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

Abstract

Factors controlling the origin of silicic magmas on Iceland are poorly constrained. Here we present new data on H 2 O 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 (δ 18 O<2%) at shallow depths (≥1.8 km). These rhyolites are hot (935-1008°C), relatively dry (H 2 O<2.7wt%), and oxidized (QFM= +1.4). Cooler (874-902°C), wet (H 2 O= 4-6.3 wt%), and non-oxidized (~QFMto QFM-1) off-rift rhyolites (Öræfajökull, Hekla) originate through differentiation deeper in the crust (≥4km) with almost no or little assimilation of high-T, altered crust, as reflected by slightly lower to normal δ 18 O 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.