Analysis of boron in fluid inclusions by microthermometry, laser ablation ICP-MS, and Raman spectroscopy: Application to the Cryo-Genie Pegmatite, San Diego County, California, USA

Boron contents in magmatic fluids are largely unknown, even though B is an important constituent in granitic magmas and, potentially, in ore fluids. Owing to the fact that fluid inclusions from Li-rich pegmatites are typically B, Li, and Na ± F-rich aqueous solutions, salinity derived from microthermometry via the freezing point depression (NaCleq) may be inaccurate. In this study, the ratio of the integrated areas of the Raman bands of [B(OH)3]aq0 at ~ 878 cm− 1 (B[3]O stretching) and of H2O at ~ 3400 cm− 1 (OH stretching) were used for quantitative analysis of B concentrations in fluid inclusions in quartz. Then, B concentration measured via Raman spectroscopy was used as an internal standard for LA-ICP-MS analysis instead of using Na contents derived from NaCleq obtained by microthermometry. This study revisits, validates, and optimizes the methodology for analysis of boron in fluid inclusions from granitic pegmatites and from other boron-rich environments by combining results from Raman spectroscopy, LA-ICP-MS and microthermometry.Raman analyses of synthetic solutions that approximate the composition of pegmatitic fluid inclusions were used to evaluate the effects of NaCl (1.1–12 mass%), LiCl (1.5–16), Li2CO3 (0.05–0.6), NaF (0.6–1.7), and LiF (0.006–0.06) on H3BO3 solutions ranging from 0.28 to 2.0 mass% of H3BO3 for the analysis of H3BO3 by Raman spectroscopy. The errors in H3BO3 concentration induced by NaCl, LiCl, and LiF, the most common salts in pegmatitic fluids, were all ≤± 6%. Addition of Li2CO3 in excess of ~ 0.1 Li2CO3/H3BO3 ratio and addition of NaF in excess of ~ 0.2 NaF/H3BO3 ratio caused a systematic decrease in the intensity of the νsB[3]O band at ~ 878 cm− 1 (I878) that resulted in the B concentration being underestimated by as much as 80%. This is related to an increase of solution pH that decreases the stability of [B(OH)3]aq0. The effect of the inclusion-host quartz lattice orientation on the Raman signal was also tested. When the sample is cut perpendicular to the quartz c-axis or if its c-axis is co-planar with the laser polarization plane, the lattice effects on borate calibration become negligible.

► A new methodology for analysis of boron in fluid inclusions is proposed.
► H3BO3 contents are calibrated via Raman spectroscopy using synthetic solutions.
► Errors induced by solutes NaCl, LiCl, and LiF on borate calibration are <± 6%.
► Effects of host-crystal lattice on borate calibration can be minimized.
► The freezing point of aqueous fluids is strongly affected by borate.