Boron concentrations and isotopic composition of the Icelandic mantle: Evidence from glass inclusions in olivine

Quenched tholeiitic glasses and glassy rims of tholeiitic lava flows and pillow lavas from the neovolcanic rift zone on Iceland (Reykjanes and Hengill fissure swarms) contain olivine phenocrysts (Fo(87-91)) with abundant primary glass inclusions. These inclusions and host glasses were analyzed by ion microprobe for boron concentrations and isotopic compositions. Inclusions are believed to represent primary or near-primary mantle melts that have not been modified at shallow levels. Boron concentrations and B/K ratios in these melt inclusions are highly variable (0.18-1.35 ppm B, B/K = 2.8 x 10(-4)-6.1 x 10(-3)), whereas their delta(11)B values are nearly constant (-11.3, 1 sigma(n) +/- 1.9 parts per thousand). This indicates that the Icelandic mantle is likely to have a constant boron isotope composition similar to that previously proposed for the primitive mantle (delta(11)B similar to -10 parts per thousand). The Hengill host glasses are characterized by low concentrations of B (0.26-0.42 ppm) and delta(11)B values similar to melt inclusions (-11.3, 1 sigma(n) +/- 1.3 parts per thousand). In contrast, the Reykjanes host glasses have higher delta(11)B values (similar to -3 parts per thousand) at the same relatively low concentrations of B (0.43-0.44 ppm), which are likely to be due to interactions of the ascending magmas with the crustal rocks hydrothermally altered at low temperatures by seawater-derived fluids. Boron concentrations and B/K ratios in melt inclusions and matrix glasses correlate with TiO2, K2O and P2O5 contents, and (La/Sm)(n) ratios. Boron contents recalculated on the basis of melt inclusion compositions for the Icelandic mantle are slightly different for the Reykjanes and Hengill areas (0.21, 1 sigma(n) +/- 0.05 and 0.13, 1 sigma(n) +/- 0.03 ppm B, respectively). This is likely to reflect the presence of zones within the Icelandic mantle variously enriched and/or depleted in boron of similar isotopic composition.