Introduction

Except for radiogenic components, the amount of noble gases in the solid crust is safely assumed to be insignificant in terrestrial noble gas inventory. In general, except for in situ derived radiogenic and nucleogenic noble gases, trapped noble gases in crustal rocks are of an atmospheric origin. Because of the higher concentration of U, Th in the crust than in the mantle, however, nucleogenic Ne isotopes are often discernible in crustal materials. Near the Earth's surface, down to a few meters in depth, cosmic ray-induced cosmogenic Ne and He isotopes can also be observed. In deepocean bottom where sedimentation rate is extremely slow, extraterrestrial noble gases carried by cosmic dusts become conspicuous. In this chapter, we will discuss these unique features of noble gases in the crust.

Sediments

Noble gases in sediments once attracted much interest of noble gas geochemists for the following reasons. First, sediments could be a major reservoir for Xe and may account for the “missing Xe”, a long-standing puzzle in noble gas geochemistry (cf. Section 7.5). Because of the fine particle size, sedimenting particles would very effectively adsorb noble gases, especially the heaviest noble gas Xe, during sedimentation. Adsorption of atmospheric noble gases would also be substantial after emplacing sediments on the surface. It is also conceivable that they may have trapped a considerable amount of noble gases dissolved in water in the rock fabrics.