Origin of high whole-rock δ18O values in rhyolites from northeast Iceland

The origin of silicic magma in basalt-dominated oceanic settings is fundamental to our understanding of magmatic processes and formation of the earliest continental crust. Particularly significant is magma-crust interaction that can modify the composition of magma and the dynamics of volcanism. This thesis investigates silicic magma genesis on different scales in two ocean island settings. First, volcanic products from a series of voluminous Neogene silicic centres in northeast Iceland are investigated using rock and mineral geochemistry, U-Pb geochronology, and oxygen isotope analysis. Second, interfacial processes of magma-crust interaction are investigated using geochemistry and 3D X-ray computed microtomography on crustal xenoliths from the 2011-12 El Hierro eruption, Canary Islands.The results from northeast Iceland constrain a rapid outburst of silicic magmatism driven by a flare of the Iceland plume and/or by formation of a new rift zone, causing large volume injection of basaltic magma into hydrated basaltic crust. This promoted crustal recycling by partial melting of the hydrothermally altered Icelandic crust, thereby producing mixed-origin silicic melt pockets that reflect the heterogeneous nature of the crustal protolith with respect to oxygen isotopes. In particular, a previously unrecognised high-δ18O end-member on Iceland was documented, which implies potentially complex multi-component assimilation histories for magmas ascending through the Icelandic crust. Common geochemical traits between Icelandic and Hadean zircon populations strengthen the concept of Iceland as an analogue for early Earth, implying that crustal recycling in emergent rifts was pivotal in generating Earth’s earliest continental silicic crust.Crustal xenoliths from the El Hierro 2011-2012 eruption underline the role of partial melting and assimilation of pre-island sedimentary layers in the early shield-building phase of ocean islands. This phenomenon may contribute to the formation of evolved magmas, and importantly, the release of volatiles from the xenoliths may be sufficient to increase the volatile load of the magma and temporarily alter the character and intensity of an eruption.This thesis sheds new light on the generation of silicic magma in basalt-dominated oceanic settings and emphasises the relevance of magma-crust interaction for magma evolution, silicic crust formation, and eruption style from early Earth to present.