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| Titel |
Fluid circulations in the depths of accretionary prism: the record of quartz from the Shimanto Belt, Japan |
| VerfasserIn |
Hugues Raimbourg, Maxime Vacelet, Claire Ramboz, Vincent Famin, Romain Augier, Giulia Palazzin |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092689
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| Publikation (Nr.) |
 EGU/EGU2014-7049.pdf |
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| Zusammenfassung |
| Fluids present in the depths of subduction zones play a major role on seismogenesis, although
fluid circulations paths and physico-chemical conditions are still largely unknown. Two main
reservoirs of water, either in the pores of sediments or bound to hydrous minerals, release
large amounts of water in the relatively shallow and deep domains of subduction zones,
respectively. The usual model of circulation assumes then a bottom-up circulation driven by
fluid pressure gradients.
This study aims at reassessing this model, using the record of rocks from a
paleo-accretionary prism, the Shimanto Belt in Japan. These rocks, buried to 5kbars and 300Ë
C (Toriumi and Teruya, Modern Geology, 1988), were affected by pervasive fracturing
throughout their history, from burial to exhumation. The quartz filling these fractures and the
fluid inclusions that it contains keep the track of the fluid associated with the rock
evolution.
Using a combined approach of microstructural observations by optical microscopy and
cathodoluminescence (CL), and chemical characterization by electron and ion microprobe
as well as microthermometry, we show that there are actually two distinct fluids
that have cyclically wetted the rock at depth. The first one is an “external” fluid
penetrating through macroscopic fractures and precipitating a quartz blue in CL. In
contrast, a “local” fluid attended the formation of quartz brown in CL, precipitating in
microfractures or associated with ductile recrystallization. The two fluids are also chemically
distinct: Both have a salinity close to seawater, but the local fluid is fresher than the
external one. In addition, the external fluid is richer in aluminum than the local one.
Finally, the external fluid is very slightly depleted in δ18O, although the difference is
probably not significant and the first-order isotopic signal is a buffering by host
rock.
Our interpretation of microstructures and chemical signatures is that the external fluid is
seawater, penetrating to accretionary prism depths during transient phases of large-scale
fracturing and fluid circulation. Macroscale fractures then close, permeability drops, and the
fluid is progressively reequilibrated at depth with water produced in-situ by metamorphic
reactions. The general scheme is therefore a top-down circulation, contrasting with the
usually proposed bottom-up flux. We finally discuss geodynamical scenarios, such
as during the postseismic phase or in association with thermal anomalies, where
such a counter-intuitive top-down flux of water could prevail in subduction zones. |
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