Effects of heterogeneous hydration in the incoming plate, slab rehydration, and mantle wedge hydration on slab-derived H ₂O flux in subduction zones

We quantify the effects of heterogeneous hydration in the incoming oceanic plate, rehydration in the subducting slab, and hydration in the lowermost mantle wedge on the pattern of H ₂O release from the slab in the forearc and arc regions of subduction zones. The release and uptake of H ₂O by dehydration and hydration reactions, respectively, are calculated using a subduction zone thermal model and thermodynamic calculations for the distribution of mineralogically bound H ₂O for idealized lithologies. We consider two end-member models for the initial hydration state of the incoming plate: (1) uniform hydration, in which H ₂O is stored homogeneously within each lithologic unit, and (2) localized hydration, in which H ₂O is stored at its saturation level in discrete zones. The former is commonly assumed in thermo-petrologic models, while the latter approximates the effect of localized hydration along faults in the oceanic plate inferred from geophysical observations. Our modeling results show that for the same bulk H ₂O content, localized hydration results in shallower H ₂O release compared to uniform hydration, and that the H ₂O flux off the subducting slab beneath the forearc and arc regions can be almost twice as large from a locally hydrated slab as from a uniformly hydrated slab. Rehydration can occur in the subducting slab and delays the liberation of H ₂O. The effect of pervasive rehydration in the slab on the pattern of fluid release in the forearc and arc regions is relatively small if the slab is uniformly hydrated, but it is large if the slab is locally hydrated. Hydration of the overlying mantle also delays the liberation of H ₂O, but the volume of H ₂O absorbed in the mantle is small compared to that released from the subducting slab, thereby implying that mantle wedge hydration has only a modest effect on the pattern of fluid release.