Sources of Li isotope bias during SIMS analysis of standard glasses

Abstract

The 7Li/6Li ratios in fourteen USGS, MPI-DING, and NIST glass reference materials (RMs) were analyzed by LG-SIMS to assess compositional matrix effects, evaluate RM δ7Li homogeneity, and calculate the useful yield of lithium in silicate glasses. The analyzed RMs cover a range of SiO2 contents from 45.5 to 75.6% (komatiite to rhyolite), the largest compositional range yet evaluated for δ7Li matrix effects. We observe a matrix-induced bias (matrix effect) of up to 18‰ over the studied range that has a negative linear correlation with SiO2 content, demonstrating that SiO2 content is a critical factor that must be considered when using silicate glasses to standardize for silicate samples of unknown δ7Li composition. All RMs were found to be sufficiently reproducible to aid in standardization of variable SiO2 contents and appear isotopically homogeneous at the precision of our measurements. Lithium ionizes very efficiently in these RMs, with measurable yields >5% in all fused-rock RMs and > 15% in low SiO2 fused-rock RMs measured at high beam intensities. Fused-rock RMs ionize lithium 2-3× more efficiently than synthetic glass RMs, which may be related to differences in major oxide chemistry and the reduced nature of the synthetic glasses relative to their fused counterparts.