B–O–Th isotope systematics in Icelandic tephra

We report new boron concentrations and isotope compositions measured by ion microprobe on Holocene tephra samples from 6 Icelandic volcanoes. The B concentrations range over almost a factor of 10, from 1.50 ±0.10 to 13.10 ± 0.10 ppm in basalt and obsidian, respectively. The highest δ11B ( + 16.9 ± 2.2 and + 6.1 ± 1.6‰) measured in rhyolites from the rift-related Askja and Krafla volcanoes. In contrast, the lowest δ11B values (− 5.3 and − 5.2‰) observed in samples with the some of the highest δ18O (+ 4.92 and + 5.18‰, respectively) and (230Th/232Th) (1.067 and 1.030, respectively), correspond to a “normal-mantle” signature. The variations of boron isotope compositions in the Icelandic tephra and their negative correlations with both δ18O and (230Th/232Th) strongly support the mixing between mantle derived basalts and crustal rhyolites produced from anatexis of hydrothermally altered basaltic crust. The low δ18O in combination with high δ11B values in rhyolite is best explained by the presence of seawater derived B in a hydrothermal fluid that is mostly of meteoric origin.