliths plot up to 200 °C above a 42 mW/m 2 geotherm for a given pressure. This reflects heating and local perturbation of the thermal gradient of these mantle regions. The ƒO 2 values of 15 samples from Finsch range from FMQ − 2.5 to FMQ − 4.7. There is a steady decrease in ƒO 2 of about − 1.0 log unit per GPa with depth with the deepest sample lying ~ 0.5 log units above the Fe–wüstite (IW) oxygen buffer. Under these conditions, the composition of a possible coexisting C–O–H fluid phase would be a mixture of CH 4 and H 2O in almost equal proportions. Other deep samples reported in the literature were all high-temperature sheared peridotites and record somewhat higher ƒO 2 values at similar depths, like that observed for five metasomatized peridotites from Finsch.