2 - Carbonaceous chondrites

Summary

Introduction

The class of meteorites known as carbonaceous chondrites was originally an assemblage of three groups that were associated on the basis of relatively high abundances of carbon and water [2.1]. Now, however, the class comprises eight individual groups, most of which are not noticeably enriched in either carbon or water [2.2]. They are considered collectively as a class on the basis of their mostly unfractionated bulk chemical composition relative to that of the Sun, and the individual groups are now thought not to be genetically related or to come from the same parent object. CI chondrites are the most primitive of the eight groups, and have a composition identical to that of the solar photosphere for all but the most volatile of elements (Figure 2.1).

Each carbonaceous chondrite group is assumed to originate from a separate parent body; associations of groups (clans, or supergroups) are thought to be of parent bodies that formed in similar regions of the solar nebula at similar times [2.2]. Figure 2.2 is a family tree for carbonaceous chondrites, showing where individual groups might cluster together in clans [2.2]. Association into clans tends to be based on commonalities in chemistry of the clan members, rather than mineralogy or texture. The numbers and names of specimens in each group are recorded and regularly updated at the Meteoritical Bulletin website [2.5].

Carbonaceous chondrites are composed of several distinct components [2.6–2.8]: fine-grained darkmatrix, chondrules, high-temperature inclusions and opaque minerals. There is a variation in chemistry and relative abundance of these components among the different carbonaceous chondrite groups.

Petrologic types and subtypes are applied to carbonaceous chondrites (Figure 2.2 and Table 2.1). CI, CM and CR chondrites include only petrologic types 1 and 2; their finegrained matrices are phyllosilicate-rich, indicating that the meteorites have been altered by aqueous fluids. McSween [2.7] suggested that the trend from type 2 to type 1 was one of increasing aqueous alteration. The other carbonaceous chondrite groups show little evidence for extensive aqueous alteration, and the major matrix minerals are olivines and sulphides [2.6]. It is mostly the CK chondrites which have specimens with petrologic type 4 and above.

There have been several thorough reviews of the mineralogy and petrology of carbonaceous chondrites [e.g., 2.6, 2.9–2.11] from which much of the information in this chapter is drawn.