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

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• 作者：Maryjo Brounce ; Maureen Feineman ; Peter LaFemina ; Andrey Gurenko
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2012
• 出版时间：1 October, 2012
• 年：2012
• 卷：94
• 期：Complete
• 页码：164-180
• 全文大小：1020 K

文摘

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 Lakag¨ªgar (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 ¦Ä¹¹B_NBS-951 ranges from ?7.8¡ë to ?16.5¡ë in the melt inclusions and ?10.5¡ë to ?16.9¡ë in the matrix glasses. In contrast to previous studies of boron in basaltic melt inclusions from other fissure swarms in Iceland (, Chem. Geol. 135, 21-34), the Lakag¨ªgar melt inclusions display a significant range of boron concentrations and isotope ratios at constant K₂O 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 ¦Ä¹¹B signature such as that observed in the Lakag¨ªgar melt inclusions and tephra host glasses. Considering boron concentrations and isotope ratios in the Lakag¨ªgar glasses and previously studied altered Icelandic basalts, together with volatile equilibration depths of the Lakag¨ªgar melt inclusions, we propose that (a) mantle-derived magmas formed beneath Lakag¨ªgar 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 (b) the magma subsequently underwent extensive mixing and homogenization prior to eruption, quite possibly within the magma chamber beneath the Gr¨ªmsv?tn 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 H₂O, CO₂, S, F, and Cl, which were responsible for the majority of the considerable casualties attributed to the Lakag¨ªgar eruption, were added together with isotopically light B by assimilation of hydrothermally altered crustal materials.