The Commission’s Subcommittee for the Isotopic Composition of the Elements has carried out its biennial review of isotopic compositions, as determined by mass spectrometry and other relevant methods. This involves a critical evaluation of the published literature, element by element, and forms the basis of the Table of Isotopic Compositions of the Elements as Determined by Mass Spectrometry presented here. New guidelines have been used to arrive at the uncertainties on the isotopic abundances and there are numerous changes to the table since it was last published in 1991. Atomic Weights calculated from this table are consistent with Ar(E) values listed in the Table of Standard Atomic Weights 1997.

Isotopic compositions of the elements 1997

Noble gases within the crust originate from three main sources: the atmosphere, introduced into the crust dissolved in groundwater; the mantle, in regions of magmatic activity; and those produced in the crust by the result of radioactive decay processes. The continental crust contains approximately 40% of the terrestrial radioelements (Rudnick and Fountain 1995) that produce noble gases and, after the mantle and the atmosphere, forms the third major terrestrial noble gas reservoir (neglecting the core). In addition to these sources, contributions from interplanetary dust particles (IDP), cosmic ray interaction with the crustal surface and anthropogenic noble gases can in some cases be a significant source of noble gases in crustal materials. The use of noble gases to understand the role of fluids in different geological settings relies on their low natural abundance and chemical inertness. The low abundance of noble gases in crustal systems and their distinct isotopic character means that contributions from these different sources can often be resolved and quantified. With this, information is gained about the source of associated fluids, the environment from which they originated the physical manner in which they have been transported to the sampling site and the different phases that may have interacted within the crustal fluid system. This is only possible, however, with a detailed understanding of the processes that control the concentration and isotopic composition of the noble gases in different crustal environments.

The first part of this chapter deals with the three different mechanisms of noble gas production within the crust—radiogenic, nucleogenic, fissiogenic (Fig. 1⇓). We show how production ratios are affected not only by the source region radioelement concentration, but in the case of nucleogenic reactions, also by the spatial distribution and concentration of the target elements. For completeness we consider cosmogenic noble gas production rates and Interplanetary …

Production, Release and Transport of Noble Gases in the Continental Crust

In contrast to most other fields of noble gas geochemistry that mostly regard atmospheric noble gases as ‘contamination,’ air-derived noble gases make up the far largest and hence most important contribution to the noble gas abundance in meteoric waters, such as lakes and ground waters. Atmospheric noble gases enter the meteoric water cycle by gas partitioning during air/water exchange with the atmosphere.

In lakes and oceans noble gases are exchanged with the free atmosphere at the surface of the open water body. In ground waters gases partition between the water phase and the soil air of the quasi-saturated zone, the transition between the unsaturated and the saturated zone. Extensive measurements have shown that noble gas concentrations of open waters agree well with the noble gas solubility equilibrium according to (free) air/(free) water partitioning, whereby the aquatic concentration is directly proportional to the respective atmospheric noble gas abundance (Henry law, Aeschbach-Hertig et al. 1999b).

In applications in lakes and ground waters the gas specific Henry coefficient can simplifying be assumed to depend only on temperature and salinity of the water. Hence the equilibrium concentrations of noble gases implicitly convey information on the physical properties of the water during gas exchange at the air/water interface, i.e., air pressure, temperature and salinity of the exchanging water mass. The ubiquitous presence of atmospheric noble gases in the meteoric water cycle defines a natural baseline, which masks other noble gas components until their abundance is sufficiently large that these components can be separated against the natural atmospheric background. For most classical geochemical aspects this typical feature of natural waters may look at first sight as a disadvantage. In fact it turns out to be advantageous because in most cases the noble gas abundance in water can be understood as a binary mixture of two …

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J.D Ross

Papers

Helium and neon isotope geochemistry of some ground waters from the Canadian Precambrian Shield