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–iron^4,5, and Fe/Fe + Mg ratios in olivine and rhombic pyroxene^6,7. 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.