Possible Correlation between Classifications and Potassium–Argon Ages of Chondrites

Abstract

ACCORDING to Kirsten et al.1 and Hintenborger2, the frequency distribution curves of the potassium–argon ages of stone meteorites show two distinct maxima. One occurs at ages close to 4.5 × 109 years, the other occurs around 1 × 109 years. However, if one classifies the chondrites into the Urey–Craig3 L- and H-groups a relationship between classifications and gas retention ages is apparent (Fig. 1). The classification in L- and H-groups (that is, hypersthene-olivine and bronzite-olivine chondrites respectively) is based on the following chemicalmineralogical parameters: bulk iron3, specific gravity4, metallic nickel–iron4,5, and Fe/Fe + Mg ratios in olivine and rhombic pyroxene6,7. Only well-classified chondrites have been included. Carbonaceous chondrites and enstatite chondrites have been excluded since they constitute separate groups7. Fig. 1 shows the frequency distribution of chondrites with well-known classification and well-determined age. The age values were taken from Anders8 and Kirsten et al.1. The diagram shows that the substantial degassing, which leads to an apparent potassium–argon age of about 1 × 109 years, is, on the basis of the chondrites investigated so far, considerably more frequent for the L-group of chondrites than for the H-group. The uranium–helium ages are also plotted in the diagram, showing a similar effect. New potassium–argon age measurements9 seem to yield some more H-group chondrites with low potassium–argon ages. However, recent uranium–helium measurements10 again seem to support an age-classification relationship as indicated in Fig. 1. The purpose of this communication is to direct attention to the apparently more frequent, substantial degassing of L-group chondrites in comparison to H-group chondrites, and to emphasize the necessity for more potassium–argon gas retention age measurements of chondrites, for which the classification into L- and H-groups is well known.