During its short history as a subaerial island (<20 Ma), Iceland has had a remarkably dynamic history, experiencing voluminous magmatism, the generation of an unusual abundance of rhyolite (~10% of its crust); and extreme climate change. Using >1300 zircon O isotope analyses from volcanic, intrusive, and detrital samples, we investigate silicic magma petrogenesis through space (on-rift vs off rift) and time (~15 Ma to present). Zircon O compositions are distributed among four major age/climate divisions in Iceland’s history: (1) >8.5 Ma, warm-temperate; (2) 8.5 -3.3 Ma, onset of cooling; (3) 3.3-0.7 Ma, initiation of full glaciation; (4) <0.7 Ma, maximum glaciation through present conditions. δ18
O is remarkably constant through Iceland’s first ~15 Ma followed by a notable decrease < 0.7 Ma. For all bins >0.7 Ma, the zircon median δ18
O is ~3 ‰ (25th
percentile range: ~2.5 to 3.5 ‰). For <0.7 Ma, the median is ~2 ‰ (25th
percentiles: 1.2 to 3.8 ‰). Zircon from active on-rift Krafla and propagating-rift Torfajökull have medians of 1.6 and 1.3 ‰ respectively, by far the lowest (>1 ‰) of 11 volcanoes in this study (~15 Ma to present). In contrast, active off-rift Öræfajökull and transitional-rift Hekla exhibit higher values (medians: 4.0 and 3.8 ‰) that are similar to those of highest δ18
O volcanic centers through Icelandic history. This on- vs off-rift distinction among rhyolites is consistent with previous interpretations that assimilation/anatexis of hydrothermally altered crust is prevalent on-rift, while fractional crystallization of mantle-derived magma dominates off-rift (e.g., Martin and Sigmarsson 2007; Schattel et al. 2014),
The sudden decline in zircon δ18O in the late Pleistocene in on-rift settings suggests that either (1) petrogenetic processes may be different for these young systems; or (2) the isotopic composition of altered, assimilated, crust has changed. We favor the latter, hypothesizing that as δ18O of meteoric water fell during glaciation, so did the δ18O of hydrothermally altered crust. Rhyolites derived from this altered crust have δ18O that is distinctly lighter than other typical rhyolites in Icelandic history (as evidenced in the zircon record). Late Pleistocene rhyolites generated off-rift lack the low δ18O climate signal, consistent with minimal influence of altered crust.