Rare gas fractionation patterns in terrestrial samples and the Earth‐atmosphere evolution model

Abstract

Rare gas abundances in various terrestrial materials can be classified into three distinct fractionation patterns: type 1 shows a progressive enrichment in the heavier gases (shales, natural gases, holocrystalline submarine basalts, seawater and groundwater), type 2 shows a large enrichment of Ne and a slight apparent enrichment of Xe (glassy margins of submarine basalts, a subaerial basalt, an olivine xenolith from Hawaii), and type 3 shows large enrichments of Ne, Kr, and Xe relative to Ar (thucolite and a shale sample). The type 1 pattern is formed by the low‐temperature adsorption of rare gases usually via the rare gases dissolved in seawater. The type 2 pattern is interpreted as a combination of high‐temperature diffusion, solubility, and bulk melting in the mantle. The type 3 pattern probably represents a hybrid pattern. These patterns are consistent with an earth‐atmosphere evolution model which assumes (1) that the present rare gas abundances in the atmosphere closely approximate the rare gas inventory of the whole earth except for xenon and (2) that the average rare gas abundance pattern in the solid earth is roughly similar to that in the atmosphere except for Xe.