Primordial Ce isotopic composition of the solar system

Ce and Nd isotope ratios and REE contents of one carbonaceous chondrite (Murchison), 3 ordinary chondrites (Granès, Holbrook and Barwise) and 4 achondrites (Camel Donga, Juvinas, two pieces of Millbillillie) were measured. For the Ce isotope analysis of meteorite samples the new chemistry for extracting Ce from the gel formed after decomposition of ∼1-g chondrite sample with Mg content of 20% was developed with a recovery yield of 80%. The initial Ce isotope ratio of each meteorite sample was calculated with the assumption that the age of meteorite samples is 4.56 Ga, and the average value was obtained to be ¹³⁸Ce/¹⁴²Ce-0.0225321 ± 0.0000007 (2σ), which is considered to be the primordial Ce isotopic composition of the solar system. The average of the present-day Ce isotope ratios of the chondrites of this study and that previously published by Shimizu et al. is 0.0225653, which suggests that the estimation of the Ce isotope ratio of the present-day bulk Earth of 0.0225652 that was published by Makishima and Nakamura is convincing. The Ce isotope ratio of the present-day bulk Earth of 0.0225652 gives the ¹³⁸La/¹⁴²Ce ratio of the bulk Earth to be 0.00306 ± 0.00006 (2σ), which corresponds to La/Ce = 0.375 ± 0.007 (2σ). We propose that the La/Ce ratio of 0.375 represents that of the bulk Earth or CHUR.