In Situ Boron Isotope Determination in Amphibole by LA-MC-ICP-MS: Matrix Effects and New Potential Reference Materials

Abstract

The B isotope systematics (δ¹¹B) in amphibole offer insights into fluid- and melt-dominated processes in the crust-mantle system. We developed an analytical strategy for in situ B isotope determination in amphibole using LA- MC-ICP-MS on material with a B fraction of a few μg g⁻¹ using 10¹³ Ω resistors. Sources of error deriving from matrix effects were evaluated. Two Ca-amphiboles, one from Pargas, Finland (PRG), and another from the Finero Complex (S. Alps, FIN), were characterised as potential reference materials. PRG has a B mass fraction of 6.75 ± 1.10 μg g⁻¹ and a δ¹¹B value of -16.58 ± 0.06‰, while FIN has 3.27 ± 0.68 μg g⁻¹ of B and a δ¹¹B value of -5.90 ± 0.09‰. A standard-sample-standard bracketing method, using USGS BHVO-2G basaltic glass as the bracketing ‘standard’ (calibrator), provided accurate and reproducible in situ δ¹¹B measurements with 2s of ± 2.86‰ (n = 87) for PRG and ± 3.96‰ for FIN (n = 70). These Ca-amphiboles are suitable reference materials to assess accuracy during in situ B isotope determination with LA-MC-ICP-MS. The potential of in situ B isotope determination in Ca-amphibole as a geochemical tool to investigate petrogenetic processes of a well-characterised amphibole-bearing gabbro from the Adamello batholith (C. Alps) has been tested successfully.