Lava shields and fissure eruptions of the Western Volcanic Zone, Iceland: Evidence for magma chambers and crustal interaction

Volcanic eruptions in Iceland occur either from fissures or central vents (lava shields). Within the post-glacial Western Volcanic Zone, the Thjófahraun fissure-fed lava field and Lambahraun lava shield were both erupted ~ 4000 yrs B.P. with eruptive centers separated by only ~ 25 km. Thjófahraun erupted ~ 1 km3 of pāhoehoe and 'a'ā lava from a 9-km long fissure, whereas the Lambahraun lava shield erupted > 7 km3 of low effusion-rate pāhoehoe. Thjófahraun lavas contain higher K, Rb, Y and Zr, and lower CaO than Lambahraun lavas at the same MgO, with variations broadly consistent with evolution by low-pressure crystal fractionation. Lambahraun spans a larger range of MgO, which generally decreases over time during the eruption. Lambahraun samples with high Al2O3 and low TiO2 and FeO likely reflect up to 15% plagioclase accumulation. In addition, all samples from Lambahraun exhibit increasing CaO and Nb/Zr with decreasing MgO and overall incompatible-element enrichments greater than predicted by crystal fractionation alone. Although the increase in Nb/Zr and other incompatible elements could be explained by gradually more incompatible-element enriched parental magma being supplied to the magmatic system during the course of the Lambahraun eruption, this process requires very small-scale trace element heterogeneities in the mantle that are apparently decoupled from isotopic variations and a systematic relationship between parental magma composition and extent of differentiation. Alternatively, correlations among incompatible element concentration, increasing differentiation and time during the eruption can be related by concurrent wallrock assimilation and crystallization during melt migration through the crust. Geochemical modeling of assimilation of wallrock clinopyroxene concurrent with crystallization of olivine (± plagioclase) effectively reproduces the observed chemical variations of Lambahraun samples. Similar chemical characteristics exist in several other Western Volcanic Zone lava shields but not in fissure eruptions. Magmas that fed fissure eruptions may also have been modified by interaction with the crust prior to aggregation in crustal magma chambers, but the geochemical signature of this process is obscured by magma mixing. In contrast, Icelandic lava shields that preserve deeper level processes may not have developed shallow magma chambers; rather they probably represent slow effusion from magma systems that are continually being recharged and reacting with the crust during the course of their eruptions.