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

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Brounce, Maryjo, Feineman, Maureen, Lafemina, Peter, Gurenko, Andrey
• Other Authors: Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2012
• Subjects: [SDU]Sciences of the Universe [physics]; Laki; Lakagígar; Iceland
• Online Access: https://insu.hal.science/insu-03619304; https://doi.org/10.1016/j.gca.2012.07.002

International audience 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 1.25 ppm in the melt inclusions, and from 1.25 to 1.65 ppm in the matrix glasses, while δ 11 B 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 2 O wt.%, 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 11 B 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 ∼3-4 km or more, probably during magma accumulation in sills formed at the boundaries of low-density hyaloclastite layers; and ...