526 



Annals New York Academy of Sciences 



history (e.g., during close approaches to the Sun), or if its parent body happened 

 to remain at a temperature somewhat above, say, 200° K., where sHght, but 

 continuous argon losses by diffusion would occur. 



That the short exposure ages are not due to diffusion losses at perihelion has 

 been shown conclusively at least for Cold Bokkeveld (Anders, 1962c). Here, 

 the content of a nonvolatile cosmogenic nuclide, Al^*^, is consistent with the Ne'-^ 



Table 5 

 Ages of Carbonaceous Chondrites 



*Zahringer (1962). 



tStauffer (1961). 



J Gerling and Rik (1955). 



Table 6 

 Carbon Isotopic Composition in Carbonaceous Chondrites (Boato, 1954) 



content, so that diffusion losses of the latter seem to be ruled out. The short 

 exposure age (0.1 to 0.2 m.y.) would seem to suggest a lunar origin, as proposed 

 by Urey (1962), but this hypothesis has its difficulties (Anders, 1962f). 



Other isotope measurements exist that have a bearing on the origin of car- 

 bonaceous chondrites. Boato (1954) has measured the carbon isotopic compo- 

 sition in these meteorites (table 6). The C^'/O^ ratio is variable from meteor- 

 ite to meteorite, and even within the same meteorite (Cold Bokkeveld). It 

 is known that living organisms have a preference for C^', so that biogenic ma- 



