Chemical and isotopic fractionation by grain size separates

Abstract

Fractionation of refractory elements according to grain size is argued to occur during their growth. Two major modes should exist: (1) during thermal condensation sequences whenever the condensing phase (e.g. Mg2SiO4) does not alloy with the precondensed phase (e.g. MgAl2O4); (2) during accretion of gaseous atoms in the nonequilibrated interstellar medium. Processes dynamically sorting grains according to size (e.g. sedimentation) therefore are potentially capable of achieving fractionations normally attributed to separations of dust and gas. Elemental fractionation patterns according to volatility may be established in this way. I consider the first mode during supernova condensation; however, it also can occur in an equilibrium solar condensation sequence owing to an overlooked freedom in that simplified description. The second mode has also not been considered owing to the common chemical assumption that the dust in the early solar system was evaporated before recondensing. © 1980.