Precise and accurate determination of boron concentration in silicate rocks by direct isotope dilution ICP-MS: Insights into the B budget of the mantle and B behavior in magmatic systems

International audienceWe determine the bulk boron concentration of silicate rocks using a new method that does not require sample evaporation. After spiking (10B enriched NIST SRM 952) and HF attack, the rock solution is directly aspirated into the ICP-MS, and B concentration is determined by isotopic dilution. This technique is applied to samples with B contents in the range of 0.1-30 ppm. The very low blank (< 0.5 ng B) makes this procedure particularly suitable for samples with low B content. Repeated analysis of the basaltic reference sample BHVO-2 yields a B concentration of 2.81 ± 0.09 ppm (2σ), giving a 3% relative error. This concentration, like those obtained for other basaltic and andesitic reference materials (BIR-1a, BCR-2, JB-1 and JA-1), agrees well with previously published values. The method is applied to 1) peridotite xenoliths from Mongolia and Russia, and 2) fresh lavas from Piton de la Fournaise, the active volcano on Réunion Island. We use the xenolith data to examine the behavior of boron during mantle melting and enrichment processes (i.e. metasomatism), and to estimate the primitive mantle B content (0.26 ± 0.04 ppm), while the lava data allows us to quantify B loss due to magmatic degassing (between 10 and 30%), using the composition of a primitive melt inclusion as the reference for an initial magma composition. Our knowledge of the degassing history of the samples allows us to quantify loss during lava emplacement and cooling (4-10%), continuous open-system degassing (10%) and shallow closed-system degassing (5-8%). Our results indicate that 1) B abundances of ocean island basalts are ca. 60-75% less than estimates based on elements with similar incompatibility (e.g., Nb and Ta), 2) this deficiency could reflect source depletion and/or magmatic degassing, and 3) magma degassing accounts for less than half of the inferred boron anomaly seen on primitive mantle-normalized patterns, implying that the mantle sources of ocean island basalts must be depleted in boron. The robust technique presented in this study could provide useful constraints on boron distribution within the Earth's mantle as well as on its behavior in magmatic systems