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 filling 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-, Sinanotubes 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.2

Noble gas enrichments in porewater of estuarine sediments and their effect on the estimation of net denitrification rates

“…During emptying of the rocks (for example during compaction and diagenesis), Xe slowly diffuses out the rock, producing a relative enrichment compared to lighter and faster noble gas Ar. Partial or incomplete filling and emptying of the rock could create the excesses of Ne and Xe, compared to Ar (Torgersen et al, 2004). Although the model of Torgersen et al (2004) is appealing, we need porosities of less than 1% and timescale of the order of hundreds of thousands or millions of years to produce the Ne and Xe enrichments observed in the StLawrence sediments.…”

“…Anomalous neon enrichments in terrestrial sedimentary rocks are common (e.g., Podosek et al, 1980;Matsubara and Matsuda, 1995) and they have been explained by diffusive processes (Matsubara and Matsuda, 1995;Pinti et al, 1999) or enhanced solubility (e.g., Pinti et al, 2004). Torgersen et al (2004) developed a model to explain the contemporary enrichments of lighter noble gas Ne and heavier Xe in sedimentary rocks. In their model, gases diffuse through half-spaces to fill the rock.…”

The noble gas geochemistry of natural CO2 gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA

“…Atmosphere-derived Ne, Kr and Xe excesses have been reported in secondary silica phases, volcanic glasses, tektites, and in MORB and OIB basalts (Matsuda et al, 1989;Matsubara and Matsuda, 1995;Pinti et al, 1999). Torgersen et al (2004) have proposed that variable enrich- ments of noble gases can be accounted for by incomplete emptying followed by partial filling of lithic grains and half spaces, similar to the mechanism proposed by Pinti et al (1999) for air diffusion into fresh pumice. Pinti et al (op cit) further demonstrate that both elemental and isotopic compositions show clear mass dependent fractionation both between the elemental ratios and isotopic compositions of Ne and Ar.…”

“…6) may be caused by the release of atmosphere-derived Xe and Kr that is adsorbed and trapped in organic-rich sediments. 130 Xe/ 36 Ar enrichment factors up to $200 relative to water in equilibrium with air have been observed (Fanale and Cannon, 1971;Kennedy et al, 1990;Torgersen and Kennedy, 1999;Kennedy et al, 2002;Torgersen et al, 2004). Direct evidence that Kr and Xe excesses survive sedimentary consolidation and low degrees of thermal alteration is documented in Kr and Xe excesses observed in gas released by the biogenic breakdown of coal (Zhou et al, 2005).…”

Identifying and quantifying natural CO2 sequestration processes over geological timescales: The Jackson Dome CO2 Deposit, USA

“…For example, diffusion combined with adsorption in a porous media could result in diffusive separation and elemental fractionation (Torgersen et al, 2004). In a system of labyrinths-with constrictions and/or C-and Si-nanotubes, heavier noble gases with larger atomic radius can be trapped in these structures due to a restricted throat radius on the order of the atomic radius (Torgersen et al, 2004).…”