Rare earth element abundances in the CK chondrites including the Kobe meteorite

The Kobe (CK4) meteorite and other CK chondrites are characterized with respect to rare earth element (REE) abundances using 12 bulk samples from 10 CK chondrites including Ningqiang (CK3), Karoonda (CK4), Kobe (CK4), Y-693 (CK4), A-882113 (CK4), Maralinga (CK4), Y-82102 (CK5), Y-82105 (CK5), Y-82191 (CK6) and A-881551 (CK6). REE, Ba, Sr, Ca, Mg, Fe, Rb and K were analyzed by high-precision isotope dilution. The CK chondrites examined exhibit systematically higher REE abundances compared to ordinary chondrites, comparable to CV and CO, flat REE patterns with minor negative anomalies up to -15% on Ce and -25% on Eu. These chondrites also exhibit an appreciable light/heavy REE gap and Yb anomaly compared to Cl chondrites. CK3-4 chondrites in general are found to exhibit a larger negative Ce anomaly (mean anomaly -9.6 ′3.8%) compared to CK5-6 chondrites (3.3 ′ 1.0%), and some CK5-6 show a slightly light-REE depleted pattern with less-pronounced negative Ce anomaly. It is suggested that these CK chondrites had a larger negative Ce anomaly prior to early thermal metamorphism, which yielded the minor redistribution of light REEs. No clear correlation is found between Sm and Eu/Sm or between Eu and Sr. From these observations, it is suggested that Eu existed in the trivalent state due to higher oxidizing conditions, suppressing redistribution of REEs among minerals compared to metamorphosed ordinary chondrites during the early thermal metamorphic event on the parent body. The systematic REE anomalies observed in CK chondrites are considered to be inherited properties from the refractory precursors produced in the early solar nebula.