Gases geochemistry

Ozima, M. and Podosek, F. (2002) Noble Gas Geochemistry, 2nd edition. Cambridge Cambridge University Press, 286 pp. 

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. 

A number of physical constants and conversion factors frequently useful in noble gas geochemistry calculations are collected in Table 1.1. These values have been used for all the calculations in this book. 

The opposite is true, however, for isotopic data (Table 1.3). Isotopic comparisons are of major concern in noble gas geochemistry, both terrestrial and extraterrestrial, and experience has shown that as error limits have shrunk in response to technological improvement, the arguments have followed the error limits down to progressively finer levels. In many instances involving comparison of air and sample compositions, or even only the use of air in instrumental calibration, the uncertainties in air isotopic composition are not inconsequential. 

The Henry constant J Cis a function of T but not P. (In some theoretical treatments, the Henry constant is the ratio of fugacity to quantity adsorbed, i.e., the inverse of the sense used here.) It is generally expected that adsorption will be governed by Henry s law at sufficiently low pressures. It is possible to construct theoretical models for adsorption in which an isotherm does not reduce to Henry s law, Equation (2.3), even in the limit P —> 0, but it is not clear that such situations obtain in practice and doubtful that they are important in noble gas geochemistry. 

The Langmuir model also provides a convenient basis for estimating when Henry s law or saturation effects can be expected. If an individual attachment site has an area a 2 x 10 15 cm2, the order of atomic cross-sectional area (cf. Table 2.1), then Ns 5 x 1014 atoms/cm2 = 2 x 10 5cm3STP/cm2. Surface concentrations approaching this order of magnitude can be expected to exhibit saturation behavior. Conversely, much lower concentrations indicate 9 1 and lead to the expectation of Henry s law behavior. Possible adsorption effects important in noble gas geochemistry always involve much lower concentration than this illustrative value, which is one reason why Henry s law violation is not expected. 

Since the Rayleigh distillation is likely to be the most common fractionation process in noble gas geochemistry, we will give some quantitative discussion of this process next. Suppose atoms (or isotopes) and / are escaping at a rate k and kj from a reservoir which contains , and rij numbers of the atoms, we have... 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

A noteworthy feature of such studies is that they frequently make the most stringent demands encountered in noble gas geochemistry for high-precision absolute elemental abundances. Particularly in marine studies the effects of interest are manifested in elemental abundance variations measured in percent. For Ar, accuracy at the required level can be achieved in gas chromatography for mass spectrometric abundance determinations at the percent level, isotope dilution is usually necessary. The technical problems are also different from those in most other noble gas spec-... 

Because of its low natural abundance and, in normal practice, severe interferences in mass spectrometric analysis, in the early noble gas analyses of sea water 3He was either ignored or used only as the spike in isotopic dilution. In the past decade, however, 3He data have assumed a position among the most important in terrestrial noble gas geochemistry. 

Begemann, F. (1994) Indigenous, extraneous noble gases in terrestrial diamonds. In Noble Gas Geochemistry and Cosmochemistry, J. Matsuda, Ed., pp. 217-27. Tokyo Terra Scientific Publ. Co. 

Podosek, F. A., Brannon, J. C., Bematowicz, J. T., Brazzle, R., Grauch, R., Cowsik, R., Hohenberg, C. M. (1994) Geochronology of tellurium ores and the double-beta decay lifetime of 130Te. In Noble Gas Geochemistry and Cosmochemistry, J. Matsuda, Ed., pp. 89-113, Tokyo Terra Scientific Publishing Co. 

Noble Gas Geochemistry discusses the fundamental concepts of using noble gases to solve problems in the earth and planetary sciences. 

There have been many developments in the use of noble gases since publication of the first edition of this book in 1983. This second edition has been fully revised and updated. The book will be invaluable to graduate students and researchers in the earth and planetary sciences who use noble gas geochemistry techniques. 

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