Volatile elements in chondrites: metamorphism or nebular fractionation

Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 13000C for > 106 yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 10000C for 1 week. Karoonda, metamorphosed at 6000C for many millennia, is less depleted in Bi and Tl than Allende heated at 6000C for 1 week. Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation, and changed little if at all during metamorphism. For enstatite chondrites, the evidence is still incomplete, but seems to favor a nebular origin of the volatile pattern. The general constancy of Tl/Rb, Tl/Cs and Tl/U ratios in terrestrial and lunar rocks suggests that loss of volatile metals such as Tl is rare during normal magmatism or metamorphism. Only impact melts show such loss with any frequency. (author)