Chemical fractionations in meteorites—I. Condensation of the elements

Abstract

The condensation history of a cooling gas of cosmic composition is outlined. Two condensation sequences are derived: one for fast cooling, with the formation of pure elements and compounds and another one for slow cooling, with the formation of solid solutions. These sequences bear a striking resemblance to the depletion patterns observed in ordinary chondrites. The elements Pb, Bi, In and T1, which are most strongly depleted are among the last ones to condense, with condensation temperatures <500 °K at a total pressure of 6·6 × 10 −3 atm. A somewhat less strongly depleted group (Zn, Cd, Ag, and probably Te and Se) condense near 680 °K, also the temperature at which FeS forms according to the reaction Fe + H 2S = FeS + H 2. Finally, elements showing only moderate depletion (Sn, Ga, Ge, Au, and Cu) condense between 700 °K and 1000 °K. Manganese and the lighter alkalis (Na, K, and Rb), elements depleted in carbonaceous but not in ordinary chondrites, lie still higher in the sequence, between 1100 ° and 1300 °K. It is proposed that the fractionation patterns observed in chondrites arose in the solar nebula during condensation and accretion. Subsequent heating in a meteorite parent body does not appear to have effected any further fractionations, with the possible exception of the most volatile trace elements, the noble gases and Hg.