A UNIQUE ULTRAREFRACTORY INCLUSION FROM THE MURCHISON METEORITE

Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered a refractory inclusion, HIB-11, that is unique in terms of its texture, mineral compositions, and bulk composition. It consists o f anhedral, Y-rich (1.6 wt% Y2O3) perovskite and lathlike spinel grain s enclosed in a matrix of fine-grained, Sc-rich (10.5 wt% Sc2O3 avg.), Ti-rich (12.6 wt% TiOZ avg., reporting all Ti as TiO2) clinopyroxene. The chondrite-normalized rare earth element (REE) pattern is complex, with light REE (LREE) at similar to 10x Cl, abundances increasing fro m Gd through Ho (the latter at similar to 10(4)x Cl), decreasing throu gh Yb at 200x Cl, and Lu at similar to 400x Cl. The pattern reflects s everal stages of high-temperature volatility fractionation. Removal of Lu and Er from the source gas in the first condensation event was fol lowed by partial to complete removal of the somewhat less refractory h eavy REE, Gd through Ho, in the HIB-11 precursors by condensation from the fractionated residual gas in a second event. Both of these events probably reflect condensation of REE into ZrO2 or a mixed Zr-, Sc-, T i-, Y-oxide at temperatures too high for hibonite stability. A second, lower-temperature component, which was subsequently added, had fracti onated (Nd-poor, Ce-rich) LREE abundances that resulted from condensat ion from a gas that had undergone prior removal of the more refractory LREE, resulting in enrichment in Ce and the most volatile REE, Eu and Yb. The aggregate was then melted and quickly cooled, forming a fine- grained spherule. This is the first reported inclusion in which the tw o most refractory REE, Lu and Er, are strongly fractionated from the o ther REE. An absence of mass fractionation among the Ti isotopes indic ates that HIB-11 is not an evaporative residue, implying that volatili ty fractionation of trace elements took place during condensation. The fact that the two most refractory heavy REE could be separated from t he other, only slightly less refractory heavy REE suggests that a wide variety of REE patterns is possible, and that ultrarefractory inclusi ons with other unusual REE patterns, important recorders of nebular co ndensation, may yet be discovered.