The generation and evolution of silicic magma and juvenile crust: Insight from the Icelandic zircon record

Abstract

Iceland’s great abundance of silicic rock (10-13% of subaerial exposures), coupled with the unusual thickness of the island’s crust, hints at continental nucleation and permanent crust construction in an oceanic environment. This phenomenon is rare on modern Earth, but it is often postulated that Icelandic magmatism is a modern analogue for ancient (Hadean) crustal construction. Understanding the processes that form abundant, juvenile, silicic crust in oceanic Iceland has important implications for understanding Earth’s early, and ongoing, history.

The Icelandic crust is juvenile; there is neither ancient nor appreciable felsic crust nor sediments to complicate the magmatic history. Investigations into the origins of Icelandic silicic magma are therefore more focused than the global approach to the issue. Despite this, the issue remains highly contentions and hotly debated.

Studies of zircon have elsewhere underpinned understanding of silicic magma processes and crustal evolution, but no such research has been carried out in Iceland until now. This investigation into the origins and evolution of Iceland’s silicic crust is the first comprehensive study of Icelandic zircons. Elemental and isotopic geochemistry and U-Th-Pb geochronology of zircon from intrusive and extrusive rocks and sediments, coupled with supporting bulk-rock elemental and isotope geochemistry and petrography, reveal that: (1) very low 18O/16O supports partial melting of crust hydrothermally altered by meteoric water as a primary driver of Icelandic felsic petrogenesis; (2) uniformity of O isotopic compositions demonstrates neither a change in magmatic processes nor a change in the meteoric-water composition imprinted on magmas throughout Iceland’s history, despite extreme climate variability; (3) increased variability in Hf isotope ratios suggest changes to Iceland’s mantle source with time, (4) Iceland is not an appropriate modern analogue for the Hadean, as their zircon records are quite distinct; (5) zircons reflect tens of thousands of years of activity preceding individual rhyolitic eruptions, and indicate the longevity of silicic magma systems at individual central volcanoes may reach three million years; (6) Icelandic zircons form a compositional category that is distinguishable from typical continental and oceanic zircon populations.