Lithium Isotope Compositions Measured by Multi-Collector ICPMS and the Question of Light Isotope Fractionation at High Temperatures

Abstract

We report on initial efforts to measure the isotopic composition of lithium using a VG Plasma 54-30 multi-collector magnetic sector ICP-MS. Precision of multiple replicate and duplicate measurements for a variety of sample types ranges from +0.98%0 to +2.1%o (2~ population) level, with a mean uncertainty of + 1.6%o. The method allows for the rapid (c. 9 min/sample) analysis of small samples (c. 40 ng Li). The technique has been applied to a suite of basalts from the Kilauea Iki lava lake, Hawaii. The samples, which range from olivine-rich, high-Mg lava to highly differentiated liquids, have 56Li (6Li/7Li relative to the L-SVEC standard) of -4 .6 to -3.0%0. The data indicate, given our current capacity to resolve differences in ~6Li, a lack of Li isotope fractionation concomitant with crystal-liquid fractionation at temperatures greater than 1000~ This conclusion has been tacitly assumed but never demonstrated, and is important to the interpretation of Li isotope results from such geochemically complex situations as island arcs.