DEVELOPMENT OF δ7LI AND δ11B ANALYTICAL PROCEDURES ON SHRIMP-RG WITH APPLICATION TO CRUST-MANTLE EVOLUTION AND METASOMATISM STUDIES

Isotopic ratios of light stable elements are useful geochemical fingerprints for constraining the conditions of fluid mediated mass transfer processes in the crust and upper mantle. Specifically, δ⁷Li and δ¹¹B isotopes are used to explore crust-mantle evolution and subduction fluid cycling. Minerals, such as micas, formed during fluid infiltration events often display micron-scale chemical zoning which require high spatial resolution measurement techniques, such as Secondary Ionization Mass Spectrometry (SIMS), to target discrete zones within minerals. We present a newly developed SIMS protocol for δ¹¹B and δ⁷Li analysis using the SHRIMP-RG ion microprobe at Stanford University. Several factors specific to SIMS measurements can cause significant instrumentation mass fractionation for δ¹¹B and δ⁷Li. The most significant on SHRIMP-RG is the effect of ambient magnetic fields on measurement reproducibility. Additionally, we found that variation in voltage applied to the first of a triplet of quadrupole lenses, critical for focusing secondary ions into the mass analyzer, correlated with fractionation of light isotopic ratios by up to 20‰. To minimize isotope fractionation from quadrupole voltage and ambient magnetic field, we conducted systematic experiments varying the voltage applied to horizontal Helmholtz coils mounted on the top and bottom of the source chamber. By applying a voltage of -250 V to the Helmholtz coils, measured δ¹¹B in NIST SRM-611 chips mounted across a 15 mm diameter area are reproducible to -0.53 ± 1.23‰ (n=52; 1σ, s.d.). Additionally, we performed δ¹¹B measurements on muscovite from the Pala pegmatite (Stewart Mine, San Diego County; Stanford Mineralogical Collection #30207) that we analyzed as a secondary reproducibility reference material to determine SIMS artifacts due to crystal-orientation for mica analyses. δ¹¹B values for samples analyzed parallel and perpendicular to mica cleavage are 0.69±1.82‰ (n=11; 1σ, s.d.) and 1.30 ±1.83‰ (n=9; 1σ, s.d.), respectively. We conclude that (1) SHRIMP-RG analysis of δ¹¹B can yield accurate and reproducible results; (2) mica orientation does not affect SIMS measurement reproducibility. This study highlights the ability to analyze isotopic zoning in minerals to characterize metasomatism in subduction zones.