Abstract
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.
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