HISTORY OF SILICIC MAGMATISM IN ICELAND ILLUMINATED BY ISOTOPES (U-PB, U-TH, O, HF) IN THE ZIRCON RECORD

Zircon, a robust geochronometer and recorder of magmatic evolution, is especially valuable in Iceland where high heat flux and hydrothermal alteration complicate investigations of silicic melts. Using whole rock Hf and Nd isotopes and in situ zircon U-Pb and U-Th ages, O isotopes, and Hf isotopes, we examine contributions from crustal (O) and mantle (Hf, Nd) sources to silicic magmas through time. We analyzed >1000 zircons from 9 volcanic, 6 intrusive, and 10 detrital systems, covering almost all of Iceland’s history (15.6 to 0 Ma) and tectonic settings. Igneous samples are used for focused studies; detrital zircons provide a broad survey of Iceland’s rock record of magmatic history. Zircon ε_Hf (n = 336) ranges from +9 to +17, and correlates with tectonic setting in active volcanic systems (+13 to +16 on rift; +10 to +13 off rift). Whole rock ε_Nd and ε_Hf (and zircon ε_Hf in igneous samples) are positively correlated, falling within and below the lower portion of the range for Icelandic basalts (+11 to +19). Zircon ε_Hf ranged from ~ +13 to +17 early in Iceland’s early history (16-11 Ma), and by ~10 Ma minimum ε_Hf extended to +12. Around 7 Ma, this range began expanding toward even lower values, but maximum values remained consistent. During the late Pleistocene, ε_Hf has ranged from +9 to 17. Our data suggest Miocene zircons record a depleted mantle source, while a new, less depleted mantle source began contributing ~7 Ma. Alternatively, the lower- ε_Hf end member from early Icelandic history became more important after that time. Oxygen isotopes (zrc; n = 753) have a mean δ¹⁸O of 3.0 ‰. Approximately 98% of analyses fall below the δ¹⁸O range of mantle magma-equilibrated zircon (+5.3 ± 0.3 ‰); 90% of analyses fall between +0.2 and 4.7. While low δ¹⁸O does not preclude fractional crystallization of basalt contributing to silicic melt generation, it requires an influence of partial melting or assimilation of crust hydrothermally altered by meteoric water throughout Iceland’s subaerial history. Intriguingly, unlike Hf, O is not correlated with time in the Icelandic zircon record. Regardless of timing of zircon growth with respect to surface conditions and presumed variation in meteoric δ¹⁸O (~16 Ma: temperate climate; ~7 Ma: onset of cooling, ~2.5 Ma: full glaciation, ~0 Ma: modern conditions), the average zircon δ¹⁸O has remained ~3.0 ‰.