Models for the fractionation of moderately volatile elements in the solar nebula

We test the validity of the idea that the abundances of moderately volatile elements in chondritic meteorites reflect global condensation and coagulation in an evolving solar nebula and explore what constraints these abundances might place on nebular parameters. The abundances of moderately volatile elements were identified as particularly suitable for modeling the nebula because the data represent a simple pattern explicable in terms of a straightforward hypothesis which has implications for global evolution. The models incorporate a correspondingly simple prescription, derived from theory, for the rate at which condensible material is decoupled from the evolving nebular gas to become part of the surviving planetary system. It is concluded from model results that cooling, diminishing nebular mass and coagulation of solids, through their mutual dependences on accretion rate and opacity, lead naturally to the chondritic depletion patterns of moderately volatile elements. In particular, the trends found in CO and CV meteorites are readily and accurately produced; those of the CM meteorites are less so. The CM meteorites and ordinary chondrite patterns may require a more complicated model, but the full range of possible results from even the simplest model has yet to be completely determined.