INVESTIGATION OF OXYGEN ISOTOPES IN ICELANDIC ROCKS TO UNCOVER RHYOLITE FORMING PROCESSES

Iceland is the product of voluminous magma generation resulting from the combined influences of a mid-ocean spreading center joined with a mantle hotspot—conditions unique on modern Earth. Due to the combination of both features, Iceland has thick crust and a higher abundance of silicic rock than is normally associated with either ridges or hotspots. However, the processes that lead to silicic magma formation under conditions such as those in Iceland are debated. One of the best ways to distinguish between fractional crystallization and partial melting, the two main processes invoked to explain silicic magma formation on Iceland, is to examine O isotope (δ¹⁸O) values in rocks and minerals. Silicates altered by low-¹⁸O waters inherit a low δ¹⁸O value. There is an apparent shift in δ¹⁸O toward lower values in zircons derived from silicic melts over the last ~3 Myr, coincident with the onset of Northern Hemisphere glaciation. Hyaloclastite, a rock produced when volcanoes erupt under ice or into water, also appeared in Iceland ~3 Myr ago. Due to their hydrous nature, hyaloclastites are more easily melted or assimilated into shallow crust magmas, thereby potentially being a notable contributor to silicic magmas <3 Ma. Hyaloclastites also potentially retain a low δ¹⁸O value due to the hydrous nature of their formation and alteration, and meteoric waters during glacial times have even lower δ¹⁸O than is typical for Iceland during interglacials. To further assess a potential contribution of hyaloclastites to <3 Ma silicic magmas, we collected 39 hyaloclastite samples from across Iceland prioritizing both geographic and chronologic variability. δ¹⁸O values of whole rock and fresh glass fragments were analyzed via laser fluorination at the Northern Illinois University Stable Isotope Lab. Preliminary data suggests that sample glass fragments and whole rock powders are statistically indistinguishable and have a δ¹⁸O value of 5.1±0.2‰ (n=8). Mantle-derived basalt has δ¹⁸O~5.5‰; our data suggest little-to-no contribution of low-¹⁸O material to either the parent magma or from low-¹⁸O waters incorporated syn- or post-eruption. Additionally, surface hyaloclastites do not initially appear to be a contributor to low-¹⁸O silicic magmas. Future findings will further elucidate the role of hyaloclastites in Iceland’s petrogenesis.