Light-dark structure and rare gas content of the carbonaceous chondrite Nogoya

Abstract

In the present study it was found that the type II carbonaceous chondrite Nogoya had a light-dark structure (Figure 1). Separate 50-mg samples of both portions were analyzed for their rare gas contents, using procedures described by Heymann and Mazor [1966]. The dark portion was found to be an order of magnitude richer in rare gases than the light portion (Table 1). The noble gases in both portions of Nogoya consist of three components, cosmogenic (c), radiogenic (r), and primordial (p). Since the light portion contains the lower primordial gas contents, the cosmogenic and radiogenic components in it are less masked. Nevertheless, only Nec21 and Arr40 can be estimated unambiguously, for the following reasons. If all the He4 were radiogenic, the U, Th-He4 age of Nogoya light would be considerably above the 4.5 by age limit of ordinary chondrites (assuming that Nogoya has about the same U and Th contents as ordinary chondrites). Hence, one suspects that part of the He4 and, consequently, of the He3 is primordial. This conclusion is supported by the following consideration. With a Hec3 production rate of 2.0 × 10−8 cc STP/g/m.y., the maximum radiation age of Nogoya is 0.55 m.y. Assuming (Ne20/Ne21)p, is 338 (the value of atmospheric Ne as determined by Eberhardt et al. [1965]), Nec21 amounts to 0.066 × 10−8 cc STP/g. This value corresponds to a radiation age of only 0.25 m.y., using Anders' [1964] Nec21 production rate of 0.266 × 10−8 cc STP/g/m.y. Hence, roughly one-half of He3 in the light portion appears to be primordial. With (He4/ He3)p ∼ 3000, we calculate that approximately one-third of the He4 in the light fraction is primordial. Whether this is the genuine Hep content of the pure light material, or has arisen from accidental contamination with gas-rich dark material, cannot be concluded with certainty. Judging from the occurrence of dark inclusions in the light portion (Figure 1) accidental contamination is the more likely source.