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Indications for aqueous CH4-bearing excess fluid
during HP-metamorphism in subduction zone eclogites of the Raspas
R. J. Bakker (University of Leoben, Austria)
Subproject within the SFB 574 "Volatiles and Fluids in Subduction
To get a better understanding of the role of fluid phases in subduction
settings, the knowledge of the composition and fluid/rock ratio of high
pressure fluids is a prerequisite. Fluid inclusions are preserved relics of the
fluid present at different stages during the subduction cycle and thus the
only direct evidence for the original paleofluids.
Geological map of the Raspas Metaophiolite Complex, Ecuador
(modified after Gabriele et al. 2003)
Fluid inclusion investigations have been obtained in eclogites of the
Raspas Complex in Ecuador. Associated with the eclogites are metapelites,
blueschists and partly serpentinized peridotites (Gabriele et al. 2003, Eur. J.
Min. 15: 977-989). The complex is regarded as a metaophiolite, representing
oceanic lithosphere subducted to a depth of about 70 km. Geochemically
different protoliths can be assigned to the eclogites: MORB, OIB and
metasomatized eclogites which are cut by zoisite veins representing former
fluid pathways. In all the geochemically different eclogite types, primary fluid
inclusions could be investigated in omphacite, zoisite, garnet and quartz.
Preliminary results from microthermometry and Raman spectroscopy on
primary fluid inclusions in the eclogite-facies minerals yield a rather
homogeneous fluid composition in the system
H2O-NaCl-CH4. Raman spectroscopy combined
with a heating-freezing stage allow to define the melting temperatures of ice
and CH4-clathrate giving a low salinity for these primary fluid
inclusions. A consistent volume fraction of the vapour bubble in all primary
inclusions in the eclogite-facies minerals indicate homogeneous entrapment.
The majority of fluid inclusions also contain tiny solids and most of them
have been identified as calcite by Raman spectroscopy. In a mm-thin
eclogite-facies zoisite vein, beside the previously mentioned inclusion type,
a second type of fluid inclusion coexists which contains CH4
with some traces of ethane and graphite. Broad zoisite veins partly with
interstitial albite, however, must have been formed later at decreasing
pressures. The vein zoisites have similar homogeneous fluid inclusion
composition in the system H2O-NaCl-CH4 like the
eclogite-facies minerals, indicating homogeneous fluid composition during
the eclogite-facies stage and during subsequent exhumation. Such
homogeneous aqueous fluid composition can be best explained by
aqueous fluid infiltration from an external source where dehydration
of (OH)-bearing minerals takes place. Deserpentinization of the underlying
oceanic mantle could be a realistic source for the liberated H2O
and CH4 of the infiltrating fluid. This model gets supported by
the fact that the serpentinized peridotites of Ecuador, geochemically defined
as depleted MORB-mantle peridotites, have been subducted as well, reaching
A comparison with eclogite-facies fluid composition from other eclogite
complexes shows that low-salinity aqueous fluids with methane are rather
the exception. Only in the Dabie-Sulu terrane of eastern China (Fu et al.
2003, J. Metam. Geol. 21: 561-578) also CH4-rich fluid
inclusions of pre- to syn-peak metamorphic origin have been identified. Fu
et al (2003) related the formation of methane to the serpentinization of
peridotites prior to or during subduction.
Omphacite with primary fluid inclusion clusters in its core.
Primary fluid inclusions in omphacite,
oriented parallel to the c-axis
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