Oxygen isotopes in melt inclusions and glasses from the Askja volcanic system, North Iceland

Primitive melt inclusions trapped during the earliest stages of fractional crystallisation are able to preserve oxygen isotope ratios inherited from mantle-derived melts. However, assimilation of low-δO hydrothermally altered crustal material and mixing with magmas held in shallow reservoirs may exert a strong control on the δO of melt inclusions trapped during later stages of crystallisation. Oxygen isotope ratios in olivine- and plagioclase-hosted melt inclusions and glasses from tephra samples collected from the Askja central volcano and Askja volcanic system indicate significant differences in the mechanisms of magma supply and storage between the northern and southern segments of the Askja volcanic system. Melt inclusions from the Holuhraun fissure eruption, ~20km south of Askja, mostly preserve δO signatures of +4.1‰ to +5.4‰, suggesting that this magma underwent minimal modification by magma mixing or crustal assimilation prior to its eruption. By contrast, melt inclusions and glasses from the Nýjahraun fissure eruption, ~60km north of Askja, have δO between +3.1‰ and +4.0‰. These relatively evolved melt inclusions (~3.9-4.3wt.% MgO) were probably trapped during late-stage fractional crystallisation in a shallow magma storage zone. Melt inclusions from two phreatomagmatic tuff sequences within the Askja caldera have δO between +2.1‰ and +5.2‰, and this variability cannot be explained by mixing with low-δO rhyolitic or andesitic contaminants in the upper crust. Instead, mixing of the ascending magmas with hydrated, low-δO basaltic magmas is invoked, thus acquiring a low δO signature with minimal modification to the magma's bulk composition. Such magma bodies are likely to be found throughout the upper 11km of the crust beneath Askja. Assimilation of low-δO meta-basalt in the upper crust is also likely to affect the δO of ascending magmas.