Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.266n&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 266
Computer Science
Scientific paper
It is now being accepted that chondrules were formed from pre- existing solids through complete or incomplete melting and that chondrules and fine-grained matrix materials were formed in oxidizing conditions where olivine solid solution was stable and not in the solar nebula of the average composition. As has been pointed out (e.g. 1-3), chondrules and matrix should have been genetically related because of constant bulk composition of chondrites in spite of various chondrule/matrix ratios. The recently calculated phase diagram of the olivine solid solution system (4) well explains their genetic relationships. At pressures above the triple point of forsterite (4x10^-4 bar, 1890 degrees C), the phase diagrams are the same as that at 1 bar. At pressures between the triple points of forsterite and fayalite (5x10^-8 bar, 1205 degrees C), liquid-bearing fields lie below the gas and solid field in the Fe-rich portion of the system. At pressures below the triple point of fayalite, the phase diagram has a gas-solid loop with a large Mg/Fe difference between coexisting solid and gas (5). X(sub)Mg of chondrules in ordinary chondrites ranges from 0.7 to 0.95 and that in carbonaceous chondrites from 0.5 to 1.0, and mostly between 0.8 and 0.90 for both C and O chondrites. Although chondrules are of multi-component, their compositions can be roughly shown by the olivine system. At pressures of 10^-4-10^-5 bar, olivine (X(sub)Mg<0.9) melts to become magnesian olivine and iron-rich liquid when heated, and X(sub)Mg of olivine and liquid are not so largely different. In this case, vaporization hardly takes place because olivine and liquid are below the vaporous curves of the system. Type I were formed in such a condition. At higher temperatures or lower pressures, liquid is not stable and highly magnesian olivine and gas with highly ferrous olivine composition are stable. Vaporization of the fayalite component takes place in this condition, resulted in remaining highly magnesian olivine. Therefore, Type II were formed at higher temperature or lower pressure than Type I. The present model does not require reduction for the formation of Type I (3,6). Difference in pressure is due to the degree of enrichment of the dust component relative to hydrogen gas. This model is consistent with the facts that (a) olivines in Type I chondrules are weakly zoned but those in Type II are strongly zoned, (b) liquid (groundmass of chondrules) in Type I is essentially FeO-free of which Mg/Fe distribution between olivine and liquid is often smaller than equilibrium value, but liquid in Type II is rich in FeO of which Mg/Fe distribution is in equilibrium, and (c) liquid in Type I chondrules are rich in refractory elements (Al, Ca, Ti, and Cr) but that in Type II is rich in mildly volatile elements (Fe, Mn, and alkalis) (6,7). Presence of alkalis in some Type I chondrules may be due to phase diagram of complex systems. Matrix olivine (mainly Fo(sub)50) are thought to be condensates from gas originated from partial evaporation of the olivine component at high temperatures responsible for formation of Type I chondrules. Condensation of iron-rich olivine from gas with iron-rich olivine composition has been experimentally shown (8). This model well explains the fine- grained but euhedral nature of matrix olivine. References: (1) Anders, E. (1987) Phil. Trans. R. Soc. Lond. A323, 287-304. (2) Nagahara, H. (1990) Meteoritics 25, 389-390. (3) Grossman, J. N. (1991) Meteoritics 26, 340-341. (4) Nagahara, H. et al. (1992) LPSC XXIII, 959-960. (5) Nagahara, H. et al. (1991) Meteoritics 26, 376. (6) Jones, R. (1990) Geochim. Cosmochim. Acta. 54, 1785-1802. (7) Jones, R. (1989) Proc. 19th Lunar Planet. Sci. Conf., 523-536. (8) Nagahara, H. et al. (1989) Nature, 315, 516-518.
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