Modeling the magmatic plumbing system beneath an off-rift volcanic deposit on Iceland, using textural analyses and geothermobarometry

Recommended Citation Burney, David. "Modeling the magmatic plumbing system beneath an off-rift volcanic deposit on Iceland, using textural analyses and geothermobarometry." MS ii To Laura and Hazel. Without you this never would have happened. iii ABSTRACT The emplacement of tholeiitic magmas along two NE-SW trending rift zones is the dominant mechanism of crustal accretion on Iceland. Small volumes of transitional to alkaline magmas erupt through older crust in several off-rift settings, including the Snaefellsnes Peninsula in western Iceland where the basement is formed by 6-8 Ma flood basalts. In this study I investigated how these off-rift magmatic plumbing systems compare to those in the main rift zones, given the significant differences in crustal structure and degree of crustal extension, through application of quantitative textural analysis and mineral geothermobarometry. My focus is Vatnafell, a sub-glacial eruptive unit (414 ± 11 ka) at the western end of the off-rift Ljósufjöll volcanic system in the Snaefellsnes volcanic zone. Samples are highly phyric (~14% phenocrysts), with large phenocrysts (1-12 mm) of clinopyroxene, olivine, and plagioclase. Crystal size distributions for olivine and clinopyroxene both show kinked profiles, indicating two distinct populations. Glomerocrysts in which large clinopyroxene oikocrysts enclose smaller rounded olivine chadacrysts are common, and a small horizon strongly enriched in large (> 5 mm) olivine and clinopyroxene crystals was found near the base of the unit. These observations suggest incorporation of wehrlitic and olivine gabbroic cumulates by the host magma. Analyses show a bimodal composition for clinopyroxene (cores: mg# 83-88; rims/groundmass mg# 72-77), and calculations suggest crystallization of cores at or near the Moho in the deep crust (~25 km). Olivine rim diffusion profiles & CSD slopes have been used to estimate residence times and ascent rates of the crystalline cargo, and indicate rapid ascent (weeks-months) soon after the incorporation of the wehrlitic cumulate. These data have shown that the magmatic system beneath Vatnafell, with its lack of shallow magma reservoirs, and Moho depth crystallization iv followed by a rapid ascent and eruption, is unlike magmatic systems present at the main rift zones. v PUBLIC ABSTRACT Magmatism on Iceland is dominated by the eruption of large volumes of tholeiitic basalts at the three major rift zones that represent the locations of plate separation and crustal growth. Minor amounts of transitional to alkaline lavas are also erupted in off-axis zones, notably along the Snaefellsnes Peninsula which is >100 km from the rifts. An important question …