Chemical fractionations in meteorites—II. Abundance patterns and their interpretation

Abstract

Abundances of thirty-one volatile elements in meteorites are reviewed. In carbonaceous chondrites of Types I, II, and III and enstatite chondrites of Type I, abundances decrease by constant factors, in ratios of 1/0·6/0·3/0·7. In ordinary chondrites and Type II enstatite chondrites, nine elements (Au, Cu, F, Ga, Ge, S, Sb, Se and Sn) likewise are depleted by constant factors of ~0·25 and ~0·5, while eighteen others (Ag, Bi, Br, C, Cd, Cl, Cs, H, Hg, I, In, Kr, N, Pb, Te, Tl, Xe and Zn) show more drastic depletions, to factors of 0·002. Apparently chondrites are a mixture of two types of material: a low-temperature fraction (= matrix) that retained most of its volatiles and a high-temperature fraction (= chondrules, metal grains) that lost them. Evidence is presented that these fractionations occurred in the solar nebula as it cooled from high temperatures. They cannot have been produced in the meteorite parent bodies. The following accretion temperatures are inferred from the abundance patterns: carbonaceous chondrites, ≤400 °K; Type I enstatite chondrites, 400–480 °K; ordinary and Type II enstatite chondrites, 530–650 °K, unequilibrated ordinary chondrites, ≤530 °K. These temperatures are much higher than the present black-body temperature in the asteroid belt, 170 °K. However, they could have been reached during collapse of the solar nebula or the high-luminosity, pre-main-sequence stage of the protosun. This model also accounts for the presence of organic compounds and primordial noble gases in meteorites; the extreme depletion of Pb, Bi, Tl, and In in ordinary chondrites; and the low alkali content of the earth and carbonaceous chondrites. The following tentative distance assignments are given: enstatite chondrites come from the inner fringe of the asteroid belt; ordinary chondrites, from the center and inner half; carbonaceous chondrites, form the outer fringe or from comets.