Elements mass-dependent isotope fractionation

We have known for many years that large isotopic fractionations of heavy elements like Pb develop in the source regions of TIMS machines. Nonetheless, most of us held fast to the conventional wisdom that no significant mass-dependent isotopic fractionations were likely to occur in natural or laboratory systems for elements that are either heavy or engaged in bonds with a dominant ionic character. With the relatively recent appearance of new instrumentation like MC-ICP-MS and heroic methods development in TIMS analyses, it became possible to make very precise measurements of the isotopic ratios of some of these non-traditional elements, particularly if they comprise three or more isotopes. It was eminently reasonable to reexamine these systems in this new light. Perhaps atomic weights could be refined, or maybe there were some unexpected isotopic variations to discover. There were. 

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 origin of mass-dependent isotopic fractionation in FUN CAIs is commonly (and somewhat casually) assumed to be the result of Rayleigh-type distillation, while the inclusions were molten. It is true that a strong case for distillation has been made in the case of the so-called HAL-type hibonites (see Section 1.08.7), based on trace element and isotopic properties (Lee et al, 1979, 1980 Davis et al, 1982 Ireland et al, 1992 Floss et al, 1996). Such an origin is problematic for other FUN CAIs, however, especially those that are otherwise identical in bulk composition to non-FUN CAIs. Most notably this is true of the FoBs that also happen to have F or FUN properties (Clayton et al, 1984 Davis et al, 1991). These objects are magnesium-rich relative to other CAIs, yet distillation experiments conducted on chondritic starting materials consistently show that... 

The extent to which isotope fractionation is observed for a given element is determined by both the relative mass difference between its isotopes and the extent to which the element participates in physical processes and/or chemical reactions. For the light elements H, C, N, O, and S, variations in their isotopic composition caused by mass-dependent isotope fractionation have been extensively studied using gas source isotope ratio mass spectrometry. For most of the metallic and metalloid elements (except Li and B), the relative mass difference between the isotopes is more limited, such that the variation in isotopic composition thus created is considerably more limited. The high precision with which isotope ratios can be measured nowadays however, not only allows the small isotope fractionations to be revealed, but also quantified. Even for the heaviest naturally occurring element U, variation in its isotopic composition due to the occurrence of isotope fractionation has been demonstrated [13]. As a general rule, elements that can occur in the environment in several oxidation states tend to show more pronounced isotope 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. 

Variations in isotopic compositions that are generated by isotope fractionation associated with chemical, physical, or (on Earth) biological processes are generally of mass-dependent nature. This implies that the magnitude of an isotope effect is proportional to the mass difference of the respective isotopes. Such mass-dependent isotope effects are hence generally most prevalent for lighter elements, which feature the largest relative differences in isotopic masses, and classic stable isotope studies were therefore focused on the elements H, C, N, O, and S. However, more recent studies, often conducted by MC-ICP-MS, have shown that natural isotope fractionation is also common for many heavier elements in both terrestrial rocks and meteorites [26, 27]. 

S Secondary Ion Mass Fractionation Mass fractionation, also referred to as isotope fractionation, describes a physical process that acts to separate isotopes of the same element over time. Isotope fractionation occurs when velocity, diffusivity, or bond strength-dependent processes are in effect. As a result, mass fractionation is of particular interest in the areas of chemistry, cosmology, and geology (in particular, chronology). 

Isotopic fractionation provides illustrative examples of first-order expansions of unknown functions. In general, the mass spectrometric measurement r/ of the ratio between two isotopes of mass m( and m, of the same element, differs from the natural value R/. Only a very small fraction of the original sample produces ions and different processes taking place in different parts of the mass spectrometer act differently on the sensitivity of each isotope. We assume that instrumental isotopic fractionation is mass-dependent. 

For elements that have three or more isotopes, isotopic fractionations may be defined using two or more isotopic ratios. Assuming that isotopic fractionation occurs through a mass-dependent process, the extent of fractionation will be a function of the relative mass differences of the two isotope ratios. For example, assuming a simple harmonic oscillator for molecular motion, the isotopic fractionation of may be related to as ... 

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