MODELING SUBDUCTION ZONE METAMORPHIC DEVOLATILIZATION: IMPLICATIONS TO VOLATILE RECYCLING, ARC MAGMATISM AND SESMICITY

Realistic modeling of metamorphic devolatilization for subducted lithologies is only possible for chemical systems that closely approximate actual bulk compositions. Using the Perplex code (www.perplex.ethz.ch) we have computed dehydration for the main volatile-bearing protoliths entering subduction zones: metabasalts and serpentinites formed by hydrothermal alteration in mid-ocean ridge systems and marine sediments. For different subduction zones, the depth range of H₂O and CO₂ release varies markedly in light of differences in thermal regimes and lithologic (bulk chemical) variations. Subducted pelagic clays (pelites) are a primary H₂O source in subduction zones. For this lithology, significant dehydration occurs in forearcs and thus provides a H₂O source for serpentinization of the mantle wedge. Thermal models show that with increasing depth the slab surface undergoes a marked temperature increase at the base of the lithosphere. This temperature increase would yield significant dehydration of subducted marine clays in deep forearcs and subarcs and thus provide a significant H₂O source for arc magmas. Marked decarbonation occurs with the subduction zone thermal models of van Keken et al. [1], whereas other thermal models predict little decarbonation. For most subduction zones, the relatively cool thermal regime of serpentinites underneath the oceanic crust precludes significant dehydration on the forearc and subarc. However, within the viscous mantle wedge, serpentinites that are “down-dragged” by induced convection could provide a primary H₂O source for arc magmas. Considering all volatile-bearing protoliths, our computations imply that considerable H₂O and CO₂ are retained in subducted slabs beyond subarcs - this is compatible with deficiencies in the amounts of H₂O and CO₂ expelled by arc magmas compared to the amounts that are subducted. Dehydration of subducted marine clays and serpentinites in the mantle wedge are considered to be primary sources for dehydration-enduced seismicity. In comparing hot versus cold subduction thermal regimes, there is a correlation between the predicted depth range of metamorphic dehydration and the depth distribution of earthquake hypocenters. [1] van Keken, P.E., Kiefer, B., and Peacock, S.M., Geochem. Geophys. Geosystems G3, 3(10), 1056, doi:10.1029/2001GC000256.