The rare earth elements (La to Nd and Sm to Lu) are trace elements found at sub-ppm to ppm levels in chondritic meteorites. Although their abundances (normalized to mean chondrites) are generally smooth at the +/- 20% level, high precision analyses reveal numerous anomalies. Some anomalies (Eu) are consistent with planetary processes while others (Ce and Yb) are probably signatures of condensation processes in the solar nebula. Phosphate minerals in equilibrated ordinary chondrites, Ca, Al-rich inclusions (CAIs), predominantly found in the oxidized carbonaceous chondrites, and oldhamite (CaS), found in the reduced enstatite chondrites, are highly enriched in the REE relative to their concentration in the bulk meteorites. The REE enrichments in phosphates are consistent with redistribution during metamorphism. The REE abundance patterns observed in CAIs fall into two broad categories: (1) generally smooth, unfractionated patterns which may have Eu and Yb anomalies, and (2) highly irregular fractionated patterns which have a complex structure. The unfractionated patterns can result either from vaporization or condensation processes in the solar nebula. However, the highly irregular (Group II) patterns must result from a fractional condensation process because both the most refractory and the most volatile REE are depleted in them. The limited data available on REE abundance patterns in CaS show only relatively unfractionated patterns and no analogs to the highly irregular Group II patterns frequently seen in CAIs. We review relevant analytical data for REE in CAIs and oldhamite and then discuss several implications of the observed REE patterns for chemical and physical conditions during the formation of these meteorite components. In particular, we emphasize the apparent discrepancy between the high temperatures required for formation of the Group II REE patterns and the isotopic anomalies in Ca and Ti in these CAIs. The REE, Ca, and Ti have similar volatilities and Ca and Ti would be vaporized and isotopically homogenized at the temperatures needed to explain the Group II patterns. This problem is unresolved and is an important question in cosmochemistry.
