NDLI: Redox state of deep off-craton lithospheric mantle: new data from garnet and spinel peridotites from Vitim, southern Siberia

Content Provider	Springer Nature Link
Author	Goncharov, A. G. Iov, D. A.
Copyright Year	2012
Abstract	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.
Starting Page	731
Ending Page	745
Page Count	15
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	164
Issue Number	5
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	2012-06-01
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Oxygen fugacity Off-craton lithosphere Mantle xenolith Garnet-spinel transition Garnet peridotite Central Asia Mineral Resources Mineralogy Geology
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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