Vaporization experiments in the system plagioclase-hydrogen

Vaporization experiments in the system plagioclase (An_)-hydrogen were carried out at temperatures between 1200 and 1475℃ and in the pressure range from 10^ to 10^ bar. After partial vaporization, originally homogeneous plagioclase became zoned with increase of CaO and decrease of Na_2O from core to rim. The compositional change plotted on Si-Na-Ca, Al-Na-Ca, and Al-Na-Si diagrams revealed that plagioclase vaporized incongruently to form a gas with an atomic ratio close to Si : Al : Na=1 : 1 : 1 and non-stoichiometric residue over wide ranges of total pressure and oxygen fugacity. Increase of total pressure, which corresponds to increase of hydrogen pressure in the present experiments, enhanced vaporization; the Na_2O content at the surface of residue decreased and the depth of the Na_2O-depleted zone from the surface to the interior of the charges increased. Hydrogen may play the role of catalyst for the vaporization reaction. Another explanation for the role of hydrogen is that plagioclase reacts with carbon from the sample container and hydrogen to form a gas, carbon monoxide or dioxide, and methane. Non-stoichiometric composition of plagioclase in mare basalts may be due to vaporization in the high vacuum of the lunar surface

An electron microscopic study of gas condensates in the system Mg-Si-O-H

Condensates of MgSiO_3 and SiO_2 from a gas formed by evaporation of enstatite at an H_2 pressure of 4.4×10^ bar and a temperature of 1525℃ by B. O. MYSEN and I. KUSHIRO (Am. Mineral. (in press), 1988) and I. KUSHIRO and B. O. MYSEN (Advances in Physical Geochemistry, New York, Springer (in press), 1988) were investigated with an analytical transmission electron microscope (ATEM), a scanning electron microscope (SEM) and an electron probe microanalyzer (EPMA). With decreasing temperature at an approximately constant total pressure the Mg/(Mg+Si) atomic ratio of the condensate (mixture of MgSiO_3 and SiO_2 polymorphs) decreases first, then increases, and finally reaches a constant value. This compositional change of the condensate is inconsistent with the equilibrium condensation model. The TEM studies suggest that metastable condensation of coesite and probably of protoenstatite and cristobalite took place. Coesite probably condensed by heterogeneous nucleation on protoenstatite. Fibrous quartz was also formed by heterogeneous nucleation on molybdenum fibers which condensed from a molybdenum vapor by a partial evaporation of a Knudsen cell used in the experiment. Heterogeneous nucleation might have played an important role in condensation process in the solar nebula. The texture of the experimental clinopyroxene condensate is different from that in interplanetary dust particles (J. P. BRADLEY et al., Nature, 301,473,1983)

Petrology of chondrules, inclusions and isolated olivine grains in ALH-77307 (CO3) chondrite

A petrological study has been conducted on ALH-77307 (CO3) chondrite. Chondrules are classified into magnesian type and iron-rich type. The magnesian chondrules contain olivine and clinoenstatite with a small amount of groundmass, whereas the iron-rich chondrules contain only olivine as phenocrysts in the groundmass. Inclusions show gradual changes in crystallinity, grain size of constituents, texture and shape. The difference among the inclusions is considered to be due to the difference of heating temperature of the precursor finegrained materials. Intensive heating of the inclusions might have formed the magnesian chondrules. Isolated olivine grains are also classified into magnesian and iron-rich types, which are thought to have been derived from the magnesian and iron-rich chondrules, respectively. Genetical relations of chondrules, inclusions and isolated olivine grains are discussed by the process of condensation, successive heating, break up, and accreation

Investigations on the Experimentally Produced Chondrules: Chemical Compositions of Olivine and Glass and Formation of Radial Pyroxene Chondrules

Recently, textures of chondrules have been reproduced experimentally by cooling liquids of chondrule compositions, and thereby the cooling rate of chondrules during their crystallization has been estimated (TSUCHIYAMA et al. : Earth Planet. Sci. Lett., 48,155,1980). In the present study, the chemical compositions of olivine and glass of run products in the previous experiments were determined. Olivine crystals are normally zoned. The chemical compositions of glasses are extremely enriched in SiO_2 due presumably to disequilibrium crystallization of olivine by rapid cooling. Similar glasses have been observed in unequilibrated chondrites. Additional experiments were also performed to produce radial pyroxene texture which could not be produced in the previous experiments. The radial pyroxene was formed by reaction between radial olivine and residual SiO_2-rich liquid without changing its texture. This process might be a possible mechanism of formation of radial pyroxene texture of chondrules

Chemical compositions of Antarctic meteorites I

Sixty-four chondrites (14H, 25L, 17LL and 8C), 32 achondrites and 2 rare stony-iron meteorites from Antarctica were analyzed by standard wet chemical analysis method. In the present paper the analyses of the chondrites and the stony-iron meteorites are reported. Many Antarctic meteorites are more or less weathered or oxidized. Analyses of fresh and weathered parts of three chondrites indicate that abundances of most major elements are essentially the same between the two parts except for the proportions of Fe, FeO and Fe_2O_3. The analyses of the chondrites show that H chondrites and L and LL chondrites are distinct in several components (e. g., Na_2O is separated at 0.85wt%). When iron and FeS are subtracted, however, all the ordinary chondrites have essentially the same compositions, although H chondrites are slightly higher in Mg and lower in Si than L and LL chondrites. The compositions of the eight carbonaceous chondrites indicate that they comprise both C2 and C3 types in equal number. The analyzed carbonaceous chondrites are distinctly lower in SiO_2,Na_2O and K_2O and higher in MgO, Al_2O_3 and CaO than the ordinary chondrites. The stony-iron meteorites analyzed are similar to lodranite; however, one differs from lodranite in SiO_2,Al_2O_3 and CaO, and the other differs in SiO_2 and Fe/Mg ratio

Melting Experiments on a Yamato Chondrite

One-atmosphere melting experiments have been carried out on Yamato L6 chondrite 74354 at temperatures 1125°, 1200°and 1275℃ and at Po_2 (oxygen partial pressure) between (10)^ and (10)^ atm. The compositions of olivine, Ca-poor pyroxene, Fe-Ni metal and liquid have been determined as functions of temperature and Po_2; for example, the composition of olivine varies from Fo_ to Fo_ and that of metal varies from Fe_Ni_ to Fe_Ni_6 as Po_2 varies from (10)^ to (10)^ atm at 1125℃, and from Fo_ to Fo_ and from Fe_Ni_ to Fe_Ni_ respectively as temperature varies from 1125°to 1275℃ at (10)^ atm Po_2. Based on these experimental data, Po_2 for equilibration of this chondrite is estimated as about (10)^ atm. The compositions of liquids coexisting with olivine, Ca-poor pyroxene and Fe-Ni metal at Po_2 lower than (10)^ atm are within the compositional ranges of howardite and eucrite, suggesting that chondrites may have a genetical relationship with these basaltic achondrites