Diffusive separation of noble gases and noble gas abundance patterns in sedimentary rocks

Abstract

The mechanisms responsible for noble gas concentrations, abundance patterns and strong retentivity in sedimentary lithologies remain poorly explained. Diffusion-controlled fractionation of noble gases is modeled and examined as an explanation for the absolute and relative abundances of noble gases observed in sediments. Since the physical properties of the noble gases are strong functions of atomic mass, the individual diffusion coefficients, adsorption coefficients and atomic radii combine to impede heavy noble gas (Xe) diffusion relative to light noble gas (Ne) diffusion. Filling of lithic grains/half-spaces by diffusive processes thus produces Ne enrichments in the early and middle stages of the process with F(Ne) values similar to that observed in volcanic glasses. Emptying lithic grains/half-spaces produces a Xe-enriched residual in the late (but not final) stages of the process producing F(Xe) values similar to that observed in shales. ‘Exotic …but unexceptional’ shales that exhibit both F(Ne) and F(Xe) enrichments can be produced by incomplete emptying followed by incomplete filling. This mechanism is consistent with literature-reported noble gas abundance patterns but may still require a separate mechanism for strong retention. A system of labyrinths-with-constrictions and/or C- and Si-nanotubes when combined with simple adsorption can result in stronger diffusive separation and nonsteady-state enrichments that persist for longer times. Enhanced adsorption to multiple C atoms inside C-nanotubes as well as dangling functional groups closing the ends of nanotubes can provide potential mechanisms for ‘strong retention’. We need new methods of examining noble gases in rocks to determine the role and function of angstrom-scale structures in both the diffusive enrichment process and the ‘strong retention’ process for noble gas abundances in terrestrial rocks.