Isotope-heterogeneity of the monogenetic Thjorsa lava eruption, Iceland - source and crustal influence

The Thjorsa lava in the Eastern Rift Zone (ERZ) in Southern Iceland erupted 8600 years ago. It is the largest Holocene fissure lava in Iceland with a total volume of 25 km3. It is one of several highly plagioclase-porphyritic tholeiitic fissure eruptions from the early Holocene. These porphyritic lavas belong to the Bardarbunga volcanic system which forms the highly productive eastern edge of the South Iceland microplate: The plagioclase content ranges from 4-30 vol% - about 7 vol% on average. The origin of the large plagioclase phenocrysts is uncertain, i.e. are they crystallized from the host magma or its precursors, or are they accidental xenocrysts. In this study, elemental and isotope (Sr, Nd and Pb) analyses were made on a suite of groundmass samples, and 87Sr/86Sr ratio in plagioclase/groundmass pairs. Samples are from various parts of the Thjorsa lava - including a number of samples from drill cores. Additional whole rock/glass samples from the Bardarbunga system lavas are also included for comparison. The host lava groundmass (i.e. the liquid phase) has a restricted variation in elemental and isotopic composition. The plagioclase separates show a range of 87Sr/86Sr isotopic values, but with hardly any overlap with the groundmass. The isotopic heterogeneity of the plagioclase phenocrysts covers almost the entire variation observed within the Bardarbunga spreading center during the Holocene. The plagioclases are therefore xenocrysts and significantly more isotopically primitive than the host groundmass, and derived from a different mantle source. Correlation between of cpx bearing elements (Cr/Y, Sc/Y) and isotopic ratios of Sr, Nd and Pb demonstrates addition of remelted cpx in the groundmass. Relics of Cr-rich cpx microliths and Cr- diopside xenoliths are assumed to be the leftovers and examples of the digested cpx. The ERZ is about 35 km wide with an array of sub-parallel fissure swarms and eruptive lineaments. The age of the ERZ is estimated at 2- 3 mill. years, and thus needs to accommodate rifting of similar width. Ascending magmas will, therefore, frequently penetrate the roots of earlier spreading swarms and possibly older crust. The large range of the isotope signature of the plagioclase xenocrysts indicates that they are formed by multiple intrusion events. These results indicate large-scale and rapid remobilization of crustal material by disintegration and partial assimilation of plutonic rocks, which significantly influenced the isotopic composition of the erupted lava. The mineralogical and geochemical character of this hybrid rock and its heterogeneous components sheds light on the origin of scattered chemical and isotopic data from Icelandic rift zone basalts.