The Noble Gas Systematics of Subduction

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The Earth's atmosphere and surface contain abundant water and gas, volatile species that sustain the habitability of our planet. These incompatible elements, including the noble gases and halogens, are continually released from the Earth through magmatism at mid-ocean ridges and arc volcanoes. Until recently, it has been accepted that this flux was one-way; volatiles are only released and never returned back into the Earth's mantle. However, with the advent of new high-sensitivity measurement techniques, it has become apparent that the isotopic/elemental composition of the Earth's mantle contains an Ar/Kr/Xe signature that is identical to seawater. Because this is a unique composition in the solar system, the only credible explanation is that a non-disrupted seawater-like noble gas signature survives the subduction process, generally thought to exclude more than 98% of input volatiles. Given that the noble gases are between 100 and 100,000 times more soluble in fluids than crustal minerals, even small amounts of fluid transport would be expected to fractionate the noble gas composition of the subducting rock. How then do noble gases, and halogens, escape removal and fractionation during subduction? Where do the volatile phases reside in the downgoing slab? Over what distances are they transported by metamorphic fluid flow? And, critically, what quantities of noble gas and halogens are returned back into the mantle? I propose to undertake the first systematic analytical characterization of the noble gas and halogen elemental/isotopic composition of the subducting slab. I will conduct these state-of-the-art volatile measurements at Oxford's Noble Gas Laboratory, one of only few laboratories worldwide with the required expertise and analytical technology. Measurements will be performed on minerals and fluid inclusions from a rock sample suite collected from several key high-pressure metamorphic terranes, representing different portions of the slab and preserving differing degrees of volatile-loss. Once the host phases have been identified, the pressures and temperatures that the noble gas and halogen compositions equilibrated at will be calculated using thermodynamics. Rock samples will also be collected across several representative lithological boundaries to determine fossil noble gas concentration profiles, preserving a direct record of the relative roles of fluid flow and reaction during subduction. I will construct a numerical model to deconvolve these profiles into key transport parameters, quantifying the degree to which the noble gases are decoupled from the transport of water-rich fluids in the slab. When combined with existing models of subduction dehydration systematics, this will allow, for the first time, estimates of the subducted noble gas flux to the mantle to be calculated. This novel interdisciplinary project will be the first to link the noble gases and halogens with the metamorphic evolution of the subducting slab. Accordingly, the proposed research has the potential to deliver a step change in our understanding of how volatiles are processed during subduction. Future research avenues include investigating the effects of these return fluxes on the chemical and physical evolution of the mantle, using models of whole-scale mantle convection.