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

Solution MC-ICP-MS is an established technique for high precision boron isotope measurement results (delta B-11(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 delta B-11(SRM 951) values (intermediate precision +/- 0.2 parts per thousand) for boric acid (BAM ERM-AE121 19.65 +/- 0.14 parts per thousand) and carbonate (NIST RM 8301 (Coral) 24.24 +/- 0.11 parts per thousand) reference materials. For silicate reference materials covering mafic to felsic compositions we obtain delta B-11(SRM 951) with intermediate precision < +/- 0.6 parts per thousand (2s), namely JB-2 6.9 +/- 0.4 parts per thousand; IAEA-B-5 -6.0 +/- 0.6 parts per thousand; IAEA-B-6 -3.9 +/- 0.5 parts per thousand (2s). Furthermore, splits of these same reference materials were processed by an alternative fusion and purification procedure. We find excellent agreement between delta B-11(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 delta B-11(SRM 951) values for low B-mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in delta B-11(SRM 951) from -5.6 +/- 0.3 parts per thousand to -8.8 +/- 0.5 parts per thousand (2s), implying some upper mantle delta B-11(SRM 951) heterogeneity.