ICELANDIC RHYOLITE GENERATION INFLUENCED BY GLACIAL CLIMATE IN THE LATE PLEISTOCENE: EVIDENCE FROM OXYGEN ISOTOPES AND AGES IN THE ZIRCON RECORD

Iceland is distinctive among oceanic islands for its thick crust, magmatic productivity, abundant silicic magmatism, and isotopically light oxygen compositions of rocks, minerals, and magmas. Zircon provides a robust record of O isotope compositions of melts from which it crystallizes, as it is resistant to alteration and re-equilibration. We use zircon to investigate silicic magma genesis and how it may have varied through Iceland’s history (15 Ma – present). Four clear characteristics of Icelandic δ¹⁸O_zrc are evident: [1] a majority of analyses are in the range 2-5‰ (1075 of 1253 analyses); [2] zircon crystals from active volcanoes where tectonic settings are evident reveal that on-rift volcanoes have lighter O compositions than those at off-rift volcanoes (medians: ~1.5 vs 4.0‰, respectively); [3] when binned by age, the δ¹⁸O_zrc median is remarkably consistent at 3.0-3.2‰ from 15 Ma until the late Pleistocene; [4] since ~0.7 Ma, the upper end of the δ¹⁸O_zrc range is unchanged, but the median falls to ~2.0‰ and very low values are far more common (42 of 57 volcanic δ¹⁸O_zrc values ≤ 1 ‰ are < 0.7 Ma). The most drastic change appears to have occurred at ~0.2 Ma and since that time, median on-rift δ¹⁸O_zrc is ~1.0‰. Characteristics [1] and [2] are consistent with previous interpretations that silicic magmas are generated both from juvenile magma by closed system processes (higher δ¹⁸O), and via anatexis or substantial assimilation of altered crust (lower δ¹⁸O; e.g. Martin & Sigmarsson 2010). Characteristic [3] suggests that this has been the case throughout Icelandic history, and that δ¹⁸O of juvenile magmas remained consistent. Characteristic [4] reveals that, as δ¹⁸O of meteoric water fell during glaciation, so did the δ¹⁸O of hydrothermally altered crust involved in on-rift anatexis and assimilation. Late Pleistocene and Holocene silicic magmas formed by closed-system fractionation (e.g., Oraefajökull) retain a relatively heavy signature (δ¹⁸O_zrc ≥ ~4‰), whereas magmas formed by anatexis or extensive assimilation of distinctively low δ¹⁸O crust are now more common (e.g., Krafla, Torfajökull, d¹⁸O_zrc ≤ 1.5‰). The data indicate a lag of ca. 3 Myr between onset of “light” (syn-glaciation) meteoric waters and initial magmatic signal of their effect on the crust.