Rubidium-Strontium studies on black hypersthene chondrites: effects of shock and reheating

Abstract

Whole-rock Rb-Sr data have been measured for 15 hypersthene chondrites including 10 black chondrites. The latter appear to have been the most heavily shocked and reheated. Several density fractions for two of the black chondrites, Orvinio and Farmington, were separated, and Rb-Sr analyses were performed. Whole-rock data for observed black hypersthene falls conform (within experimental error) to the 4.5-4.6 × 10⁹-year isochron defined by other chondrites. Although black ‘finds’ plot uniformly to the left of the isochron, this behavior is also shown by unshocked finds. Therefore the total-rock data do not reveal any significant shock or reheating effects. The density separates, particularly those of Orvinio, depart significantly from the 4.5-b.y. isochron. Although their ⁸⁷Rb/⁸⁶Sr ratios vary from 0.5 to 1.1, their ⁸⁷Sr/⁸⁶Sr ratios are the same as that of the total meteorite to within 0.002. This similarity results in a nearly horizontal distribution of points on a Sr evolution diagram. To a lesser extent, data from Farmington also show the same effect. It is concluded that this behavior is due to partial homogenization of the Sr isotopes within these two meteorites very recently in their history. The scatter in the data precludes any precise determination of the time of the event.