This study reports the bulk rare earth element (REEs, La-Lu) compositions of
41 chondrites, including 32 falls and 9 finds from carbonaceous (CI, CM, CO and
CV), enstatite (EH and EL) and ordinary (H, L and LL) groups, as well as 2
enstatite achondrites (aubrite). The CI-chondrite-normalized REE patterns and
Eu anomalies in ordinary and enstatite chondrites show more scatter in more
metamorphosed than in unequilibrated chondrites. This is due to parent-body
redistribution of the REEs in various carrier phases during metamorphism. The
dispersion in REE patterns of equilibrated ordinary chondrites is explained by
the nugget effect associated with concentration of REEs in minor phosphate
grains.
Terrestrial rocks and samples from ordinary and enstatite chondrites display
negative Tm anomalies of ~-4.5 % relative to ca chondrites. In contrast, CM, CO
and CV (except Allende) show no significant Tm anomalies. Allende CV chondrite
shows large excess Tm (~+10 %). These anomalies are similar to those found in
group II refractory inclusions in meteorites but of much smaller magnitude. The
presence of Tm anomalies in meteorites and terrestrial rocks suggests that
either (i) the material in the inner part of the solar system was formed from a
gas reservoir that had been depleted in refractory dust and carried positive Tm
anomalies or (ii) CI chondrites are enriched in refractory dust and are not
representative of solar composition for refractory elements. The observed Tm
anomalies in ordinary and enstatite chondrites and terrestrial rocks, relative
to carbonaceous chondrites, indicate that material akin to carbonaceous
chondrites must have represented a small fraction of the constituents of the
Earth.Comment: Geochimica et Cosmochimica Acta, in press, 58 pages, 6 tables, 14
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