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

Abstract

Abstract-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as condensation proceeds, a specified fraction (called the isolation degree, 6) of the existing condensate is steadily withdrawn from reactive contact with the residual gas, presumably as a result of the growth and aggregation of condensed mineral grains.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 6 values <0.2%, the condensation sequence is essentially independent of the isolation degree and identical to the classic condensation sequence. At 6 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 5 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 6 > 2.5; this is an upper limit on the 5' 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 6 values of 10.1% 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.

Silica‐rich igneous rims around magnesian chondrules in CR carbonaceous chondrites: Evidence for condensation origin from fractionated nebular gas

“…The required fractionation could have resulted from condensation with partial isolation (Petaev and Wood 1998) or from partial evaporation during chondrule formation.…”

“…16) is difficult to explain by equilibrium condensation from a gas of solar composition. Equilibrium condensation calculations for a gas of solar composition, which is characterized by an atomic Si/Mg ratio of ~1 (Anders and Grevesse 1989), do not predict condensation of silica at any values of dust/gas ratio or total pressure (e.g., Grossman 1972; Petaev and Wood 1998;Ebel and Grossman 2000). Instead, the last silicate condensate is enstatite that forms either by reaction between SiO (gas) and forsterite or by reaction between SiO (gas) and forsterite + melt, depending on the total pressure and dust/gas ratio (Petaev and Wood 1998;Ebel and Grossman 2000).…”

A multi‐step model for the origin of E3 (enstatite) chondrites

“…Larimer and Wasson (1988) concluded that the enstatite chondrites (EH and EL combined) had lost -58% of their solar complement of Al and Ca, 51% of their solar Mg, and 33% of their solar Si in a "refractory element" fractionation. Petaev and Wood (1998) reported that partial isolation of a refractory component over a range of temperatures produced minerals such as silica polymorph, oldhamite, Na-sulfides, metal, troilite, and enstatite, most of which are found in enstatite chondrites. Modal abundances were not given, but Petaev and Wood (1998) stated that "the matches between calculated and real mineralogies.. .…”

“…Petaev and Wood (1998) reported that partial isolation of a refractory component over a range of temperatures produced minerals such as silica polymorph, oldhamite, Na-sulfides, metal, troilite, and enstatite, most of which are found in enstatite chondrites. Modal abundances were not given, but Petaev and Wood (1998) stated that "the matches between calculated and real mineralogies.. . are not perfect.…”

“…Several models to account for the origin of the enstatite chondrites have been proposed, including: ( I ) condensation with slow nucleation kinetics (Blander, 1971); (2) condensation at high pressures (>I bar) (Herndon and Suess, 1976;Sears, 1980); (3) formation in a region of the nebula with low H20/H2 ratio (Baedecker and Wasson, 1975) or high C/O ratio (Larimer and Bartholomay, 1979;Lodders andFegley, 1993, 1997); (4) increasing the amount of a "tar" component in the nebula (Wood and Hashimoto, 1993); and (5) condensation with partial isolation of solids (Petaev and Wood, 1998). None of these models have successfully replicated the observed modal mineralogy and bulk compositions of type 3 enstatite chondrites.…”