Stable Sm isotopic analysis of terrestrial rock samples by double-spike thermal ionization mass spectrometry

Abstract

This study presents the first report on mass-dependent stable Sm isotopic variability in geological materials using the double-spike thermal ionization mass spectrometry (DS-TIMS). The precision of the stable Sm-isotopic-composition DS-TIMS analysis was ca. 0.5ε/amu. The accuracy of the analysis was demonstrated by the consistent Sm isotope fractionation behavior observed during cation-exchange column chromatography. The Sm isotope fractionation factor between the cation-exchange resin (AG 50W X8) and alpha-hydroxyisobutyric acid (HIBA) at 22.6°C was obtained from the experimental results to be α147=(147Sm/150Sm)AG50WX8/(147Sm/150Sm)HIBA=1.000076±0.000011, which corresponds to a Sm isotopic difference of 0.25ε/amu between the resin and HIBA under equilibrium condition. Mass-dependent Sm stable isotopic variations on the order of epsilon unit were observed among the analyzed geological rock samples. Six igneous rocks from various rock types showed no resolvable variation in their Sm isotopic compositions, whereas a Mesozoic seamount-type marine dolomite and an Archean banded ferruginous chert showed stable Sm isotopic compositions significantly enriched in heavy isotopes as compared to the igneous rocks. The range of the Sm isotopic variation among the geological materials herein studied was 0.9ε/amu. Our results suggest that significant stable Sm isotope fractionation occurs in the marine environment at low temperatures and the stable Sm isotopic composition may thus be used to trace the Sm cycling in the ocean and earth surface environment.