The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.

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