Refining Boron Isotopic Measurements of Silicate Samples by Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICP-MS)

Abstract

Solution MC-ICP-MS is an established technique for high precision boron isotope measurement results (δ¹¹B_{SRM 951}) in carbonates, yet its application to silicate rocks has been limited. Impediments include volatilisation during silicate dissolution and contamination during chemical purification. To address this, we present a low-blank sample preparation procedure that couples hydrofluoric acid-digestion and low-temperature evaporation (mannitol-free), to an established MC-ICP-MS measurement procedure following chemical purification using B-specific Amberlite IRA 743 resin. We obtain accurate δ¹¹B_{SRM 951} values (intermediate precision ±0.2‰) for boric acid (BAM ERM-AE121 19.65 ± 0.14‰) and carbonate (NIST RM 8301 (Coral) 24.24 ± 0.11‰) reference materials. For silicate reference materials covering mafic to felsic compositions we obtain δ¹¹B_{SRM 951} with intermediate precision < ±0.6‰ (2s), namely JB-2 6.9 ± 0.4‰; IAEA-B-5 -6.0 ± 0.6‰; IAEA-B-6 -3.9 ± 0.5‰ (2s). Furthermore, splits of these same reference materials were processed by an alternative fusion and purification procedure. We find excellent agreement between δ¹¹B_{SRM 951} measurement results by MC-ICP-MS of the reference materials using both sample processing techniques. These measurement results show that our sample processing and MC-ICP-MS methods provide consistent δ¹¹B_{SRM 951} values for low B-mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in δ¹¹B_{SRM 951} from -5.6 ± 0.3‰ to -8.8 ± 0.5‰ (2s), implying some upper mantle δ¹¹B_{SRM 951} heterogeneity.