Résumé
Oxygen fugacity (fO(2)) affects melting, metasomatism, speciation of C-O-H fluids and carbon-rich phases in the upper mantle. fO(2) of deep off-craton mantle is poorly known because garnet-peridotite xenoliths are rare in alkali basalts. We examine the redox and thermal state of the lithospheric mantle between the Siberian and North China cratons using new Fe3+/I Fe pound ratios in garnet and spinel obtained by Mossbauer spectroscopy, major element data and P-T estimates for 22 peridotite xenoliths as well as published data for 15 xenoliths from Vitim, Russia. Shallow spinel-facies mantle is more oxidized than deep garnet peridotites (average, -0.1 vs. -2.5 Delta logfO(2)(FMQ)). For intermediate garnet-spinel peridotites, fO(2) estimates from spinel-based oxybarometers are 1.5-3.2 Delta logfO(2)(FMQ) lower than those from garnet-based oxybarometers. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel-garnet reaction and concomitant changes in mineral chemistry do not keep up with P-T changes (e.g., lithospheric heating by recent volcanism) due to slow diffusion of trivalent cations and because gar-, gar-spl and spl-facies rocks may coexist on centimeter-meter scale. The spinel-based fO(2) estimates may not be correct while garnet-based fO(2) values provide conditions before the heating. The T (780-1,100 A degrees C) and fO(2) ranges of the Vitim xenoliths overlap those of coarse garnet and spinel cratonic peridotites. However, because of a higher geothermal gradient, the deepest Vitim garnet peridotites are more reduced (by 0.5-2.0 Delta logfO(2)(FMQ)) than cratonic garnet peridotites at similar depths, and the "water maximum" conditions (> 80 % H2O) in the off-craton mantle exist in a more shallow and narrow depth range (60-85 km) than in cratonic roots (100-170 km). The base of the off-craton lithospheric mantle (a parts per thousand yen90 km) at 2.5 GPa and 1,150 A degrees C has fO(2) of -3.0 a dagger logfO(2)(FMQ), with dominant CH4 and H2O and minor H-2 in the fluid. Melting near the base of off-craton mantle lithosphere may be induced by increasing water share in migrating fluids due to oxidation of methane.