Chapter 25 Formation of Liquid and Condensation of Gas in the Primitive Solar Nebula : Part VII Formation of Asteroids and Meteorites : Origin of the Solar System
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概要
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The "triple point" which defines the low-pressure limit for stability of liquid has been experimentally determined for compositions of major chondrite-forming minerals; 1×10^<-5>, 2×10^<-6> and 1×10^<-6> bar for forsterite(Mg_2SiO_4), enstatite(MgSiO_3) and diopside(CaMgSi_2O_6), respectively. Under conditions of the primitive solar nebula where there was 10^4 times as much hydrogen as silicate components, these "triple points" shift to pressures with order of 10^<-2> bar. Liquid would not have been stable. therefore, in most part of the primitive solar nebula where total pressure was estimated to be lower than 10^<-3> bar. Liquid could be formed stably, however, by heating dusts or condensation of gas when the dust/gas ratio was significantly high. Chondrules were most probably formed from stable liquid. It is suggested, therefore, that the primitive solar nebula was heterogeneous in terms of dust/gas ratio and that chondrules were formed in the regions with high dust/gas ratios. Condensation experiments were carried out at 8.0×10^<-5> to 5.3×10^<-4> bar hydrogen pressures and oxygen fugacities between those of C-CO-CO_2, and Mo-MoO_2, buffers in the Mg-Fe-Si-O-H system to simulate condensation processes in the primitive solar nebula. Vapor formed by evapolating olivine (Fo_<84>) at 1500° and 1550℃ in graphite containers condenses forsterite at about 1200℃ and then, enstatite, metallic iron and silica mineral with decreasing temperature. Vapor formed at 1400℃ condenses forsterite first, but metallic iron condenses as a second phase and is followed by enstatite. The latter sequence coincides with that obtained from thermochemical calculations by Grossman (1972) for condensation of gas of solar composition at total pressures between 10^<-4> and 10^<-5> bar. The experiments demonstrate that liquid or amorphous material does not condense from vapor but major chondriteforming minerals condense directly from vapor under conditions similar to those of the primitive solar nebula.
- 理論物理学刊行会の論文
- 1989-03-10
著者
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NAGAHARA Hiroko
Geological Institute, School of Science, University of Tokyo
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Nagahara H
Univ. Tokyo Tokyo Jpn
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Nagahara Hiroko
Geological Institute Faculty Of Science University Of Tokyo
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Kushiro Ikuo
Geological Institute, Faculty of Science,University of Tokyo
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Kushiro Ikuo
Geological Institute Faculty Of Science University Of Tokyo
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Kushiro I
Inst. Frontier Res. On Earth Evolution Tokyo Jpn
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