Isotope anomaly

The presence of 26Mg excesses correlated with Al/Mg ratios in fifteen Ca-Al-rich inclusions from the Allende and Leoville carbonaceous chrondrites has provided additional strong evidence for the in situ decay of 26A1 (see [9] for a recent review of isotopic anomalies). There are also, however, several examples of minerals whose isotopic compositions depart substantially from a unique Al-Mg isochron, even within a single inclusion [10,11]. Since deviations from the isochron may reflect either differences in the formation age of individual minerals or intrinsic heterogeneities in the initial 26A1/27A1 ratio, the value of the Al-Mg system as a chronometer for early solar system events remains unclear. 

A unique feature of the ion microprobe is the potential to measure both elemental concentrations and isotopic ratios in the same spot. This capability was particularly valuable in the present study since the possible correlations between the 26Mg/24Mg and Al/Mg ratios are central to the interpretation of Mg isotopic anomalies. The Al/Mg ratio is calculated from the... 

Another isotopic anomaly, discovered in Allende inclusions, concerns magnesium, for which an intrinsically low abundance in these samples makes its isotope ratios sensitive to small effects. Certain of the inclusions show a correlation between 26Mg and 27 Al, indicating an origin of excess 26Mg from radioactive decay of 26 A1 (mean life 1 Myr), the existence of which had previously been postulated as a heat source for meteorite parent bodies (Fig. 3.32). Other short-lived activites that seem to have been alive in the early Solar System are 10Be (mean life 2.2 Myr) from a correlation of 10B with 9Be, and 41Ca (mean life 0.15 Myr) from a correlation of... 

A few inclusions are exceptional in that they show isotopic anomalies for virtually every element studied, notably in the heavier isotopes of O, Mg, Si, Ca, Ti etc. these have been called FUN (fractionation and unknown nuclear) anomalies by Wasserburg and his colleagues and they may be due to unmodified stellar ejecta that were incorporated into the solar nebula in a solid form in which they survived. 

For more detailed discussions of isotopic anomalies and short-lived radioactivities in the Solar System, see... 

This new edition includes results from recent space missions, including WMAP and FUSE, new material on abundances from stellar populations, nebular analysis and meteoric isotopic anomalies, and abundance analysis of X-ray gas, and several extra problems at the end of chapters. 

Harrison, T. M. (1981). Diffusion of40Ar in hornblende. Contrib. Mineral. Petrol., 78,324-31. Hart, S. R. (1984). A large-scale isotope anomaly in the Southern Hemisphere mantle. 

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... 

For the following text, isotopic anomalies always stands for non-linear or non-mass dependent variations linear or mass dependent have the same meaning although mass dependent fractionation may not be strictly linear (Rayleigh). Usually, in the first approach the difference is not essential for description... 

As a concluding remark of this section, the theoretical models of nucleosynthesis within stars show that the isotopic compositions of the elements are highly variable depending on star size, metallicity, companion s presence. From the isotopic data obtained in diverse solar system materials it turns out that most of this material was highly homogenized in the interstellar medium or by the formation of the solar system. The presence of isotopic anomalies preserved in some primitive materials are the last witnesses of the initial diversity of the materials constituting our planetary system. 

Other meteorite classes like C2, CO and ordinary chondrites contain much smaller inclusions less than 1 mm (MacPherson et al. 1988) and require ion microprobe techniques to evaluate the isotopic compositions. On the least metamorphosed side. Cl have very few inclusions or oxide grains, but are the carrier of the greatest amounts of stellar nanodiamond and other carbides (Anders and Zirmer 1993). As will be shown for Cr anomalies in carbonaceous chondrites, the survival of the mineral carriers of the anomalies also depends on the metamorphic grade of the meteorites. Nevertheless, isotopic anomalies have also been formd in higher metamorphic grade from other classes, especially in the reduced enstatite chondrites. 

The excess was first suggested to have a nuclear origin in stars. Almost pure O is produced in He-buming shells in massive stars, and in supemovae. On the other hand it has been shown that non-mass dependent fractionation can be produced in the laboratory by non-nuclear processes (Thiemens and Heidenreich 1983 Thiemens 1988). Similar non-linear effects have been found for O isotopes in atmospheric gases (Schueler et al. 1990 Thiemens et al. 1995). Although stellar nucleosynthesis is indeed at the origin of the O observed in the universe, the link between O isotopic anomalies in inclusions and nucleosynthesis is still under debate (Thiemens 1999 Clayton 2002). 

Titanium-calcium. The first evidence for isotopic anomalies in the iron-group was found in Ti showing up to 10% excesses of Ti in hibonites from the Murray CM2 meteorite (Hutcheon et al. 1983 Fahey et al. 1985 Ireland et al. 1985 Hinton et al. 1987). Further studies in Murchison showed that Ti extended from -7% to +27% associated with Ca variation from -6% to +10% (Ireland 1988 Ireland 1990). Except for the magnitude of the variations, this is similar to the results from Allende inclusions. Only a few samples display mass-dependent fractionation for which it ranges up to 1.3 %/amu. In the majority of the samples, it is absent or very low (less than 1 %o/amu) for Ca-Ti. There is no correlation between the presence of linear fractionation and the magnitude of Ti effects. Ti variations are also present, but about an order of magnitude smaller than Ti. Variations affecting these two isotopes are related but not strictly correlated (Ireland 1988). 

The measurements of isotopic anomalies in meteorites has contributed greatly to the understanding of mixing processes and time scales in the formation of the solar system as well as strong constraints on presolar stellar evolution but it also left unanswered questions and revealed new complexities which are discussed here. 

There are many other questions underlying the measurement of isotopic anomalies and future planetary explorations will bring new stones to the construction. 

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