52 



and volcanism. Evidently, partial melting of primitive undifferen- 

 tiated material in asteroid al-sized bodies gave rise to the oldest 

 basalts in the solar system at about 4.5 b.y. ago. Elucidation of the 

 circumstances of early differentiation of some meteorite parent 

 bodies and the nature of the heat sources involved may have much to 

 tell us about the course of the Earth's differentiation to form the 

 mantle and core, and its subsequent thermal history. 



Undifferentiated meteorites have elemental abundances similar 

 to those found in the Sun. Among these meteorites, the carbona- 

 ceous chondrites are closest to the Sun in bulk elemental composi- 

 tion and are considered to be among the least fractionated and, 

 therefore, most primitive solid objects available for study in the solar 

 system (fig. IV-2). It is noteworthy, however, that relative to the 

 Sun, even the carbonaceous meteorites are depleted in hydrogen, 

 carbon, nitrogen, and noble gases. Observations indicating less 

 depletion of these elements in comets signify that comets are even 

 more primitive bodies than meteorites. 



Carbonaceous meteorites consist of complex assemblages of 

 relatively fine-grained mineral and organic matter that reflect a broad 

 range of elemental compositions and textures. This is indicative of 

 wide variations both in the environments of origin for the various 

 components and in the evolution of the respective parent bodies. 

 For present purposes, we consider the classification of carbonaceous 

 meteorites into three types: CI, CII, and CIII. Major differences 

 among these types lie in their content of volatile elements and miner- 

 als of high-temperature origin; these are inversely correlated. Accord- 

 ingly, the amount of organic matter increases in the order CIII, CII, 

 CI, with the CIII containing about 0.5% and the CI having about 5% 

 by weight. Similarly, minerals exhibiting a high-temperature history 

 occur most abundantly in CIII meteorites, along with metals (iron 

 and nickel). These minerals exist only in low to trace amounts in CII 

 meteorites and are virtually absent in the CI meteorites. 



Mixtures of clay-like minerals comprise the predominent miner- 

 als in CI and CII meteorites (50% to 80%) and a minor proportion in 

 some CIII meteorites. These minerals resemble terrestrial clays in 

 crystallographic structures, and the mixtures exhibit bulk elemental 

 compositions remarkably similar to the pattern of solar abundances. 

 Recent research suggests that like terrestrial clays, the clay-like 

 materials in carbonaceous meteorites were formed in an aqueous 



