Cerium isotopic fractionation during magmatic processes and the composition of the upper mantle

Abstract

Cerium (Ce) is a refractory, incompatible and redox-sensitive element and its isotopes can be used to trace planetary accretion and evolution in the early solar system. A knowledge of the isotopic composition of rocks that sample from the Earth's mantle is a prerequisite to understand curst-mantle evolution. In this study, we present the first comprehensive high-precision Ce stable isotopic compositions of different types of igneous rocks, including sixteen normal mid-ocean ridge basalts (N-MORB) from the East Pacific Rise (EPR) and Juan de Fuca Ridge (JdF), two depleted mid-ocean ridge basalts (D-MORB) from the EPR and Ecuador Rift, seven evolved lavas (basaltic andesites, andesites, and dacites) from the EPR. These igneous rocks, spanning compositions from primitive basalt to evolved dacite with MgO contents decreasing from 8.09 to 0.80 wt. %, display a limited variation in δ¹⁴²Ce from -0.038 to 0.024 ‰. Although these rocks have experienced different amounts and proportions of olivine, clinopyroxene, plagioclase, Fe-Ti oxides, and apatite fractional crystallization, no Ce isotopic fractionation was detected. The δ¹⁴²Ce values of MORBs show no correlation with La/Sm_(N) or Nb/Y, indicating that partial melting process cannot induce significant Ce isotopic fractionation. Our batch non-modal melting modelling shows that more than 95 % of the Ce budget will be extracted into melt after only 5 % degree of partial melting. Therefore, we conclude that Ce isotopic fractionation during magmatic processes is insignificant and the average δ¹⁴²Ce of studied samples of -0.005 ± 0.028 ‰ (2SD, N = 25) can be the best estimate of upper mantle's isotopic composition. Based on simple mass-balance calulation, the Ce isotopic composition of the bulk silicate Earth (BSE) is roughly estimated to be -0.008 ± 0.025 ‰ (2SD, propagated error).