Lanthanides in the Norton County achondrite

Abstract

Lanthanide concentrations in 13 fractions of the Norton County achondrite have been determined by a mass spectrometric isotope dilution method. A most remarkable fact is that the distribution of lanthanides is quite different between enstatite single crystal and polycrystalline material. This difference is interpreted in terms of different formation processes. It is thought that in the growth of single crystals, individual ions were placed one by one to build a strictly defined lattice structure, whereas the polycrystalline material was produced by the transitional formation of amorphous phases or phases of low crystallinity resulting from spontaneous cohesion of neighboring ions in the melt. The lanthanide patterns for single crystal fractions are grossly similar to each other, but there is some variation between them in fine structure of the patterns. This can be interpreted as a reflection of the fact that the partition of lanthanides into single enstatite crystals is more or less sensitive to rather subtle differences in the conditions of crystal growth. The genesis of Shalka, Johnstown (both hypersthene achondrites) and eucritic achondrites can be interpreted in terms of the precipitation of single crystals only. A great difference in partition coefficient between single crystals and polycrystalline material system appeares to be of much petrological significance, because it is thought that the difference can also bring about a great difference in efficiency of enrichment of calcium in remnant liquid. Irregular behavior of Cc, Eu, and Yb in a few single-crystal fractions was ascertained by independently processed redeterminations. The irregularities may be ascribed to single crystals favoring higher valence state of these elements relative to the melt. It is suggested that lanthanum might have been a little depleted relative to the other lanthanides in the initial melt that produced Norton County.