Source and sink of fluid in pelagic siliceous sediments along a cold subduction plate boundary

- White chert layers in on-land accretionary complex acted as fluid conduits.
- We calculated mass balance between pressure solution seam and white chert layers.
- Most of the silica in the white chert was precipitated from external fluids.
- Required fluid mass is ~ 102 times larger than that in pelagic siliceous ooze unit.

Subduction zones where old oceanic plate underthrusting occurs are characterized by thick pelagic sediments originating from planktonic ooze as well as cold thermal conditions. For a better understanding of dehydration from pelagic sediments and fluid behavior, which would play a key role in controlling the dynamics in the shallow portion of the subduction zone, as observed in the 2011 Tohoku earthquake and tsunami, we investigate cherts in a Jurassic accretionary complex in Japan. The microstructure and microchemistry of these cherts indicate dissolution of SiO2 from a pressure solution seam and precipitation of SiO2 to the “white chert layer,” which would act as a fluid conduit. The amount of water necessary to precipitate SiO2 in the white chert is ~ 102 times larger than that produced by compaction and silica/clay diagenesis. Other fluid sources, such as hydrated oceanic crust or oceanic mantle, are necessary to account for this discrepancy in the fluid budget. A large amount of external fluid likely contributed to rising pore pressure along cold plate boundaries.