Radiogenic mass-dependent isotope fractionation

Apart from fission and radiogenic components, it appears that the principal underlying relationship between terrestrial Xe and solar Xe is a strong (about 3.5%/amu) mass-dependent isotopic fractionation. This is shown in Figure 7.6. To a good approximation, atmospheric Xe is related to SUCOR Xe, a solar Xe composition calculated to be surface-correlated Xe in a lunar mare soil. However, it is also clear that slight deviation from a linear trend indicates that atmospheric Xe and solar Xe cannot be related solely by fractionation. 

The application of MC-ICP-MS has had a profound impact on isotopic research in cosmochemistry over the last two decades. This immense impact primarily reflects two factors. First, MC-ICP-MS instruments are comparatively affordable and straightforward to use. As a result, there are now many laboratories world-wide in which MC-ICP-MS instruments are in routine use on a daily basis. The second factor is the performance characteristics of the instrumental technique, which is both versatile and suitable for high-precision isotopic analysis. As such, MC-ICP-MS can been applied to resolve small natural isotopic variations for a wide range of metallic and metalloid elements. Furthermore, it is equally suitable for the analysis of radiogenic and nucleosynthetic isotope anomalies and also mass-dependent isotope fractionations. As such, the technique of MC-ICP-MS is ideally suited for exploring the wealth of isotopic variations that are present in extraterrestrial materials and many successful investigations, which have yielded novel and important results, have been carried out in the recent past. 

Apphcations of isotopes in geology fall into two broad categories radiogenic isotope stndies, where an unstable parent isotope decays to a daughter product, thus giving age information when analyzed in rocks and minerals, and stable isotope studies that generally rely on fractionation due to mass difference, and thus have been traditionally restricted to hght isotope systems (typically elements with an atomic nnmber <20). Here 1 will concentrate on mass dependent stable isotope systems. 

The technique of internal normalization is commonly apphed in both MC-ICP-MS and TIMS for the precise correction of the instrumental mass bias (see also Chapter 5) that is encountered during the analysis of radiogenic isotopic compositions [33, 34]. The ICP ion source of MC-ICP-MS, however, also features two characteristics that play an important role for isotopic analysis, where internal normalization cannot be applied. First, an ICP source operates at steady state and therefore mass fractionation is not primarily a time-dependent process, as in TIMS where the measured isotopic compositions change with time due to the progressive evaporation of a sample from the filament. The steady-state operation of an ICP ion source is beneficial for the correction of instrumental mass bias by external standardization, where the isotope ratio data obtained for a sample are referenced to the values obtained for bracketing analyses of an isotopic standard [27, 35]. Hence, this procedure is commonly termed standard-sample bracketing. 

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