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The oxidation state of metasomatized mantle wedge: Insights from C-O-H-bearing garnet peridotite (Articolo in rivista)
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- Label
- The oxidation state of metasomatized mantle wedge: Insights from C-O-H-bearing garnet peridotite (Articolo in rivista) (literal)
- Anno
- 2009-01-01T00:00:00+01:00 (literal)
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- Malaspina Nadia; Poli Stefano; Fumagalli Patrizia (literal)
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- Rivista
- Note
- ISI Web of Science (WOS) (literal)
- Http://www.cnr.it/ontology/cnr/pubblicazioni.owl#affiliazioni
- Dipartimento Scienze della Terra, Università degli Studi di Milano (literal)
- Titolo
- The oxidation state of metasomatized mantle wedge: Insights from C-O-H-bearing garnet peridotite (literal)
- Abstract
- Oxygenfugacity(fO2)isanimportantparameterindeterminingthe
relative stabilities of phase assemblages.Whereas a number of studies
have been devoted to determining the redox state of low-pressure
assemblages in the mantle system, the fO2 of the supra-subduction
mantle wedge is still poorly known. An essential input for fO2 esti-
mates is the determination of the ferric^ferrous iron content of key
mantle minerals such as garnet, which can be measured using the
'flank method'technique on an electron microprobe.We selected sam-
ples of orogenic peridotites from the ultrahigh-pressure Sulu belt
(Eastern China) and from the Ulten Zone (Italian Alps) for
detailed case studies; these correspond to slices of metasomatized
mantle wedge sampled at different depths. They are characterized
by the assemblage phlogopite þ magnesite þamphibole in equilib-
rium with olivine, orthopyroxene and Fe3þ-bearing garnet. The
'flank method' measurements indicate that these pyrope-rich garnets
contain Fe3þ/?Fe up to 0?12^0?14. For peridotite mineral assem-
blages fO2 can be evaluated from equilibria involving the Fe3þ-
garnet component skiagite (Fe2þ Fe3þ Si O ) on the basis of 3 2312
Fe3þ^Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3þ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solu- tions of Ca^Fe2þ^Mg^Al^Fe3þ-garnet.This allowed us to calcu- late garnet-peridotite fO2, given the presence of Fe3þ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ þ 2) compared with garnet peri- dotite xenoliths from the sub-cratonic mantle equilibrated at similar pressures. The determination of the oxygen fugacity of these hydrate^carbonate-bearing garnet peridotites allowed us to estimate the speciation of C^O^H metasomatic fluids derived from the sub- ducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential (mu)O2, provides a first step in unravelling the rela- tionship between (mu)O2 and the metasomatic phase assemblage (literal)
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