Isotope fractionation chemical composition

If we had chosen to describe composition in terms of elements, we would need to carry the elemental compositions of all species, minerals, and gases, as well as the coefficients of the independent chemical reactions. Our choice of components, however, allows us to store only one array of reaction coefficients, thereby reducing memory use on the computer and simplifying the forms of the governing equations and their solution. In fact, it is possible to build a complete chemical model (excluding isotope fractionation) without acknowledging the existence of elements in the first place ... 

The strong conceptual link between stable isotopes and chemical reaction makes it possible to integrate isotope fractionation into reaction modeling, allowing us to predict not only the mineralogical and chemical consequences of a reaction process, but also the isotopic compositions of the reaction products. By tracing the distribution of isotopes in our calculations, we can better test our reaction models against observation and perhaps better understand how isotopes fractionate in nature. 

Coleman ML (1998) Novel methods for measuring chemical composition of oil-zone waters Implications for appraisal and production. PETEX 98 Conf Proc Publ Abs M3 Coleman ML, Ader M, Chaudhuri S, Coates JD (2003) Microbial isotopic fractionation of perchlorate chlorine. Appl Environ Microb 69(8) 4997-5000... 

Richter FM, Davis AM, Ehel DS, Hashimoto A (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions Experiments, theoretical considerations, and constraints on their thermal evolution. Geochim Cosmochim Acta 66 521-540 Richter FM, Davis AM, DePaolo DJ, Watson EB (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim Cosmochim Acta 67 3905-3923 Rudnick RL, Fountain DM (1995) Nature and composition of the continental crust—a lower crustal perspective. Rev Geophys 33 267-309... 

A key problem in the present natural analogue study is the distinction between chemical variations related to trace element migration during basalt alteration and variations due to magmatic fractionation and other syn-intrusive processes. The detailed evaluation of the available data has shown that the chemical and isotopic composition of the HC1 residues is largely controlled by fractional crystallization and syn-intrusive assimilation of salt. In contrast, the chemical composition of the leachates is strongly modified by post-intrusive alteration (Steinmann et al. 1999). 

Quantifying the chemical and isotopic fractionations in planetary samples and in meteorites, the closest analogues to the materials that formed planets, is a necessary first step. This involves careful measurement of chemical and isotopic compositions of the various bodies and an understanding of the composition of the material from which they formed. Once these fractionations have been identified, experiments (and theory, when the relevant experiments cannot be performed) that yield similar fractionations can point to the processes and conditions that produced them. 

Some of these processes also cause measurable isotopic effects. Evaporation into empty space can cause the residual liquid or solid to become enriched in heavy isotopes. Processes that do not necessarily produce chemical fractionations can also produce isotopic effects. Diffusion is an example of such a process. Also, if the various constituents that go into making asteroids and planets have different isotopic compositions, the formation of these bodies can result in bulk compositions that are isotopically fractionated. Oxygen isotopes... 

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. 

Different isotopes of the same element differ slightly in chemical and physical properties because of their mass differences. For elements with low atomic masses, these mass differences are large enough for many physical, chemical, and biological reactions to fractionate or change the relative proportions of different isotopes of the same element in various compounds. Thus, a particular water or mineral may have a unique isotopic composition (ratio of the isotopes of an element) that indicates its source or the process that formed it. Two different processes—equilibrium and kinetic isotope effects—cause isotope fractionation. 

Besides the problem of accounting for the chemical abundances of planetary noble gases, there are characteristic differences in isotopic composition between planetary noble gases in meteorites and the solar gases that presumably represent the nebula from which meteorites formed. For Ar and Kr the differences are modest or perhaps nonexistent or can ultimately be explained in terms of a reasonable degree of mass-dependent isotopic fractionation. For Ne (Figure 3.3) and Xe (Figure 7.6), the... 

Unlike elemental concentrations, isotopic compositions are only affected a little by chemical differentiation processes. Mass-dependent isotopic fractionations can arise in chemical partitioning (cf. Section 2.9), of course, but on the scale of interest in the present context, plausible fractionation effects are small, especially at the high temperatures prevalent in the mantle. We can thus be much more confident that a noble gas isotopic composition measured in a mantle-derived sample is indeed characteristic of its mantle source. Representative mantle ranges for selected isotopic ratios are presented in Table 6.2. 

The fact that different isotopes of an element do not have the same physical-chemical properties means that kinetic and isotope exchange processes can lead to variations in isotopic composition. This phenomenon is usually referred to as isotope fractionation. Isotope fractionation, in most cases, leads to only small differences in isotopic composition. Consequently, isotope ratios are generally reported in terms of parts per thousand (per mil, o/00) differences. While it is possible to report isotope compositions in absolute terms, it has been found most convenient to report them relative to a standard with a composition typical of common natural materials. These two considerations result in the commonly used "8" notation ... 

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