[SSS04-P53] Fluid flow, fluid-rock interaction and slow earthquakes at the forearc mantle corner
Keywords:Albitization, Reaction-enhanced permeability, subduction-zone mélange, Slow earthquakes
The question is, how the updip fluid flow is formed and maintained at the slab-mantle interface? Fractures probably play an important role of fluid pathways, but the life span of individual fractures could be short for the rapid sealing by vein minerals. We propose that fluid-rock interaction in subduction-zone mélange would sustain the fluid flow if reaction-enhanced permeability takes place.
We present an example of fluid-rock interaction and reaction-enhanced permeability in subduction-zone mélanges. The Nagasaki metamorphic rocks in Kyushu, Japan, consist mainly of Late Cretaceous schists and contain serpentinite mélanges. The metamorphic pressure and temperature are close to those of the forearc mantle corner (~0.8 GPa and ~440 oC). The serpentinite typically occurs as mélanges, in which metapelitic tectonic blocks have been albitized along the rims and cracks. The albitization is considered to be the result of fluid-rock interaction. The isocon analysis indicates that the albitization involves a loss of rock volume and extraction of silica from the blocks. The volume loss increases along with the reaction progress of the albitization.
Albitization is commonly found in the subduction-zone mélanges of the blueschist to eclogite facies. This suggests that reaction-enhanced permeability widely takes place at the slab-mantle interface. As a result, the subduction-zone mélanges probably become porous media, which act as sustainable channels of the updip flow of fluids (and silica) towards the site of deep slow earthquakes.