CAIs oxygen isotopes

Substantial abundance anomalies occur among the heavy oxygen isotopes 170 and 180, which are underabundant by up to about 4 per cent relative to 160 in oxide grains of certain of the CAIs, compared with the bulk composition in which the isotope ratios are closer to a terrestrial standard. The intriguing feature of these anomalous ratios is that, in common with some other meteorites, but in contrast to terrestrial and lunar samples, the relative deviations of the two heavy isotopes are equal most normal fractionation processes would cause 180 to have twice the anomaly of 170, as indeed is observed in terrestrial samples and more differentiated meteorites, where the anomalies are also usually much smaller. While there has been speculation that there might be a substantial admixture of pure 160 from a supernova, there are fractionation mechanisms that may be able to account for the effect, e.g. photo-dissociation of molecules affected by selfshielding (R. Clayton 2002). In this case, it is possible that the terrestrial standard is enriched in the heavy O-isotopes while the inclusions have more nearly the true solar ratio. 

A test of the hypothesis that the linear trends in Figures 10-12 are due to mixing comes from comparisons with oxygen isotopes. It has been conventional wisdom that Mg does not correlate in detail with O excesses in primitive chondrite components, including CAIs. However, combining the new highly precise MC-ICPMS data with isotope ratio data for the... 

When differences in the oxygen isotopic composition of CAIs were first measured in 1973, Robert Clayton and his coworkers attributed these mass-independent variations to mixing of normal solar system gas (plotting on or above the terrestrial mass-fractionation... 

Oxygen isotopic compositions of minerals in CAIs on an oxygen three-isotope diagram. Axes are given in delta notation 81S0 =[((180/160)sampie/(180/160)s,andard)-1] x 1000, and similarly for S170. After Clayton et al. (1977). 

Oxygen isotopic compositions (relative to standard mean ocean water, SMOW) of several mineral grains in three Stardust particles, each grain denoted by a separate symbol. Open circles are from a CAI comet particle. After McKeegan et al. (2006). 

FIGURE 2.6 Theory for forming CAIs on dust surfaces at 1500-2000 K and separating oxygen isotopes in a mass-independent way [27]. 

On the basis of analyses of bulk CAIs or separated minerals, the inheritance of 0-rich condensates from supemovae appears plausible. However, the magnitude of the isotopic difference between the putative dust and gas reservoirs is a free parameter, so that the model has no predictive power. Furthermore, the observed oxygen isotope anomalies in presolar oxide grains are best understood as resulting from processing in red giant stars, and do not show 0-excesses. 

Differences between the bulk compositions of chondrites, planets and asteroids can be attributed to accretion from different batches of CAIs, chondrules and other components, which are spread along variation lines on the standard three-isotope plot. The preservation of oxygen isotopic anomalies shows that there were numerous oxygen reservoirs for the manufacture of chondrules and CAIs that were quite separate. 

For volatile-rich carbonaceous chondrites like Cl and CM chondrites, constraints on thermal histories are derived from carbonate ages and oxygen isotopic data. The former indicate that alteration began soon after CAI formation and lasted —20Myr (Endress et al, 1996 Brearley et al, 2001). Oxygen-isotopic compositions of carbonates provide model-dependent temperatures... 

Figure 6 Oxygen-isotopic compositions of individual minerals in CAIs from the CO chondrites Y-81020, Colony, Kainsaz and Ornans. Primary minerals in CAIs from the least metamoprhosed CO chondrites Y-81020 (type 3.0) and Colony (3.0) are uniformly 0-enriched, whereas CAIs from Kainsaz (3.2) and Omans (3.3) tend to show oxygen isotopic heterogeneity with spinel and high-calcium pyroxene enriched in 0 and melilite and secondary nepheline depleted in 0. Based on these observations, Wasson et al (2001) inferred that oxygen isotope exchange took place during thermal metamorphism and alteration in an asteroid (data from Itoh et al, 2000 Wasson et al, 2001).

Figure 7 (a,b) Oxygen-isotopic compositions of individual minerals in CAIs from CR chondrites (data from Aleon et al, 2002a). Most CAIs are °0-rich and isotopically... 

Oxygen-isotopic compositions of aluminum-rich chondrules from CR and CH chondrites (Krot et al, 2003c) are plotted in Figure 15(b). Three out of six chondrules analyzed exhibit large internal-isotopic heterogeneity, whereas others are iso-topically uniform all chondrules have porphyritic textures. In contrast to aluminum-rich chondrules in ordinary chondrites, oxygen isotopic heterogeneity is due to the presence of relict CAIs (Krot and Keil, 2002). 

Figure 15 Oxygen-isotopic compositions of individual minerals in (a) aluminum-rich chondrules from ordinary chondrites (Russell et at, 2000) and (h) CR carhonaceous chondrites (Krot et ah, 2002h). (c) Oxygen-isotopic compositions of individual minerals in the CAI-hearing chondmle 17 from Acfer 094 (data from Krot et al, 2003a) (Ahhreviations chd, chondmle cpx, clinopyroxene gl, glass hih, hihonite nph, nepheline ol, olivine opx, orthopyroxene pi, plagioclase px, pyroxene sp, spinel).

CIl chondrites also contain a small fraction of isolated olivine and pyroxene grains up to 400 p,m in size with chemical, oxygen isotopic compositions, and rounded inclusions of metallic Fe,Ni and trapped melt indicating that they were derived from chondrules (Leshin et al, 1997). Brearley and Jones (1998) estimate the abundance of olivine and pyroxene at <1 vol.%, but X-ray diffraction studies by Bland et al. (2002) indicate 7 wt.% olivine. The dilference probably reflects a high abundance of olivine crystallites embedded in phyllosilicates (P. A. Bland, private communication). CIl chondrites also contain rare refractory grains with oxygen isotopic compositions comparable to those in CAIs (see Scott and Krot, 2001). 

Guan Y., McKeegan K. D., and MacPherson G. J. (2000) Oxygen isotopes in calcium—aluminum-rich inclusions from enstatite chondrites new evidence for a single CAI source in the solar nebula. Earth Planet. Sci. Lett. 181, 271-277. 

Ito M., Yurimoto H., and Nagasawa H. (1999) Oxygen isotope microdistribution vi. composition in meUlite/fassaite in the Allende CAI 7R-19-1 a new evidence for multiple heating. In Lunar Planet. Sci. XXX, 1538. The Lunar and Planetary Institute, Houston (CD-ROM). 

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