Anders: Origin of Carbonaceous Chondrites 



515 



X-ray diffraction and optical studies of composite grains of olivine and Murray 

 F (a hydrated silicate), show that the olivine sometimes occurs in thin parallel 

 plates of the same crystallographic orientation, although the individual plates 

 are separated by a thin layer of exceedingly finely grained, randomly oriented, 

 Murray F mineral. The common orientation of the olivine plates can be 

 understood only if single crystal olivine served as the starting material (Du- 

 Fresne and Anders, 1962a). Still, one cannot exclude the possibility that some 

 fraction of the characteristic minerals is primordial, rather than being derived 

 from the olivine. 



Many of the other characteristic minerals, too, seem to be hydrated silicates. 

 This fact, and particularly the occurrence of MgS04 in distinct veins (figure 1) 



Table 1 

 Properties of Meteorite Parent Bodies 



Size 



Location 

 Number 

 Heat source 



Lovering (1957) 



Planetary 

 2-5 a.u. 

 One 



Long-lived 

 radioactivity 



Urey (1959) 



Lunar 



1 a.u. 



One 



Chemical reac- 

 tions; adiabatic 

 compression 

 of gases 



Fish et al. (1960); 

 Wood (1958, 1962) 



Asteroidal 

 2-5 a.u. 

 Several 

 Extinct radio- 

 activity 



Ringwood (1961) 



Lunar 

 2-5 a.u. 

 Several 

 Radioactivity 



Table 2 

 Origin of Carbonaceous Chondrites 



\. High-iron group chondrites altered by infiltration of water, carbonaceous matter, and 

 hydrogen sulfide from some other source (Urey, 1961). 



2. Primitive material accreted at low temperatures from solar nebula (Mason, 1960, 1961; 



Ringwood, 1961). Other chondrites were derived from this material by heating and 

 reduction. 



3. Primitive material expelled from the sun at high temperatures (Wood, 1958), accreted at 



low temperatures into asteroidal-sized bodies (Wood, 1958, 1962; Fish et al., 1960), 

 altered by liquid water and sulfur compounds (DuFresne & Anders, 1962a). 



suggests that licjuid water must once have acted on these meteorites. This 

 raises three interesting questions. First, what were the chemical and phys- 

 ical conditions (pH, reduction potential, and temperature) during this aqueous 

 stage, and how long did it last? Second, what was the source material of the 

 carbonaceous chondrites, i.e., where did the high temperature minerals come 

 from? And third, in what setting did this aqueous stage occur? 



Former environment. To answer the first question, one can turn to the sta- 

 bility diagrams of Garrels (1960), which give the stabihty regions for various 

 minerals and ions as a function of pH and reduction potential (Eh) . In figure 

 2 is shown a composite diagram based upon Garrels' data. Looking up the 

 stability regions of the principal constituents of carbonaceous chondrites on 

 this diagram, one finds that nearly all of them [Fe304 , (Mg,Fe)C03 , MgS04 , 

 S, organic matter] can coexist under equilibrium conditions at pH 8 to 10 and 



