Noble gases in silica and their implication for the terrestrial “missing” Xe

Abstract

Elemental abundances of noble gases in siliceous microfossils from deep‐sea sediments were determined employing the stepwise heating technique. Almost all the Ar and about 80% of the total Kr were released in the lowest temperature fraction of 800°C. Only Xe was released in higher temperature fractions, suggesting that Xe was tightly trapped inside silica. The Xe concentrations of siliceous microfossils seem to lie on the correlation line of Xe content vs. the temperature of geothermal water obtained for amorphous silica of geothermal origin. The selective trapping of Xe at high temperatures in amorphous silica indicated the possibility that amorphous silica is the reservoir of the terrestrial "missing" Xe. However, quantitative calculation showed that the total amount of Xe in amorphous silica could be at most only 5‐20 % of the total "missing" Xe.

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

“…Subsequent heating due to burial and the geothermal gradient will decrease the effective adsorption coefficients leading to F(Xe) enrichments caused by faster diffusive loss of light noble gases. Since the sediments that have been measured [4,9,24] all show a Xe enrichment ( Figure 5), we believe that sediments are deposited with an adsorbed component in noble gases and enhance their F(Xe) by noble gas emptying rather than by strong preferential adsorption.…”

“…Figure 5a shows the relative noble gas patterns of young ocean sediments [4,9,24] (recovered as piston cores from the ocean floor and therefore on the order of <100Myr). All sediments show a pattern exhibiting progressive enrichment with increasing atomic mass.…”

“…Since simple adsorption is inconsistent with light element enrichment, the higher diffusivity of Ne relative to the heavy noble gases has led to a diffusive hypothesis as a potential mechanism for Ne enrichment in rocks [9]. A passive random walk process through a restricted space (interlayer spacing of clays) was suggested [10] that would enhance separation of noble gases, as preferential adsorption would additionally slow the diffusion of one noble gas relative to another.…”

Reactions of xenon with iron and nickel are predicted in the Earth's inner core

“…The unexpected depletion of Xe in the Earth's atmosphere relative to argon (Ar) and krypton (Kr) 1,2 , the so-called "missing Xe paradox", has become one of the most challenging enigmas in the planetary sciences. Although models have proposed the possibility of Xe escape from the atmosphere into space 2,10 , the majority of researchers accept that Xe may be hidden in the interior of Earth [6][7][8][9][11][12][13] .…”

“…Attempts towards Xe capture in ices, clathrates, and sediments in crust of Earth have failed [11][12][13] . A recent experiment reported Xe reactivity with water ice, but the reaction took place at pressure and temperature conditions of 50 GPa and 1500 K found in the interiors of giant planets such as Uranus and Neptune 14 .…”