Contrasting conditions of rift and off‐rift silicic magma origin on Iceland

Abstract

AbstractFactors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H2O content, pressure, temperature, oxygen fugacity, and oxygen isotope composition of rhyolites from Askja, Öræfajökull, and Hekla volcanoes. All these parameters correlate with tectonic (rift and off‐rift) setting of the volcanoes. Askja rift rhyolites originate through extensive assimilation of high‐temperature hydrothermally altered crust (δ18O < 2‰) at shallow depths (≥1.8 km). These rhyolites are hot (935–1008°C), relatively dry (H2O < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874–902°C), wet (H2O = 4‐6.3 wt%), and non‐oxidized (~QFM to QFM‐1) off‐rift rhyolites (Öræfajökull, Hekla) originate through differentiation deeper in the crust (≥4 km) with almost no or little assimilation of high‐T, altered crust, as reflected by slightly lower to normal δ18O values (5.2–6‰). Although off‐rift rhyolites predominate during the Holocene, older silicic rocks on Iceland primarily formed in a rift setting possibly analogous to the oldest continental crust on Earth.