Solar System isotopic anomalies

Chronological studies show that the angrites are essentially as old as the solar system. Isotopic anomalies in chromium due to the decay of... 

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. 

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... 

Curiously, there are some nearby interstellar diffuse clouds displaying anomalously low isotope ratios for 7Li/6Li, with a ratio apparently as small as 2 in one case (Lemoine et at. 1994 Knauth, Federman Lambert 2003), compared to the Solar-System (and more usual interstellar) ratio of 12 the anomaly here is that the low ratio in such clouds is consistent with cosmic-ray spallation whereas that in the Solar-System is not. 

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

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... 

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. 

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. 

Clayton DD (1989) Origin of heavy xenon in meteoritic diamonds. Astrophys J 340 613-619 Clayton DD, Dwek E, Woosley SE (1977a) Isotopic anomalies and proton irradiation in the early solar system. Astrophys J 214 300-315... 

Lee DC, Halliday AN, Snyder GA, Taylor LA (1997) Age and origin of the moon. Science 278 1098-1103 Lee T (1978) A local proton irradiation model for the isotopic anomalies in the solar system. Astrophys J 224 217-226... 

Lee T (1988) Implications of isotopic anomalies for nucleosynthesis. In Meteorites and the Early Solar System. Kerridge JF, Matthews MS (eds) University of Arizona Press, Tucson, p 1063-1088 Lee T, Papanastassiou DA (1974) Mg isotopic anomalies in the Allende meteorite and correlation with O and Sr effects. Geophys Res Lett 1 225-228... 

In practice, it is not sufficient for an object to have an isotopic composition that cannot be explained by radioactive decay or mass-dependent fractionation effects. The object must also have physical and chemical characteristics making it unlikely to be a product of solar system processes. For example, millimeter- to centimeter-sized refractory inclusions from primitive chondrites have been shown to contain small (parts in 103 to 104) isotopic anomalies in many elements. However, based on the size, composition, physical characteristics, and abundance of the inclusions, it is generally believed that these objects formed within the solar system. They preserve small isotopic anomalies because they did not form from a representative sample of the bulk solar system (see Chapters 7 and 14). So, isotopic anomalies can indicate either that an object is itself presolar or that it formed in the solar system from precursor material that was not fully homogenized in the solar system. As mass spectrometry has become more precise, small isotopic anomalies of the second type have shown up in a wide variety of chondritic materials. As we discuss below and in Chapter 7, these anomalies and bona fide presolar grains can be used as probes of processes in the early solar system. 

Huss, G. R. (2004) Implications of isotopic anomalies and presolar grains for the formation of the solar system. Antarctic Meteorite Research, 17, 132-152. 

One implication of the discovery that all pre-existing solids were not vaporized and recondensed in the early solar system is the possibility that variations in the U/ U ratio could have been inherited from the Sun s parent molecular cloud. In addition, the short-lived nuclide Cm, which decays to U, might have been present. Recent studies have begun to re-investigate the assumption of constant U/ U in early solar system materials. A search for uranium isotopic anomalies in relatively large samples of a wide range of chondrites, achondrites, and terrestrial rocks found the U/ U ratio to be constant to... 

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. 

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