Cerium stable isotope analysis of synthetic and terrestrial rock reference materials by MC-ICPMS

Cerium is the most abundant rare earth element (REE) in the solar photosphere, CI chondrites, and the Earth. It has four main stable isotopes (masses: 136,138,140, and 142), with Ce-138 being the most studied species, used in geochronology and petrogenesis. In addition, more abundant Ce-140 and Ce-142 are suggested to be potentially applicable in geochemical investigations. In this work, we developed a modified four-step ion chromatography procedure for Ce chemical separation. Using a MC-ICPMS, we designed a cup configuration to measure Ce-142/Ce-140 ratio of the samples with an optimized Nd correction equation. A 0.03 parts per thousand (25D) reproducibility was obtained for Ce Ames metal standard. We analyzed ten different igneous and one sedimentary geochemical reference materials. Mean delta Ce-142 range from -.07 to 0.32 parts per thousand. Most of the samples show a heavier Ce isotopic composition than the Ce Ames standard. The majority of rocks have a homogenous delta Ce-142. The delta Ce-142 does not show any correlation with rock chemical composition including their Ce content or rock types. A carbonatite (SARM 40) has a mean delta Ce-142 of -0.07 +/- 0.13 parts per thousand (25D), lower than the other rocks, suggesting the possibility of a pronounced isotopic fractionation. Our work demonstrates the applicability of the developed methodology and the potential of Ce stable isotopes for future geochemical studies. Production of a larger database of delta Ce-142 values is required to obtain a clearer view on the similarities and differences between different geological material and explaining Ce stable isotope dynamics.

Automated summary

Cerium is the most abundant rare earth element in the solar photosphere, CI chondrites, and the Earth. A modified ion chromatography procedure for Ce separation was developed, and Ce stable isotopes were analyzed in various igneous and sedimentary geochemical reference materials. The majority of samples showed heavier Ce isotopic composition than the standard, with a homogenous delta Ce-142 in most rocks.