Imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an excellent technique for investigating the micro distribution of elements. We used this aspect to study the elemental distribution of Lithium within different components of the CM chondrites Cold Bokkeveld, Yamato 791198, and Banten. Lithium can be detected by TOF-SIMS with a high sensitivity.
During systematic search, 14 objects were detected containing enrichments of Lithium. The evaluation of TOF-SIMS spectra shows that the atomic Li/Si-ratio ranges up to 175¥CI (5.71¥10-5; Anders and Grevesse, 1989). The most important finding is that Li-enrichments are exclusively occurring in CAIs or in their alteration products. An
intensive search for Lithium in other components like
chondrules, PCP-rich objects, or accretionary dust rims was unsuccessful. Li-bearing phases include Fe-rich phyllosilicates, pyroxenes and an unidentified Al-rich phase that all occur within CAIs. Lithium is mainly enriched within the Fe-phyllosilicate rim that commonly occurs between the Wark-Lovering rim and the spinel of the interiors of Ca,Al-rich inclusions. The Fe-phyllosilicate rim is interpreted to
be an alteration product of melilite. This phase can be
characterized by high Fe and Al concentrations. Chemical analyses of this phase have been carried out by electron microprobe techniques. It is rich in FeO, Al2O3 and SiO2, whereas the Ca-content is below 1 wt%. In two cases Al-rich pyroxenes appear to be the carrier phases of Lithium. Lithium also occurs within an unidentified, probably H2O-containing, Al-rich silicate (Al2O3: 13.7 wt%, SiO2: 18.6 wt%).
CM-chondrites contain abundant components that have been modified by secondary processes. The occurrence of aqueous phases is one of the most important observations. These phases occur within chondrules, CAIs, PCP-rich objects, and within the accretionary dust mantles (Metzler et al., 1992). Some controversy exists about the time and environment of their formation. Metzler et al. (1992) suggested that at least some aqueous alteration products were produced prior to the formation of the final CM parent body (see their Fig. 20). On the other hand, it can not be ruled out that
a second period of aqueous alteration occurred on the
meteorite parent body (Lee, 1993). Also, Greenwood et
al. (1994) suggested that all phyllosilicates have been formed by aqueous processes in the parent body. Lithium enrichments in CM-chondrites were found in Fe-rich phyllosilicates exclusively located within Ca,Al-rich inclusions and are absent in the Fe-rich phyllosilicates that also occur within the host CM-chondrites. The presence of Lithium in altered phases of CAIs and the lack of Lithium within other components of the meteorites indicates that the Fe-phyllosilicates must have formed under different conditions. We suggest that the Li-bearing Fe-phyllosilicates are the result of
alteration processes in the nebula prior to incorporation of the CAIs into the meteorite parent bodies.
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