Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia
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Oxygen fugacity (fO2) affects melting, metasomatism, speciation of C–O–H fluids and carbon-rich phases in the upper mantle. fO2 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+/ΣFe 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 ΔlogfO2(FMQ)). For intermediate garnet–spinel peridotites, fO2 estimates from spinel-based oxybarometers are 1.5–3.2 ΔlogfO2(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 fO2 estimates may not be correct while garnet-based fO2 values provide conditions before the heating. The T (780–1,100 °C) and fO2 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 ΔlogfO2(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 (≥90 km) at 2.5 GPa and 1,150 °C has fO2 of −3.0 ∆logfO2(FMQ), with dominant CH4 and H2O and minor H2 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.
KeywordsOxygen fugacity Off-craton lithosphere Mantle xenolith Garnet-spinel transition Garnet peridotite Central Asia
AGG thanks I. Ashchepkov for donating Vitim xenoliths, L.-S. Doucet for logistic and technical assistance in St Etienne and D. Frost for providing a program for computing fluid compositions, and acknowledges a CMIRA fellowship of the French Rhône-Alpes region in 2010–2011 and funding from the Russian Foundation of Fundamental Research (RFBR grants 10-05-01017 and 11-05-00346). DAI acknowledges PNP grants from the French CNRS in 2011 and 2012.
- Amthauer G, Annersten H, Hafner SS (1976) The Mossbauer spectrum of 57Fe in silicate garnets. Z Kristallogr 143:14–55Google Scholar
- Ashchepkov IV, Dobretsov NL, Kalmanovich MA (1989) Garnet peridotite xenoliths from alkalic picritoid and basanitoid of the Vitim Plateau. Trans (Doklady) USSR Acad Sci, Earth Sci Sect 302(5):156–159Google Scholar
- Brey GP, Köhler T (1990) Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. J Petrol 31:1353–1378Google Scholar
- Goncharov AG, Ionov DA, Doucet LS (2012) Thermal state, oxygen fugacity and C-O-H fluid speciation in cratonic lithospheric mantle: new data on peridotite xenoliths from the Udachnaya kimberlite, Siberia. Earth Planet Sci Lett: (in press)Google Scholar
- Ionov DA, Ashchepkov IV, Stosch HG, Witt-Eickschen G, Seck HA (1993) Garnet peridotite xenoliths from the Vitim volcanic field, Baikal region: the nature of the garnet-spinel peridotite transition zone in the continental mantle. J Petrol 34(pt. 6):1141–1175Google Scholar
- O’Neill HSC, Wall VJ (1987) The olivine-orthopyroxene-spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the Earths upper mantle. J Petrol 28:1169–1191Google Scholar
- Stagno V, Frost DJ (2010) Carbon speciation in the asthenosphere: experimental measurements of the redox conditions at which carbonate-bearing melts coexist with graphite or diamond in peridotite assemblages. Earth Planet Sci Lett 300(1–2):72–84. doi: 10.1016/j.epsl.2010.09.038 CrossRefGoogle Scholar
- Taylor WR (1998) An experimental test of some geothermometer and geobarometer formulations for upper mantle peridotites with application to the thermobarometry of fertile Iherzolite and garnet websterite. N Jahrb Miner-Abh 172(2–3):381–408Google Scholar
- Woodland AB, Peltonen P (1999) Ferric iron contents of garnet and clinopyroxene and estimated oxygen fugacities of peridotite xenoliths from the Eastern Finland Kimberlite Province. In: Gurney JJ, Gurney JL, Pascoe MD, Richardson SH (eds), In: Proceedings of 7th International Kimberlite Conference, vol 2. RedRoof Design, Cape Town, pp 904–911Google Scholar
- Yaxley GM, Berry AJ, Kamenetsky VS, Woodland AB, Golovin AV (2012) An oxygen fugacity profile through the Siberian Craton—Fe K-edge XANES determinations of Fe3+/∑Fe in garnets in peridotite xenoliths from the Udachnaya East kimberlite. Lithos 140–141:142–151. doi: 10.1016/j.lithos.2012.01.016 CrossRefGoogle Scholar