Deep fluid release in warm subduction zones from a breached slab seal

Abstract

Petrological models and seismic data from subduction zones with geotherms of 7 Kkm−1 or higher suggest that slabs in these systems dehydrate effectively in the forearc. A large fluid flux is nevertheless released from these slabs at and beyond subarc depth, suggesting that large amounts of H2O can remain slab-bound to much greater depth than expected. We propose that this is due to a transient sealing effect exerted by the subducting lower crust. To test this concept, the petrological and geochemical evolution of such gabbroic crust is investigated through a textural, petrological and Li-chronometric analysis of eclogitized gabbros from an exhumed ultrahigh-pressure terrane. The samples record pristine transitions from dry, rigid gabbro to hydrated eclogite and eclogite mylonite, which occurred when these rocks resided at 90-110 km depth. The observations characterize step-by-step the deformation and overstepped mineral reactions that following the influx of external fluids along a developing network of permeable shear zones. Lithium chronometry indicates that the gabbroic rocks were breached and permeated within a few weeks at a very specific depth within the 90-110 km interval—depths where, in warm subduction zones, large fluid-filled channel system emanate from the slab. The data support a model in which fluids produced in the deserpentinizing slab mantle are trapped at high pore pressure beneath the slab Moho and are ultimately released at subarc depth where the lower crust fails and develops highly permeable fluid vents. The subducting lower crust thus may play an important role in regulating H2O and element budgets, and controlling slab rheology in warm subduction zones.