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 (δ11BSRM 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 δ11BSRM 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 δ11BSRM 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 agreement excellent agreement between δ11BSRM 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 δ11BSRM 951 values for low B-mass fraction samples. We present new data from Mid Ocean Ridge Basalt (MORB) glass, documenting a range in δ11BSRM 951 from -5.6±0.3‰ to -8.8±0.5‰ (2s), implying some upper mantle δ11BSRM 951 heterogeneity.