Time constraints on the origin of large volume basalts derived from O-isotope and trace element mineral zoning and U-series disequilibria in the Laki and Grيmsvِtn volcanic system

The 1783–1784 AD fissure eruption of Laki (Iceland) produced 15 km3 of homogeneous basaltic lavas and tephra that are characterized by extreme (3‰) 18O-depletion relative to normal mantle. Basaltic tephra erupted over the last 8 centuries and as late as in November 2004 from the Grímsvötn central volcano, which together with Laki are a part of a single volcanic system, is indistinguishable in δ18O from Laki glass. This suggests that all tap a homogeneous and long-lived low-δ18O magma reservoir. In contrast, we observe extreme oxygen isotope heterogeneity (2.2–5.2‰) in olivine and plagioclase contained within these lavas and tephra, and disequilibrium mineral-glass oxygen-isotope fractionations. Such low-δ18Oglass values, and extreme 3‰ range in δ18Oolivine have not been described in any other unaltered basalt. ergy constrained mass balance calculation involving oxygen isotopes and major element composition calls for an origin of the Laki–Grímsvötn quartz tholeiitic basaltic melts with δ18O = 3.1‰ by bulk digestion of low-δ18O hydrated basaltic crust with δ18O = − 4‰ to + 1‰, rather than magma mixing with ultra-low-δ18O silicic melt. The abundant Pleistocene hyaloclastites, which were altered by synglacial meltwaters, can serve as a likely assimilant material for the Grímsvötn magmas. 26Ra / 230Th) activity ratio in Laki lavas and 20th century Grímsvötn tephras is 13% in-excess of secular equilibrium, but products of the 20th century Grímsvötn eruptions have equilibrium (210Pb / 226Ra). Modeling of oxygen isotope exchange between disequilibrium phenocrysts and magmas, and these short-lived U-series nuclides yields a coherent age for the Laki–Grímsvötn magma reservoir between 100 and 1000 yrs. We propose the existence of uniquely fingerprinted, low-δ18O, homogeneous, large volume, and long-lived basaltic reservoir beneath the Laki–Grímsvötn volcanic system that has been kept alive in its position above the center of the Icelandic mantle plume. Melt generation, crustal assimilation, magma storage and homogenization all took place in only a few thousands of years at most.