RAPID REACTION-INDUCED BREACHING OF THE SLAB LOWER CRUST SPARKS DEEP FLUID RELEASE BENEATH ARCS

Slabs in subduction zones with geotherms of 7 K km^-1 or higher are expected to dehydrate effectively in the forearc. Thermodynamic calculations predict this for both crust and mantle sections, and earthquake distributions appear to track fluid release across the forearc [1,2,3]. Strikingly, a large amounts of H₂O is not released to the forearc mantle, but instead appears to remain slab-bound, only to become released at and beyond subarc depth [4,5]. It is possible that the apparent retention of H₂O in slabs reflects a transient sealing effect exerted by the subducting lower crust; this section of the subducting lithosphere typically undergoes delayed reaction and is effectively impermeable until then.

To test this concept, we investigated gabbros that were partially transformed to hydrous ultrahigh pressure eclogite along shear zones during subduction. The rocks were subjected to a field-based, textural, petrological analysis and and Li chemical and isotope chronometry. The observations characterize the progressive stages of transformation, and provide detailed insight into the governing feedbacks among fluid flow, deformation, and reaction. Lithium chronometry indicates that it took only a few weeks for the shear zone network to develop and for the externally derived fluids to traverse this network and drive eclogitization [6].

The switch in these rocks—from strong to weak and from impermeable to sustaining long-range fluid flow—thus was essentially instantaneous on subduction time scales. The re-equilibration of the rocks occurred well beyond equilibrium at c. 90 km depth, which is where large fluid-filled channel system typically emanate from warm slabs [4,5]. Our data suggest that the fluids that are produced in the slab mantle throughout the forearc accumulate beneath the Moho until the lower crust is breached by dynamic fluid vents and commences its delayed transformation. The subducting lower crust may thus be a exert a strong control on H₂O and element budgets, and the rheology of slabs in warm subduction zones.

[1] Hacker et al. (2003) J. Geophys. Res. 108, 2030; [2] England et al. (2004) Geophys. J. Int. 156, 377-408; [3] Abers et al. (2017) Nature Geosci. 10, 333-337; [4] Zhao (2001) Phys. Earth Planet. Interiors 127, 197-214; [5] McGary (2014) Nature 511, 338-340; Smit & Pogge von Strandmann (2020) Earth Planet. Sci. Lett. 534, 116046.