Geochemical Characteristics of the Noble Gases

Although this whole book is about noble gas geochemistry, two important general characteristics should be noted here. These features are commonly known by workers in the field but deserve explicit attention from the nonpractioner. 

the noble gases are trace elements par excellence. As an example, a not unreasonable value of Xe concentration in a rock is some 10 11 cm3 STP/g (about 3 x 108 atoms/g), or 0.00006ppb. It is nevertheless quite feasible to perform an adequate analysis on a 1-g sample of such a rock, in the sense of a sample to blank ratio in excess of 102, 5-10% uncertainty in absolute abundance, and 1% or less uncertainty in relative abundances of the major isotopes. Detection limits are much lower than this, and for the scarcer isotopes the blank and thus the quantity necessary for analysis are two to three orders of magnitude lower. It is worth noting that the reason why such an experiment is possible is the same reason why noble gases are so scarce in the first place their preference for a gas phase and the ease with which they can be separated from more reactive species. 

An important corollary of this scarceness is that many effects, which are not intrinsically characteristic of the noble gases, are best studied through them. These are effects in which small amounts of material are added to some reservoir. In many 

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