Chemical fractionations in meteorites - V. Volatile and siderophile elements in achondrites and ocean ridge basalts

Eighteen achondrites and 4 terrestrial basalts (3 ocean ridge, 1 continental) were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Co, Cs, Cu, Ga, In, Ir, Rb, Se, Tl and Zn. Samples included 7 eucrites, 5 howardites, 2 nakhlites, 2 shergottites, an angrite, and an aubrite. Light and dark portions of the gas-rich meteorites Kapoeta and Pesyanoe were analyzed separately. Nakhlites and shergottites have volatile element abundances similar to those in ocean ridge basalts; eucrites, howardites and angrites show greater depletions by an order of magnitude and less similar abundance patterns. In terms of a two-component model of planetary accretion, the parent planets contained the following percentages of low-temperature material: eucrites 0.8, nakhlites 38, shergottites 28. Nominal accretion temperatures ( ^oK) inferred from Tl contents were: eucrites 432, nakhlites 438, shergottites 433, all for an assumed nebular pressure of 10 ^-5 atm. These data appear to be consistent with the oxygen isotope composition of these meteorites. Shergottites may be genetically related to L-chondrites. The siderophile element pattern of achondrites resembles that of the Moon, but with less extreme depletions. Terrestrial basalts, on the other hand, show a different pattern, with a steep decline in the order Ag > Au > Ir. The dark portion of Kapoeta seems to contain 1-2 % chondritic material, compositionally similar to Cl chondrites. No such enrichment was found for our samples of Pesyanoe.