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, Öraefajökull, 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 < 2.7 wt%), and oxidized (QFM = +1.4). Cooler (874-902°C), wet (H 2 O = 4-6.3 wt%), and non-oxidized (~QFM to QFM-1) off-rift rhyolites (Öraefajökull, 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 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.
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