Nucleosynthetic isotope anomalies

Nudeosynthetic isotopic variations are caused by heterogeneities in the distribution of stable, non-radiogenic isotopes within the solar nebula, which reflect the incomplete mixing of material that is derived from distinct sources of element production. 

The study of nudeosynthetic isotopic variations is of interest because such anomalies provide direct evidence of the sites and mechanisms of element 

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Besides neon, xenon has turned out to be the element most diagnostic in its isotopic composition. An important role has been played by Xenon-HL, which was the first of the nucleosynthetic isotope anomalies to be discovered (Reynolds and Turner 1964). The HL component has received its name for the simultaneous overabundance of the heavy xenon isotopes (= Xe-H) and the light xenon isotopes (= Xe-L). Because the H part originally was more reliably determined, Xenon-HL was first believed to be associated with fission, possibly of a superheavy element (e.g., Anders et al. 1975), but in the end the search for its host phase led to the discovery of the existence of grains of presolar origin in primitive meteorites (Lewis et al. 1987). 

A few reports are available where TIMS has been used for cosmochemical analysis. The recent report of Yamakawa et al. (2009) deals with sequential chemical separation technique for Cr, Fe, Ni, Zn, and Cu in terrestrial and extraterrestrial silicate rocks for precise and accurate determination of elemental concentration. The chemical technique uses a combination of cation-anion exchange chromatography and Eichrom nickel specific resin. The developed method was tested to a variety of matrixes bulk meteorite (Allende), terrestrial peridotite (JP-1), and basalt (JB-lb). Concentrations of each element were determined by TIMS. The analytical procedure was further extended to obtain high-precision isotope data for Cr. The method is capable to determine the isotopic ratios of Cr/ Cr and Cr/ Cr with precision of 5 X 10 and 1 x 10, respectively. The method can be equally applicable in cosmochemical studies, like Mn-Cr chronology and investigation of nucleosynthetic isotopic anomalies in meteorites. The elemental concentrations measured by TIMS in Allende are, for example, Cr 3,600 0.007 ppm, Fe 233,400 0.009 ppm, Ni 14,020 0.006 ppm, Cu 107 0.013 ppm, and Zn 117.8 0.01 ppm. The detection of Al (0.7 Ma) in the Allende meteorite proves that nucleosynthesis is still active in the Milky Way as the half-life of A1 is much less than the timescale of galactic evolution (10 years). 

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. 

Birck JL (2004) An overview of isotopic anomalies in extraterrestrial materials and their nucleosynthetic heritage. Rev Mineral Geochem 55 25-64... 

An Overview of Isotopic Anomalies in Extraterrestrial Materials and Their Nucleosynthetic Heritage... 

In a strict sense isotopic anomalies are defined here as isotopic variations that are not understood to have been generated from a once uniform reservoir by processes acting within the solar system. They may result from incomplete homogenization of isotopically very diverse nucleosynthetic components. They potentially possess two types of information the nature of the nucleosynthetic sources of the material, and the processes acting during their transfer from within stars to their preservation in meteorites. The definition of the... 

Over the years, numerous studies of CAIs have been carried out by a variety of techniques. MacPherson et al. (1995) compiled all available data and found that the 26A1/27A1 ratios for CAIs have a bi-modal distribution (Fig. 8.27). Many have ratios near 5 x 10-5, which they interpreted as the initial ratio for the solar system (the canonical ratio). Many others have initial ratios near zero. Resetting or isotopic disturbance by secondary processes is responsible for the low ratios in most cases. But a few CAIs formed with little or no 26Al. These so-called FUN CAIs (Fig. 8.27) also exhibit large isotopic mass fractionations and isotopic anomalies reflecting different mixtures of nucleosynthetic components. In 1995, evidence for 26A1 in objects other than CAIs was rare. 

The most extreme example of isotopic anomalies is provided by the laboratory analyses of individual preserved presolar dust grains extracted from primitive meteorites (Anders and Zinner 1993). These micron-size or smaller grains of SiC, graphite, and (less commonly analyzed) refractory oxides formed in the outflows of evolved stars and their isotopic compositions of C, N, O and other major and even trace elements quantitatively reflect the unique nucleosynthetic environment of that particular star, which may differ from average solar system compositions by one or more orders of magnitude. That such... 

The alternative hypothesis, that of a nucleosynthetic component carried into the early solar system by 0-rich presolar dust (see discussion in Clayton 1993) remains plausible after all, as mentioned in the Introduction, trace amounts of presolar dust is known to exist in the matrices of chondrites. However, all the high spatial (ion microprobe) measurements of the many types of primitive refractory materials discussed here have failed to find any very extreme oxygen isotopic anomalies such as might be expected if pure 0 dust (e.g. from a supernova) were incorporated into CAIs or AOAs along with local condensates. Even among the -100 true interstellar oxide grains that... 

One striking exception was the very early discovery of I decay to Xe (Jeffery and Reynolds 1961). This discovery reflects the particular properties of rare gases which are nearly absent in telluric planetary bodies. Because they are not diluted by high abrmdances of isotopically normal noble gases, anomalies in rare noble gas components were the first to be detected. This is also the reason for the Xe record of the fission of Pu (Rowe and Kuroda 1965). From the available data on short-lived nuclides at that time, it was concluded that the last nucleosynthetic input into the protosolar cloud predated the formation of the planets by 100-200 Ma. 

Isotopic studies involving MC-ICP-MS revealed evidence for a heterogeneous distribution of Zr, Ti, Mo, and Ni isotopes in the solar system [49, 53-55, 59, 61]. Variations in relative isotopic abundances of Zr and Ti between carbonaceous chondrites and terrestrial samples provide evidence for a heterogeneous distribution of CAIs (Table 10.1) as carriers of these nucleosynthetic anomalies. This conclusion has implications for mixing processes in the solar nebula and accretion models of planets. 

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