Gas Retention Ages Research Articles

Cosmogenic and radiogenic helium, neon, and argon in amphoteric chondrites

Cosmogenic and radiogenic rare gases were determined by mass spectrometry in 16 amphoterites. The radiation, U, Th-He, and K-Ar ages were calculated. Radiation ages of amphoterites vary from 0.9 to 58 m.y. They resemble the age distribution of the hypersthene chondrites in that few amphoterites have ages in the interval 2–6 m.y. The amphoterites Appley Bridge, Hamlet, and Soko-Banja have the same fall date, October 13, but very different radiation ages—1, 31, and 58 m.y.—indicating that they did not belong to a periodic meteorite shower or stream. The same seems to be true also of Olivenza and Vavilovka, both of which fell on June 19. The Ar40 contents of 4 amphoterites differ significantly from previous determinations by other authors, a finding that supports earlier conclusions that the K abundance in some amphoterites seems to be nonuniform. Although 2 of the amphoterites had U, Th-He ages as short as 1 b.y., most others are indeed quite old. Seventy-five per cent have ages greater than 3.0 b.y., whereas only 20% of the hypersthene chondrites have ages above this limit. It is concluded that differences in chemistry and mineralogy cannot account for the systematic differences in gas-retention ages of bronzite, hypersthene, and amphoteric chondrites. Most, if not all, amphoterites seem to have escaped the reheating episode that hypersthene chondrites underwent late in their history.

Possible Correlation between Classifications and Potassium–Argon Ages of Chondrites

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.

γ Radioactivity in the Fayetteville meteorite

The Fayetteville meteorite has been shown to have an unusual rare gas content [Manuel and Kuroda, 1964], and it was thought to be of some value to measure the γ-ray activity of this meteorite. The resulting data, combined with the rare gas values reported by Manuel and Kuroda, can be used to calculate the K40-Ar40 gas retention age and the Ne21-Al26 cosmic-ray exposure age of the meteorite to establish whether this part of the past history of the Fayetteville meteorite was also unusual. The specimen of the Fayetteville meteorite used in this study was the principal mass (1.788 kg) of the recovered fall, which is under the custody of Professor D. P. Richardson, Chairman of the Department of Mathematics, University of Arkansas. The circumstances of the fall and recovery of the meteorite were reported by him [Richardson, 1935].