Physics
Scientific paper
Feb 1964
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1964gecoa..28..189m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 28, Issue 2, pp.189-207
Physics
12
Scientific paper
The composition of the equilibrium "atmosphere of crystallization" of chondritic meteorites has been calculated using thermochemical data. The following reactions form the basis for the calculations: olivine pyroxene olivine pyroxene (a) pyroxene metal gas olivine olivine 2 MgSiO 3 + 2 Fe + O 2 Fe 2 SiO 4 + Mg 2 SiO 4 (b) The exchange reaction (a) has been evaluated in terms of existing experimental and observational data and theoretical expressions for the equilibrium constants of both reactions have been derived. According to these expressions the atomic fraction Fe 2+ /(Fe 2+ + Mg) of the silicates increases with increasing oxygen fugacity at a constant activity of iron in the metallic phase. Similarly, the atomic fraction Fe/(Fe + Ni) of the metallic phase decreases with increasing fugacity of oxygen at constant silicate composition. Equilibrium log P 0 2 - T curves are presented for various silicate compositions. If free carbon and hydrogen are present reactions (a) and (b) also yield values of P co , P CO 2 and such ratios as P H 2 O / P H 2 , P CH 4 / P H 2 2 . An analogous evaluation for the carbon species should be possible if free carbon was absent during crystallization but if the amount dissolved in the metal is known. The analysis indicates that the dominant carbon species in equilibrium with bodies of chondritic composition would be CO rather than CO 2 and that P H 2 O / P h 2 < 1 for the greater part of the stability range of the assemblages. The physical environment of the crystallization of chondrites is discussed in terms of the limits imposed by the calculations. The significance of certain crystallization textures and the stability relations of hydrocarbons are also examined.
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