Cosmochemistry isotope fractionation

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

All of the bodies in the solar system formed from the same mixture of gas and dust inherited from the Sun s parent molecular cloud. The composition of the dust is best approximated by Cl chondrites. The current compositions of the bodies in our solar system came about because various chemical and physical processes fractionated the elements and isotopes in that initial composition. Understanding how and why elements and isotopes fractionate is a central theme of cosmochemistry. It is easy to visualize fractionations using certain kinds of diagrams that compare elements and isotopes with different chemical characteristics. 

Sharp, Z. (2007) Principles of Stable Isotope Geochemistry. Upper Saddle River, New Jersey Pearson Prentice Hall, 344 pp. A good recent textbook covering the basics of isotope fractionation and its application to geochemistry and cosmochemistry. 

Kinetic effects have been of great interest in cosmochemistry since the mid-1990s and we now review the generally accepted theory of evaporation and condensation in some detail. This discussion focuses on isotope fractionations, but is equally applicable to elemental fractionations. 

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. 

The Wetherill concordia and Tera-Wasserburg concordia can both be extended to include 232Th. The 232Th system simply takes the place of either 235U or 238U in the calculation of concordia. In general, this just introduces a potential additional complication because uranium can be fractionated from thorium. Fractionation of uranium from thorium would mean that the ratio of the two parent isotopes has changed and must be properly accounted for. Thus U-Th concordia treatments are rarely employed in cosmochemistry. 

Lead has four stable isotopes, ° Pb, Pb, Pb, and ° Pb, but three of these are radiogenic, such that there is no invariant isotope that can be used for internal correction of instrumental mass fractionation. This poses a significant analytical challenge for precise Pb isotopic analysis. This problem is most readily overcome with MC-ICP-MS, because the Pb isotope ratio measurements can utilize the ratio of added T1 for external normalization of the Pb isotope ratio data (see also Chapter 5). The simplicity of this procedure is the main reason why MC-ICP-MS Pb isotopic analysis is now commonly used in geo- and cosmochemistry. This approach needs to be applied with care, however, and it has been argued that Pb isotopic analysis that utilizes the double spike methodolc (either in conjunction with TIMS or MC-ICP-MS) is typically superior in accuracy and precision [86, 87]. 

---