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| Titel |
Redox state of cratonic and off-craton lithospheric mantle: new Mossbauer data from garnet and spinel peridotites |
| VerfasserIn |
A. Goncharov, D. Ionov |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250068254
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| Zusammenfassung |
| We present new data on redox and thermal state of lithospheric mantle beneath the Siberian
craton and PR-PZ mobile belts between the Siberian and North China cratons based on
studies of garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite and
the areas of Cenozoic basaltic volcanism in Russia (Vitim east of Lake Baikal) and SE
Mongolia (Dariganga). The redox conditions are estimated using Fe3+/ΣFe ratios in spinel
and garnet obtained by Mössbauer spectroscopy for over 70 new samples together
with published data for 24 Vitim peridotites (Ionov and Wood 1992; Luth et al.
1990).
Beneath the Siberian craton, fO2 decreases by several orders of magnitude with depth,
from +1 to -4 log units below the FMQ buffer, i.e. the base of the lithosphere at ~220 km is
strongly reduced, as earlier inferred for the Kaapvaal and Slave cratons. In addition, there is
strong lateral redox heterogeneity due to partial melting and metasomatism. The
redox state of deep off-craton mantle is poorly known because garnet-peridotite
xenoliths are very rare in alkali basalts. The T (780-1150Ë C) and fO2 (-1.9 to -3.0
ΔlogfO2(FMQ)) ranges of our garnet peridotites from Vitim and Dariganga overlap those of
coarse garnet and spinel cratonic peridotites worldwide, but the off-craton rocks
equilibrated at much lower pressures (1.8-2.5 GPa). Because of a higher geothermal
gradient, the deepest off-craton garnet peridotites are more reduced (by 0.5-2.0
ΔlogfO2(FMQ)) than shallow cratonic garnet peridotites at the same depth. The shallow
spinel-facies mantle beneath Vitim is more oxidized than deep garnet peridotites (average
-0.1 vs. -2.5 ΔlogfO2(FMQ)). Importantly, fO2 estimates for garnet-spinel Vitim
peridotites from spinel-based oxybarometers are 1.5-3.2 ΔlogfO2(FMQ) lower than
garnet-based values. The gar-spl rocks are out of chemical and phase equilibrium
because lithospheric heating by recent volcanism was not fully compensated by slow
cation diffusion. The spinel-based fO2 may be erroneous (e.g. they yield unrealistic
C-O-H fluid compositions) while garnet-based fO2 refer to conditions before the
heating.
Oxygen fugacity controls speciation of C-O-H fluids coexisting with mantle rocks. The
"water maximum" conditions (>80% H2O) beneath the Vitim and Dariganga regions exist in
a more shallow and narrow depth range (60-85 km) than in the central Siberian
and other cratons (100-170 km). In general, uppermost spinel peridotites coexist
with CO2-rich fluids and the region near the base of the lithospheric mantle (~90
km) at 2.5 GPa and 1150Ë C has fO2 of -3.0 log units below FMQ, with dominant
CH4 and H2O and minor H2 in the fluid. If fO2 continues to decrease with depth at
the same rates, metallic iron (controlled by the IW buffer) may be stable below
-4 ΔlogfO2(FMQ) at 1250Ë C and -¤3GPa, i.e. -¥100 km, in off-craton mantle,
and at -5 ΔlogfO2 (FMQ), 1450Ë C and -¥8GPa (-¥250 km) in cratonic mantle. |
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