Examination of analytical conditions of cerium (Ce) isotope and stable isotope ratio of Ce in geochemical standards

摘要

The cerium isotope ratio (delta Ce-142), a novel proxy reflecting the paleoredox condition, yields more quantitative information related to the redox condition than Ce anomaly alone. However, sample amounts and analytical precision represent bottlenecks for Ce isotope measurements using multi-collector (MC-)ICP-MS. Sample treatment processes and analytical conditions of MC-ICP-MS using desolvating nebulizer were examined for this study. Our new cation exchange process removes most of the Ba, which is worthwhile for REE measurement. The examination of analytical conditions, including the sampling cone, torch Z position, Guard Electrode, Ar sweep, and N-2 gas greatly decreases the oxide formation ratio from approximately 1% (typical wet plasma mode) to less than 0.05%, which caused the great increase in the precision of Ce isotope measurement. The best reproducibility for delta Ce-142 achieved in repeated measurements of NIST solution was +/- 0.020 parts per thousand. The typical reproducibility was about +/- 0.030 parts per thousand. The required Ce amount of the measurement was also decreased from 0.2 mu g to 0.05 mu g. Moreover, the current study measured delta Ce-142 of 5 USGS and 17 GSJ standard rocks. The Ce stable isotope ratio of standard igneous rocks shows no significant isotope fractionation, although delta Ce-142 of JDo-1 and JMn-1 are fractionated respectively from standard NIST solutions of +0.134 +/- 0.025 parts per thousand and +0.110 +/- 0.025 parts per thousand. The delta Ce-142 fractionation of sedimentary rocks can result from the adsorption reaction of Ce.