Lithium isotope analysis of olivine by SIMS: Calibration of a matrix effect and application to magmatic phenocrysts

High-spatial resolution analysis of light element isotope variations by secondary-ion mass spectrometry (SIMS) has numerous applications in geochemistry and cosmochemistry. Recent attention has focused on Li-7/Li-6 variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas, and on minerals from mantle samples to determine source-region processes and the recycling history of mantle reservoirs. In these studies the effect of mineral composition on the Li-7/Li-6 ratio Measured by SIMS has been considered secondary, and generally disregarded. We show, Using a Suite of nine olivines analyzed by MC-ICP-MS or TIMS, that there is a Substantial effect of composition on the Li-7/Li-6 ratio of olivine measured by SIMS. For magnesian olivine (74<Fo<94) the effect is a linear function of composition, with (delta Li-7 increasing by 1.3 parts per thousand for each mole percent decrease in forsterite component. At higher Fe contents, the relationship ceases to be linear. The composition range over which linear behavior is exhibited appears to depend oil instrumental conditions. A calibration of this matrix effect over the linear range is presented, assuming the measurement of Li-7/Li-6 relative to ail olivine standard of known composition. Application of this calibration to a Suite of olivines separated from basaltic lavas from Ko'olau, Hawai'i demonstrates that the matrix effect is responsible for a geologically spurious correlation between delta Li-7 and Mg#. However, after correction, the olivines retain evidence of Li isotope heterogeneity, the degree and nature of which differs in each of the four separates Studied. These results emphasize the importance of compositional correction for SIMS measurement of 17 Li in olivine, particularly in zoned crystals, and support previous conclusions that Li isotope variability in igneous materials is Subject to late-stage disturbance. The significant matrix effect demonstrated for olivine suggests that matrix effects in other minerals require further evaluation. (C) 2008 Elsevier B.V. All rights reserved.