| Summary: | The 1783–1784 AD fissure eruption of Laki (Iceland) produced 15 km^3 of homogeneous basaltic lavas and tephra that are characterized by extreme (3‰) ^(18)O-depletion relative to normal mantle. Basaltic tephra erupted over the last 8 centuries and as late as in November 2004 from the GrÃmsvötn central volcano, which together with Laki are a part of a single volcanic system, is indistinguishable in δ^(18)O from Laki glass. This suggests that all tap a homogeneous and long-lived low-δ^(18)O 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-δ^(18)O_(glass) values, and extreme 3‰ range in δ^(18)O_(olivine) have not been described in any other unaltered basalt. The energy constrained mass balance calculation involving oxygen isotopes and major element composition calls for an origin of the Laki–GrÃmsvötn quartz tholeiitic basaltic melts with δ^(18)O = 3.1‰ by bulk digestion of low-δ^(18)O hydrated basaltic crust with δ^(18)O = − 4‰ to + 1‰, rather than magma mixing with ultra-low-δ^(18)O silicic melt. The abundant Pleistocene hyaloclastites, which were altered by synglacial meltwaters, can serve as a likely assimilant material for the GrÃmsvötn magmas. The (^(226)Ra /^(230)Th) activity ratio in Laki lavas and 20th century GrÃmsvötn tephras is 13% in-excess of secular equilibrium, but products of the 20th century GrÃmsvötn eruptions have equilibrium (^(210)Pb /^(226)Ra). Modeling of oxygen isotope exchange between disequilibrium phenocrysts and magmas, and these short-lived U-series nuclides yields a coherent age for the Laki–GrÃmsvötn magma reservoir between 100 and 1000 yrs. We propose the existence of uniquely fingerprinted, low-δ^(18)O, homogeneous, large volume, and long-lived basaltic reservoir beneath the Laki–GrÃmsvötn volcanic system that has been kept alive in its position above the ...
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