Thermal state, oxygen fugacity and COH fluid speciation in cratonic lithospheric mantle: New data on peridotite xenoliths from the Udachnaya kimberlite, Siberia

Abstract

Oxygen fugacity (fO2) and temperature variations in a complete lithospheric mantle section (70–220km) of the central Siberian craton are estimated based on 42 peridotite xenoliths in the Udachnaya kimberlite. Pressure and temperature (P–T) estimates for the 70–140km depth range closely follow the 40mW/m2 model conductive geotherm but show a bimodal distribution at greater depths. A subset of coarse garnet peridotites at 145–180km plots near the “cold” 35mW/m2 geotherm whereas the majority of coarse and sheared rocks at ≥145km scatter between the 40 and 45mW/m2 geotherms. This P–T profile may reflect a perturbation of an initially “cold” lithospheric mantle through a combination of (1) magmatic under-plating close to the crust–mantle boundary and (2) intrusion of melts/fluids in the lower lithosphere accompanied by shearing. fO2 values estimated from Fe3+/∑Fe in spinel and/or garnet obtained by Mössbauer spectroscopy decrease from +1 to −4 Δlog fO2 (FMQ) from the top to the bottom of the lithospheric mantle (∼0.25log units per 10km) due to pressure effects on Fe2+–Fe3+ equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO2 distribution with depth similar to those in the Slave craton. Published fO2 estimates for Udachnaya xenoliths based on C–O–H fluid speciation in inclusions in minerals from gas chromatography are similar to our results at ≤120km, but are 1–2 orders of magnitude higher for the deeper mantle, possibly due to uncertainties of fO2 estimates based on experimental calibrations at ≤3.5GPa. Sheared peridotites containing garnets with u-shaped, sinusoidal and humped REE patterns are usually more oxidized than Yb, Lu-rich, melt-equilibrated garnets, which show a continuous decrease from heavy to light REE. This further indicates that mantle redox state may be related to sources and modes of metasomatism.