Sedimentary noble gases

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This paper presents experimental data for the trapped noble gas contents in a variety of shales and related samples. These data, along with those previously available, indicate that the normal sedimentary rock pattern is one of progressive enrichment of the heavier gases in comparison with the proportions in air, from which sedimentary rock gases were presumably acquired. Within this normal pattern, however, variations considerably beyond an order of magnitude characterize both abundance and composition, so that multiple processes appear to be necessary to account for trapping of noble gases in sedimentary rocks. Relative to the normal pattern a number of cases of anomalous Ne enhancement occur. This unexplained phenomenon, previously thought to be exotic, must now be considered unexceptional. Noble gases in air occur in approximately the ‘planetary’ proportions characteristic of meteorites, except for more than an order of magnitude deficiency of Xe. It is commonly held that atmospheric noble gases are indeed planetary, the Xe deficiency in air being made up by the inventory in sedimentary rocks. We consider that while this hypothesis cannot be proved false the available data do not support it. At least equal weight should be given to the alternatives: either that (preferentially) Xe has failed ever to become part of the atmosphere or that the total terrestrial noble gas inventory is simply not planetary.