LITHIUM ISOTOPE BIAS DURING SIMS ANALYSIS OF SILICATE GLASS STANDARDS

The element lithium diffuses rapidly through solid materials at elevated temperatures, and the two stable isotopes of lithium (⁶Li and ⁷Li) can diffuse at significantly different rates. For these reasons, geologists and material scientists will measure lithium gradients, and ⁶Li/⁷Li ratios, across boundaries of different lithium concentrations to determine rates and timings of lithium diffusion. Secondary Ion Mass Spectrometry (SIMS) has the precision required for such work, and though the method is subject to a mass bias that is dependent on material composition, commonly referred to as the ‘matrix effect’, this mass bias can be corrected for through the use of compositionally matched standards.

An array of USGS, MPI-DING, and NIST silicate glass reference materials was analyzed by SIMS to better characterize the compositional matrix effects and micron-scale heterogeneities that might impact ⁶Li/⁷Li ratios as measured on a SIMS instrument. The analyzed glasses cover a range of compositions from 45.5 to 75.6% SiO₂ (komatiite to rhyolite), and include both synthetic glass mixtures and undoped remelts of preexisting rocks. Though the other major elements composing these glasses were examined, we found that SiO₂ content is the dominant control on the magnitude of the matrix-induced bias over the studied range, and a linear relationship exists between SiO₂ content and matrix-induced mass bias. Lithium ionizes very efficiently by SIMS in glasses under the analyzed conditions, with measurable yields exceeding 5% in all samples, and exceeding 10% at high beam intensities in low SiO₂ glasses. The ⁶Li/⁷Li ratio is also demonstrated to be stable over wide ranges of beam intensity and during long periods of sputtering, making SiO₂ content the most important variable for accurate reconstruction of ⁶Li/⁷Li ratios by SIMS in silicate glasses.