Stepwise Degassing

Component separation is often particularly prominent when one or more of the components present is in situ. There is no requirement that different components reside in different minerals indeed, it is often not known at all where the components reside. An in situ component will be sited wherever its parent element is, and this siting might be rather different from that of some other in situ component or a trapped component, as illustrated in Fig. 2.12. Radiogenic gases are common in situ components, and stepwise heating is frequently applied to resolving them from other components. 

The numeral 3 indicates 300°C extraction, whereas the numeral 4 represents a 400°C extraction, and so on. Reproduced from Bematowicz et al. (1978). 

To proceed with a quantitative illustration, it is necessary to adopt some specific model for gas release. It is usually assumed that Fick s law applies, and the problem can be treated as volume diffusion (Section 2.7). As a first approximation, it is often assumed that the sample is composed of uniform spheres of radius a. If it is further assumed that at some initial time / = 0 the gas concentration is uniform, and that the concentration of gas is always nil at the surface of the sphere. Then, after diffusion to time t, the fraction / of the original gas that remains in the sphere is (Carslaw Jaeger, 1959) 

The extent of diffusion is conveniently parameterized by the dimensionless variable 

In actual experiments, of course, things are not quite so simple. In the first place, one does not observe the instantaneous release but only its integration in discrete temperature steps. Problems also arise because of the recoil effect mentioned earlier and various interferences and other uncertainties arising in the neutron irradiation. 

---

Albarede, F. (1978). The recovery of spatial isotope distributions from stepwise degassing data. Earth Planet. Sci. Letters, 39, 387-97. 

This was the first extinct radioactivity detected (Jeffrey and Reynolds 1961) and was made possible by the early high sensitivity of rare gas measurements and the low abrmdance of Xe in rocks. I has only one stable isotope at mass 127. Its abundance is measured as Xe after exposing a sample to an adequate neutron flux. The correlation between Xe and Xe observed in a stepwise degassing of a sample demonstrates that the excess Xe results from decay (Fig. 9h). Results in primitive meteorites and inclusions show that i29j/i2tj j.jose to 10 . Chronometry with I- Xe has been widely used in meteorite work (Reynolds 1963 Hohenberg 1967) but occasionally has some diflflculties to agree with the other chronometers due to the sensitivity of I to secondary processes and water alteration (Pravdivtseva et al. 2003 Busfleld et al. 2004 see also Swindle and Podosek (1988) for an extensive review). ... 

Figure 10. Comparison of thermal release of Xe from Orgueil nanodiamonds (Huss and Lewis 1994b) with those from artificial nanodiamonds implanted with noble gas ions (according to Koscheev et al. 2001 sample UDD-1-1). Both show two release peaks. Shown is the percent release per °C temperature interval in stepwise degassing.

A very instructive example of stepwise photoreduction porphyrin - chlor-in - isobacteriochlorin is provided by tin(IV) porphyrins [238, 239], e.g. Sn(OEP)2+ is photoreduced in the presence of SnCl2 2HzO (in which Sn(II) acts as an electron donor and H20 as a proton donor) in degassed pyridine to tin (IV) octaethylchlorin and subsequently to tin (IV) octaethylisobacteriochlorin The use of SnCl2 2D20 led to the formation of deuteriated reduced products. 

Stepwise, reduce the initial degassing period (20 min) to find the required degassing time for the specific conditions. The required degassing period must be redetermined, if... 

Ru]-II, however, was suitable for the RCM of 121 and 122, despite the low yield of 122. Therefore, in the one-pot procedure, after the consumption of 118 by the first metathesis reaction using [Ru]-Ia and degassing excess 1,3-butadi-ene, [Ru]-II was added in four portions to the resulting reaction mixture. Subsequent desilylation of the tcrt-butyl-dimethylsilyl (TBDMS) group afforded cyclic diene 119 in a better overall yield than in a stepwise manner. 

---