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

摘要

Oxygen fugacity (fO_2) 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/m~2 geotherm whereas the majority of coarse and sheared rocks at ≥145km scatter between the 40 and 45mW/m~2 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. fO_2 values estimated from Fe~(3+)/σFe in spinel and/or garnet obtained by M?ssbauer spectroscopy decrease from +1 to-4 δlog fO_2 (FMQ) from the top to the bottom of the lithospheric mantle (~0.25log units per 10km) due to pressure effects on Fe~(2+)-Fe~(3+) equilibria in garnet. Garnet peridotites from Udachnaya appear to be more oxidized than those from the Kaapvaal craton but show fO_2 distribution with depth similar to those in the Slave craton. Published fO_2 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.