The absence of lithium isotope fractionation during basalt differentiation: New measurements by multicollector sector ICP-MS

We report measurements of the isotopic composition of lithium in basalts using a multicollector magnetic sector plasma-source mass spectrometer (MC-ICP-MS). This is the first application of this analytical technique to Li isotope determination. External precision of multiple replicate and duplicate measurements for a variety of sample types averages +/-1.1 parts per thousand (2 sigma population). The method allows for the rapid (similar to 8 min/sample) analysis of small samples (similar to 40 ng Li) relative to commonly used thermal ionization methods. The technique has been applied to a suite of samples from Kilauea Iki lava lake, Hawaii. The samples range from olivine-rich cumulitic lava to SiO2- and K2O-enriched differentiated liquids, and have delta(7)Li (per mil deviation of sample Li-7/Li-6 relative to the L-SVEC standard) of +3.0 to +4.8. The data indicate a lack of per mil-level Li isotope fractionation as a result of crystal-liquid fractionation at temperatures greater than 1050 degrees C, This conclusion has been tacitly assumed but never demonstrated, and is important to the interpretation of Li isotope results from such geochemically complex environments as island arcs. Copyright (C) 1999 Elsevier Science Ltd.