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 a nd pillow lavas from the neovolcanic rift zone on Iceland (Reykjanes a nd Hengill fissure swarms) contain olivine phenocrysts (Fo(87-91)) wit h 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-pr imary mantle melts that have not been modified at shallow levels. Boro n concentrations and B/K ratios in these melt inclusions are highly va riable (0.18-1.35 ppm B, B/K = 2.8 x 10(-4)-6.1 x 10(-3)), whereas the ir delta(11)B values are nearly constant (-11.3, 1 sigma(n) +/- 1.9 pa rts per thousand). This indicates that the Icelandic mantle is likely to have a constant boron isotope composition similar to that previousl y proposed for the primitive mantle (delta(11)B similar to -10 parts p er thousand). The Hengill host glasses are characterized by low concen trations of B (0.26-0.42 ppm) and delta(11)B values similar to melt in clusions (-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 asce nding magmas with the crustal rocks hydrothermally altered at low temp eratures by seawater-derived fluids. Boron concentrations and B/K rati os in melt inclusions and matrix glasses correlate with TiO2, K2O and P2O5 contents, and (La/Sm)(n) ratios. Boron contents recalculated on t he basis of melt inclusion compositions for the Icelandic mantle are s lightly 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 v ariously enriched and/or depleted in boron of similar isotopic composi tion.