Primitive achondrites ureilites

Figure 11 Oxygen isotopic compositions of whole-rock primitive achondrites. Data for a given class are much more scattered than data for differentiated achondrites shown in Figure 10, as a result of incomplete melting and homogenization. Several genetic associations are implied by the data (i) aubrites and enstatite chondrites (ii) acapulcoites and lodranites (iii) lAB irons and winonaites and (iv) ureilites and dark inclusions in carbonaceous chondrites. This and Figure 10 are drawn to the same scale for comparison (source Clayton and Mayeda, 1996).

The ureilites are the most remarkable of the primitive achondrites. They are coarse-grained, carbon-bearing ultramafic rocks, and are residues of extensive partial melting that has removed a basaltic component. Their oxygen isotopic compositions are highly variable, and extend along an extrapolation of the CCAM line. The isotopic variability is probably inherited from heterogeneous carbonaceous chondrite precursors (Clayton and Mayeda, 1988). 

The ureilite group is the second largest achondrite group, and exhibits some characteristics of both primitive and differentiated achondrites. However, the dominant silicates of ureilites seem to require a high-temperature, igneous origin, and no tme primitive materials related to the ureilites are known (e.g., Goodrich, 1992). 

The trapped noble gases in ureilites are part of the properties that make this class of achondrites enigmatic, as they often have been called because they combine properties of primitive and not-so-primitive meteorites. Among the primitive ones is the high abundance of noble gases that approach or in some cases even surpass those found in the most primitive chondrites (Gobel et al. 1978). This, as well as the similarity in isotopic composition and elemental abundance pattern to the Q component has repeatedly been cited as evidence for a close relationship between those two components (Ott et al. 1984, 1985b Busemann et al. 2000). 

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