Noble gas transport during devolatilization of oceanic crust

Abstract:

Here we examine the role of slab dehydration in determining the elemental pattern of recycled noble gases. As a first step, we apply newly reported measurements of He-Ne-Ar (light noble gases) solubility and diffusivity in amphibole to parameterize a 1D diffusive-reaction transport model that simulates noble gas behavior during fluid loss from down-going oceanic crust. Recent experiments demonstrate that noble gases are highly soluble in ring-structured minerals, such as amphibole and other common hydrothermal products in slabs [1]. These results suggest that ring-structured minerals have the potential to strongly influence the budget of noble gases input into subduction zones and the elemental fractionations associated with volatile loss from slabs

New measurements of He-Ne-Ar solubility in a suite of amphiboles have been completed utilizing the methodology described in [1]. These new measurements confirm that all light noble gases are highly soluble in amphibole, and that noble gas solubility correlates with the availability of unoccupied ring sites. New experimental measurements of He and Ne diffusivity have also been completed using a step-degassing approach at the Berkeley Geochronology Center. These measurements suggest that vacant ring sites in amphibole act to slow noble gas diffusion. We combine the newly acquired He and Ne diffusivity measurements with literature values for Ar diffusivity [2] to parameterize the diffusive-reaction transport model.

Application of these data to the diffusive-reaction transport model yields several new insights. The relative mobility of Ne compared to Ar allows for efficient extraction of Ne from “hot” slabs by shallow depths (<50 km), while Ar is effectively retained to deeper depths, potentially past sub-arc conditions. Noble gas partition coefficients sharply increase with depth, following their increasing non-ideality in supercritical fluids, causing noble gases to partition back into minerals from any fluids retained in slabs at depth. The efficiency of noble gas extraction is particularly sensitive to the thermal regime and porosity of the slab (i.e. cold slabs with low porosity have the potential to recycle significant quantities noble gases).

Refs: [1] Jackson et al. (2013). Nat.Geosci. 6, 562-565. [2] Harrison (1981). CMP 78, 324-331