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| Titel |
Witness of fluid-flow organization during high-pressure antigorite dehydration |
| VerfasserIn |
Vicente López Sánchez-Vizcaíno, José Alberto Padrón-Navarta, Carlos J. Garrido, María Teresa Gómez-Pugnaire |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043399
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| Zusammenfassung |
| The link between devolatilization reactions and fluid flow is crucial to unravel important
geodynamic processes in subduction zones as deformation and element transfer is extremely
controlled by the presence of water. At high confining pressure, significant fluid pressure
gradients are expected in a reacting rock being dehydrated, because of its rather limited
permeability [1]. Compactation-driven fluid flow seems to be an intrinsic mechanism
occurring at devolatilization of viscolastic rocks. Nevertheless, and despite the
important implications of this coupled deformation/fluid-migration mechanism for
fluid transport, a conclusive confirmation of these processes by petrological and
textural evidences in metamorphic terrains has been hampered by the scarcity of
devolatilization fronts in the geological record. Evidences of high-pressure antigorite
dehydration found at Cerro del Almirez (Betic Cordillera, Spain) [2] represent a
noteworthy exception. Here, the transition between the hydrous protolith (antigorite
serpentinite) and the prograde product assemblage (olivine + orthopyroxene + chlorite,
chlorite harzburgite) is extremely well preserved and can be surveyed in detail.
The maximum stability of the antigorite has been experimentally determined at
~680ºC at 1.6-1.9 GPa [3]. Antigorite dehydration is accompanied by release of high
amounts of high–pressure water-rich fluids (~ 9 wt.% fluid). Distinctive layers
(up to 1 m thick) of transitional lithologies occur in between atg-serpentinite and
chl-harburgite all along the devolatilization front, consisting of (1) chlorite-antigorite
olivine-serpentinite, which gradually changes to (2) chlorite-antigorite-olivine-orthopyroxene
serpentinite. These transitional lithologies are more massive and darker in color than
atg-serpentinite and largely consist of coarse sized grains of antigorite and chlorite (250-500
μm). Antigorite in these assemblages is characterized by microstructural disorder
features, which are lacking in antigorite far from the devolatilization front [4]. The
sharp appearance of chlorite (Chl-in), crosscutting the serpentinite foliation, and
coarsening of olivine define the upper limit of the transitional lithologies, whereas
the lower limit (Atg-out) is gradational to chl-harzburgite. The modal increase of
orthopyroxene is concomitant with the gradual disappearance of antigorite. The gradual
disappearance of antigorite over short distances leads to the final prograde assemblage
in the Chl-harzburgite with two contrasting textures: (1) coarse granular texture
and (2) an intriguing spinifex-like texture (arborescent growth of centimeter-sized
olivine and orthopyroxene). Both textures alternate at the meter to tens of meters
scale over the entire massif. We interpret these textures as the result of contrasting
pore fluid overpressure, reaction rates and fluid-flow organization shortly after the
antigorite breakdown. These observations will be discussed on the frame of the
reaction kinetic and the propagation of deformation associated to fluid pressure
gradients.
[1] Connolly, Journal of Geophysical Research 112 (B8), 18 (1997).
[2] Trommsdorff, López Sánchez-Vizcaíno, Gómez-Pugnaire et al., Contrib Mineral Petr
132 (2), 139 (1998).
[3] Padrón-Navarta, Hermann, Garrido et al., Contrib Mineral Petr 159 (1), 25
(2010).
[4] Padrón-Navarta, López Sánchez-Vizcaíno, Garrido et al., Contrib Mineral Petr 156
(5), 679 (2008). |
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