NOBLE GASES TRACE DEHYDRATION OF SUBDUCTED OCEANIC CRUST

Volatile elements are transported from Earth’s surface reservoirs back into the mantle during subduction of oceanic lithosphere (e.g. Schmidt & Poli 1998). Here, we investigate the degree to which the fate of slab-bound noble gases and water are linked through the subduction process. Both water and noble gases are soluble in ring-structured minerals, such as amphibole, that are common constituents of subducted oceanic lithosphere. Heating and burial during subduction liberates noble gases and water from minerals through a combination of diffusion and dissolution. Combining a kinetic model, parameterized for noble gas fractionation in amphibole (Jackson et al. 2013), with thermodynamic phase equilibria calculations, we quantify the effect of subduction dehydration on the elemental composition of slab-bound noble gases. Results show that post-arc slab water and noble gas fluxes are highly correlated. Hot subduction zones, which likely dominate over geologic history, efficiently remove noble gases and water from the down-going slab; furthermore, kinetic fractionation of noble gases is predicted to occur beneath the forearc. Conversely, hydrated portions of slab mantle in cold subduction zones transport noble gases and water to depths exceeding 200 km. Preservation of seawater-like abundances of Ar, Kr and Xe in the convecting mantle (Holland & Ballentine 2006) implies that recycling of noble gases and water occurred during cold subduction and that the subduction efficiency of these volatile elements has increased over geological time, driven by secular cooling of the mantle.

Holland, G. and Ballentine, C. (2006). Nature 441, 186-191; Jackson et al. (2013). Nat. Geosci. 6, 562-565; Schmidt, M. & Poli, S. (1998). EPSL 163, 361–379