Oxygen fugacity (fO 2) affects melting, meta-somatism, 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 Fe 3? /RFe ratios in garnet and spinel obtained by Mössbauer 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 DlogfO 2 (FMQ)). For intermediate garnet–spinel peridotites, fO 2 estimates from spinel-based oxybarometers are 1.5–3.2 Dlog-fO 2 (FMQ) lower than those from garnet-based oxybarom-eters. These rocks may be out of phase and chemical inter-mineral equilibrium because the spinel–garnet reac-tion 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 °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 DlogfO 2 (FMQ)) than cratonic garnet peridotites at similar depths, and the ''water maximum'' conditions ([80 % H 2 O) 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 (C90 km) at 2.5 GPa and 1,150 °C has fO 2 of -3.0 DlogfO 2 (FMQ), with dominant CH 4 and H 2 O 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.