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Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...1..129c&link_type=abstract
Treatise on Geochemistry, Volume 1. Editor: Andrew M. Davis. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 71
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17
Scientific paper
Oxygen isotope abundance variations in meteorites are very useful in elucidating chemical and physical processes that occurred during the formation of the solar system (Clayton, 1993). On Earth, the mean abundances of the three stable isotopes are 16O: 99.76%, 17O: 0.039%, and 18O: 0.202%. It is conventional to express variations in abundances of the isotopes in terms of isotopic ratios, relative to an arbitrary standard, called SMOW (for standard mean ocean water), as follows:The isotopic composition of any sample can then be represented by one point on a "three-isotope plot," a graph of δ17O versus δ18O. It will be seen that such plots are invaluable in interpreting meteoritic data. Figure 1 shows schematically the effect of various processes on an initial composition at the center of the diagram. Almost all terrestrial materials lie along a "fractionation" trend; most meteoritic materials lie near a line of "16O addition" (or subtraction). (4K)Figure 1. Schematic representation of various isotopic processes shown on an oxygen three-isotope plot. Almost all terrestrial materials plot along a line of "fractionation"; most primitive meteoritic materials plot near a line of "16O addition." The three isotopes of oxygen are produced by nucleosynthesis in stars, but by different nuclear processes in different stellar environments. The principal isotope, 16O, is a primary isotope (capable of being produced from hydrogen and helium alone), formed in massive stars (>10 solar masses), and ejected by supernova explosions. The two rare isotopes are secondary nuclei (produced in stars from nuclei formed in an earlier generation of stars), with 17O coming primarily from low- and intermediate-mass stars (<8 solar masses), and 18O coming primarily from high-mass stars (Prantzos et al., 1996). These differences in type of stellar source result in large observable variations in stellar isotopic abundances as functions of age, size, metallicity, and galactic location ( Prantzos et al., 1996). In their paper reporting the discovery of 18O in the Earth's atmosphere, Giauque and Johnston (1929) refer to nonuniform distribution of oxygen isotopes as a "remote possibility," whereas Manian et al. (1934) sought to find variations in oxygen isotope abundances in meteorites as evidence for an origin outside the solar system.In addition to the abundance variations due to nuclear processes, there are important isotopic variations produced within molecular clouds, the precursors to later star-formation. The most important process is isotopic self-shielding in the UV photodissociation of CO (van Dishoeck and Black, 1988). This process results from the large differences in abundance between C16O, on the one hand, and C17O and C18O on the other. Photolysis of CO occurs by absorption of stellar UV radiation in the wavelength range 90-100 nm. The reaction proceeds by a predissociation mechanism, in which the excited electronic state lives long enough to have well-defined vibrational and rotational energy levels. As a consequence, the three isotopic species - C16O, C17O, and C18O - absorb at different wavelengths, corresponding to the isotope shift in vibrational frequencies. Because of their different number densities, the abundant C16O becomes optically thick in the outermost part of the cloud (nearest to the external source of UV radiation), while the rare C17O and C18O remain optically thin, and hence dissociate at a greater rate in the cloud interior. The differences in chemical reactivity between C16O molecules and 17O and 18O atoms may lead to isotopically selective reaction products. This scenario has been suggested to explain meteoritic isotope patterns, as discussed below (Yurimoto and Kuramoto, 2002).Stable isotope abundances in meteoritic material provide an opportunity to evaluate the thoroughness of mixing of isotopes of diverse stellar sources. Molybdenum presents a good test case: it has seven stable isotopes, derived from at least three types of stellar sources, corresponding to the r-process, s-process, and p-process. Presolar silicon carbide grains, extracted from primitive meteorites, contain molybdenum that has been subject to s-process neutron capture in red-giant stars, resulting in large enrichments of isotopes at masses 95, 96, 97, 98, and severe depletions (up to 100%) of isotopes at masses 92 and 94 (p-process) and 100 (r-process) (Nicolussi et al., 1998). Complementary patterns have been found in whole-rock samples of several meteorites, with >1,000-fold smaller amplitude, suggesting the preservation of a small fraction of the initial isotopic heterogeneity ( Yin et al., 2002; Dauphas et al., 2002). Oxygen is another element for which primordial isotopic heterogeneity might be preserved. This is discussed further below.It would be highly desirable to have samples of oxygen-rich mineral grains that have formed in stellar atmospheres and have recorded the nucleosynthetic processes in individual stars. Similar samples are already available for carbon-rich grains, in the form of SiC and graphite, primarily from asymptotic giant branch (AGB) stars and supernovae (Anders and Zinner, 1993). These presolar grains have provided a wealth of detailed information concerning nucleosynthesis of carbon, nitrogen, silicon, calcium, titanium, and heavier elements (see Chapter 1.02). It is thought that such carbon-rich minerals should form only in environments with C/O>1, as in the late stages of AGB evolution, or in carbon-rich layers of supernovae. By analogy, one would expect to form oxide and silicate minerals in environments with C/O<1, as is common for most stars. Indeed there is evidence in infrared spectra for the formation of Al2O3 (corundum) and silicates, such as olivine (Speck et al., 2000) around evolved oxygen-rich stars. However, searches for such grains in meteorites have yielded only a very small population of corundum grains, a few grains of spinel and hibonite, and no silicates ( Nittler et al., 1997). The observed oxygen isotopic compositions of presolar corundum grains show clear evidence of nuclear processes in red-giant stars, and have had significant impact on the theory of these stars ( Boothroyd and Sackmann, 1999).There are several possible reasons for the failure to recognize and analyze large populations of oxygen-rich presolar grains:(i) they may not exist: oxygen ejected in supernova explosions may not condense into mineral grains on the short timescale available;(ii) they may be smaller in size than can be detected by applicable techniques (˜0.1 μm); and(iii) they may be destroyed in the laboratory procedures used to isolate other types of presolar grains.
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