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| Fluid evolution in the
central American subduction zone recorded by authigenic minerals:
Gas hydrate-derived carbonates
X. Han1,2, E. Suess2
1Key Lab of Submarine Geosciences, SOA, Hangzhou 310012, China
2GEOMAR Research Center, Kiel Germany
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¡¡¡¡The evolution of fluid in subduction zones is an
important aspect of the accretionary process, however, it remains
poorly understood, because it is not feasible to directly monitor
and determine fluid composition, pathways and reactions over long
periods or over great depths. Authigenic carbonates precipitated
by subduction-induced dewatering on the other hand provide potentially
new insights into the dewatering history of the subducted slab.
Hence, our objective is to determine the record of fluid evolution
preserved by carbonates.
¡¡¡¡The research area is located at Costa Rica margin, where the Cocos
Plate is subducted underneath the continental margin. Dewatering
of the subducted slab generates many mud diapers on the margin,
and several scarps were left by subducted seamounts. More than 300
authigenic carbonate samples were collected from these mounds and
scarps for detailed analyses. Based on the morphology, petrology
and mineralogy, 5 types were classified: micritic concretion, seepage-associated
crusts, chemoherms, gas hydrate-associated carbonates, and dolostones.
Stable C- and O-isotopes analyses reveal that all types are depleted
in ¦Ä13C and enriched in ¦Ä18O. This indicates
that all of carbonate samples are methane-derived and that the precipitating
fluids have a heavier¦Ä18O-isotope composition than today¡¯s
ocean. Two end members of the C-source were identified: thermogenic
and biogenic methane. The C source of gas hydrate carbonate, which
has the ¦Ä13C in the range of -6¡ë~-23¡ëPDB is thermogenic,
while the ¦Ä13C of chemoherms and seepage associated crusts
are in the range from -48¡ë~-53¡ëPDB, indicating that the methane
is biogenic. The other carbonate types have ¦Ä13C indicating
that they are mixtures generated from biogenic and thermogenic methane.
The equilibrium ¦Ä18O of the precipitating fluids shows
that the fillings of chimney-like concretions precipitated from
the clay-dehydration water. Seepage-associated crusts and aragonite
cements of the chemherms bear the equilibrium ¦Ä18O in
the range of today¡¯s ocean and the glacial ocean (0~1¡ë SMOW) indicating
that the ¦Ä18O of the precipitating fluids have been equilibrated
with the bottom water. The other carbonate types could have been
precipitated from hydrate water, or mixtures of clay-dehydration
water and seawater. ¦Ä18O values of concretions from different
depths in a core as well as different generations of carbonates
show that the equilibrium ¦Ä18O of the precipitating fluid
is getting lighter with time.
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