CLIMATE INFLUENCE IMPRINTED ON LATE PLEISTOCENE ICELANDIC RHYOLITES? EVIDENCE FROM OXYGEN ISOTOPES AND AGES IN THE ZIRCON RECORD

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. δ¹⁸O 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 δ¹⁸O is ~3 ‰ (25^th-75^th percentile range: ~2.5 to 3.5 ‰). For <0.7 Ma, the median is ~2 ‰ (25^th-75^th 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 δ¹⁸O 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.