Rare Earth Element Abundances in Refractory Inclusions from Y-81020 (co3.0) Chondrite: Evidence of Ree Fractionation under Variable Conditions

Introduction: In the course of the systematic study on refractory inclusions in the Ningqiang meteorite [1], ion microprobe analyses of rare earth elements (REEs) have been performed and correlated positive anomalies in Ce (+Eu) +Yb have been found in some inclusions [2, 3]. Two different patterns are recognized: one is HREE-depleted patterns with positive anomalies in Ce +Eu +Yb (named as modified Group II) and the other is almost flat (unfractionated) patterns with positive anomalies in Ce +Eu +Yb (named as modified Group I). It was suggested that condensation from a fractionated gas, from which ultra-refractory HREE and some amount of LREE had been condensed and removed, would produce modified Group II patterns [4]. This REE fractionation condition is similar to that for the Group II pattern [5] but the temperature is slightly lower where LREE are also partly condensed. It was also suggested that incomplete separation of such dust from the gas would produce modified Group I patterns [4]. In order to see if such correlated positive anomalies in Ce +Eu +Yb are common among refractory inclusions from different meteorite groups, we further conducted REE analyses on refractory inclusions from Yamato 81020 (CO3.0) chondrite, one of the most primitive CO chondrites. Samples and analytical conditions: So far 10 inclusions have been analyzed for REE abundances: five AOAs or AOA-like inclusions (RI#5, RI#9, RI#25, RI#28 and RI#38), one melilite-rich (type A) inclusion (RI#8), one hibonite-melilite inclusion (RI#14), one melilite-spinel inclusion (RI#16), one meli-lite-perovskite inclusion (RI#17) and one pyrox-ene-spinel-plagioclase inclusion (RI#1). Most of them are small (typically ~100 μm in size) except for RI#8. RI#8 (~500 μm in diameter) consists mostly of gehlenitic melilite in the inner part surrounded by rim layers of spinel +perovskite +melilite +diopside. Its central part was possibly a cavity space now filled with fragments of this inclusion or matrix-like materials. REE analyses were preformed for the inner melilite and perovskite-rich part of the rim. RI#14 (~100 μm x ~60 μm in size) consists gehlenitic melilite, hibonite and