B–O–Th isotope systematics in Icelandic tephra

Abstract

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 δ 11B ( + 16.9 ± 2.2 and + 6.1 ± 1.6‰) measured in rhyolites from the rift-related Askja and Krafla volcanoes. In contrast, the lowest δ 11B values (− 5.3 and − 5.2‰) observed in samples with the some of the highest δ 18O (+ 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 δ 18O and ( 230Th/ 232Th) strongly support the mixing between mantle derived basalts and crustal rhyolites produced from anatexis of hydrothermally altered basaltic crust. The low δ 18O in combination with high δ 11B values in rhyolite is best explained by the presence of seawater derived B in a hydrothermal fluid that is mostly of meteoric origin.