Geochemical Journal, Vol. 53, 2019
Ryoichi Nakada1*, Naoko Asakura1, and Kazuya Nagaishi2
1Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, Kochi 783-8502, Japan
2Marine Works Japan Ltd., Nankoku, Kochi 783-8502, Japan
(Received March 26, 2019; Accepted July 3, 2019)
The cerium isotope ratio (δ142Ce), 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 N2 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 δ142Ce achieved in repeated measurements of NIST solution was ±0.020‰. The typical reproducibility was about ±0.030‰. The required Ce amount of the measurement was also decreased from 0.2 μg to 0.05 μg. Moreover, the current study measured δ142Ce of 5 USGS and 17 GSJ standard rocks. The Ce stable isotope ratio of standard igneous rocks shows no significant isotope fractionation, although δ142Ce of JDo-1 and JMn-1 are fractionated respectively from standard NIST solutions of +0.134±0.025‰ and +0.110±0.025‰. The δ142Ce fractionation of sedimentary rocks can result from the adsorption reaction of Ce.
Key words: Cerium isotope ratio, MC-ICP-MS, analytical condition, geochemical standards, desolvating nebulizer