The condensation with partial isolation (CWPI) model of condensation in the solar nebula

We have developed a nebular condensation model and a computational routine that potentially can account for the unequilibrated mineral assemblages in chondritic meteorites. The model assumes that as condensation proceeds, a specified fraction (called the isolation degree, xi) of the existing condensate is steadily withdrawn from reactive contact with the residual gas, presumably as a result of the growth and aggregation of condensed mineral grains. The isolated condensates may remain in the condensing system as coarse inert objects; whereas, the mineral grains that are still in reactive contact with residual nebular gases are in the form of fine dust. This paper describes the condensation with partial isolation (CWPI) model of condensation and uses it to study condensation in a nebula of solar composition at a total pressure of 10(-5) bar. The systematic isolation of condensates from residual nebular gases has profound effects on the condensation sequence. At xi values <0.2%, the condensation sequence is essentially independent of the isolation degree and identical to the classic condensation sequence. At xi values >2.5%, the condensation sequence is also independent of the isolation degree and closely resembles the "inhomogeneous accretion model" or "chemical disequilibrium model" of condensation. In the intermediate range of xi values, the character of the condensation sequence is very sensitive to the degree of chemical fractionation caused by condensate isolation. The mineralogy of chondritic meteorites is not consistent with condensation sequences having xi > 2.5; this is an upper limit on the xi values that is characteristic of condensation in the solar nebula. The mineralogy and chemistry of carbonaceous and enstatite chondrites can be explained by accretion of isolated condensates formed at xi values of less than or equal to 0.1% and 0.7-1.5%, respectively, providing that segregation of the inert coarse objects and fine reactive dust occurred in the nebula. Segregation of these two categories of condensate may have been responsible for the observed volatility-based chemical fractionations among chondritic meteorites.