Anders: Origin of Carbonaceous Chondrites 



517 



glass into the meteorite the temperature of Mighei could never have exceeded 

 206° C. for as long as 48 hours. Other time-temperature combinations can be 

 read off the graph, although it is doubtful whether any extrapolation beyond 

 the measured points is valid. One can infer that temperatures were much lower 

 from the fact that the characteristic minerals are quite finely grained, judging 

 from the diffuseness of their x-ray diffraction patterns. It seems likely that 

 the aqueous stage occurred at approximately room temperature. There is 

 hope of obtaining a more accurate value by measuring the O'YO^'' fractiona- 

 tion between carbonate and magnetite (Clayton, 1962). Presumably the 



Figure 1. A fragment of Orgueil, showing white vein of magnesium sulfate running hori- 

 zontally across specimen. This vein must have deposited from water solution, thus offering 

 evidence of the onetime presence of liquid water in the meteorite parent body. (Reproduced 

 from DuFresne and Anders, 1962a, with permission of the editor.) 



carbonate was made during the aqueous stage, by the action of CO2 on basic 

 oxides. The CO2 was, in turn, probably evolved from the interior of the body 

 during reduction of iron oxides to metallic iron. If the carbonate and mag- 

 netite reached isotopic ecjuiUbrium during the aqueous stage, the temperature 

 of this stage may be determined by means of Urey's paleotemperature method. 



A clue to the duration of the aqueous stage is given by the relatively high 

 degree of ordering of the Ca++ and Mg++ ions in the dolomite from Orgueil 

 and Ivuna. From a comparison with terrestrial dolomites. Goldsmith has 

 estimated a formation time of > 10^ years. 



Ancestral material of carbonaceous chondrites. It is a little harder to get an 

 answer to the second question, concerning the origin of the high temperature 

 minerals. Edwards and Urey (1955) and Urey (1961) have pointed out that 



