COPPER ISOTOPE CHARACTERISTICS OF COPPER-RICH MINERALS FROM THE HOROMAN PERIDOTITE COMPLEX, HOKKAIDO, NORTHERN JAPAN

Abstract

The copper isotope ratios of primary and secondary native copper grains in well-characterized plagioclase lherzolites from the Horoman peridotite complex, Hokkaido, northern Japan, were measured by femtosecond-pulsed laser ablation-multiple collector-inductively coupled plasma-mass spectrometry (fs-LA-MC-ICP-MS). The whole-rock copper isotope value of the fresh fertile plagioclase lherzolite was also determined. The primary native copper and whole-rock samples yield homogeneous δ 65 Cu (where δ 65 Cu = [{( 65 Cu/ 63 Cu) sample /( 65 Cu/ 63 Cu) NIST-SRM976 } − 1] × 1,000) values between −0.03 and 0.14‰, implying that there is no significant copper isotope fractionation during high-temperature magmatic processes. These and previously published results suggest that the copper isotope range of terrestrial igneous copper is from −0.27 to 0.27‰. In contrast, secondary native copper grains affected by serpentinization have obviously lower δ 65 Cu values (−0.67 to −0.41‰) than primary native copper and whole-rock samples, though the compositional differences between primary native copper and secondary native copper are significantly small, suggesting that selective preferential oxidation or leaching of the heavier copper isotope occurs during low-temperature serpentinization processes. Because secondary low-temperature processes probably disturb the original copper isotope values of primary copper-rich minerals, in situ determination of copper isotope values for intact mineral grains by microanalytical techniques such as LA-MC-ICP-MS has become increasingly important to understand primary isotope signatures of rock and ore samples.