Insights into crustal assimilation by Icelandic basalts from boron isotopes in melt inclusions from the 1783-1784 Lakagígar eruption

The boron isotope system has great potential for tracing alteration and assimilation in basaltic systems due to the very low concentrations of B in mantle-derived melts and the strong isotopic contrast between the mantle and surface materials. However, variability in B concentrations and isotope ratios in basalts can also be interpreted to reflect inputs from enriched regions of the mantle, as the extent of mantle heterogeneity with respect to boron remains poorly delineated. We have determined boron concentrations and isotope ratios in fresh, glassy, plagioclase-hosted melt inclusions and unaltered scoriaceous matrix glasses from four localities associated with the 1783-1784 Lakagígar (Laki Fissure) eruption, Iceland. Boron concentrations range from 0.59 to 1.25ppm in the melt inclusions, and from 1.25 to 1.65ppm in the matrix glasses, while δ ¹¹B _NBS-951 ranges from -7.8‰ to -16.5‰ in the melt inclusions and -10.5‰ to -16.9‰ in the matrix glasses. In contrast to previous studies of boron in basaltic melt inclusions from other fissure swarms in Iceland (Gurenko and Chaussidon, 1997, Chem. Geol. 135, 21-34), the Lakagígar melt inclusions display a significant range of boron concentrations and isotope ratios at constant K ₂Owt.%, which is more consistent with B addition by assimilation of altered basalt than it is with mixing between depleted and enriched mantle sources. Assimilation of freshwater-altered crustal materials at depth may impart a light δ ¹¹B signature such as that observed in the Lakagígar melt inclusions and tephra host glasses. Considering boron concentrations and isotope ratios in the Lakagígar glasses and previously studied altered Icelandic basalts, together with volatile equilibration depths of the Lakagígar melt inclusions, we propose that (a) mantle-derived magmas formed beneath Lakagígar assimilated ∼5-20% altered crust at a depth of ∼3-4km or more, probably during magma accumulation in sills formed at the boundaries of low-density hyaloclastite layers; and (b) the magma subsequently underwent extensive mixing and homogenization prior to eruption, quite possibly within the magma chamber beneath the Grímsvötn central volcano, assimilating an additional ∼10% of altered crust at that time, for a total of up to 30% crustal assimilation. We hypothesize that volatiles including H ₂O, CO ₂, S, F, and Cl, which were responsible for the majority of the considerable casualties attributed to the Lakagígar eruption, were added together with isotopically light B by assimilation of hydrothermally altered crustal materials.