Date of Degree
MS (Master of Science)
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 are also erupted through older crust in several off-rift settings, including the Snæfellsnes Peninsula in western Iceland where the basement is formed by 6-8 Ma flood basalts. In this study, we are investigating how these off-rift magmatic plumbing systems compare to those in the main rift zones, given the significant differences in crystal structure and degree of crustal extension, through application of quantitative textural analysis and mineral geothermobarometry. Our focus is Vatnafell, a sub-glacial eruptive unit (414 ± 11 ka) at the western end of the off-rift Ljósufjöll volcanic system in the Snæfellsnes 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 olivine gabbroic/wehrlitic 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). More extensive analyses of mineral compositions have been used to calculate residence times and ascent rates of the crystalline cargo, and indicate rapid ascent soon after the incorporation of the wehrlitic cumulate. These data have been used to develop a more complete picture of an extension-limited off-rift magmatic plumbing system and allow a more detailed comparison with plumbing systems beneath the extension-dominated main rift zones on Iceland.
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 addressed, is whether the magmatic plumbing systems are significantly different in the off-axis zones compared to main rifts, given the different tectonic environment (e.g. minimal crustal extension, thicker lithosphere, lower geothermal gradient, lower magmatic productivity, and a different crustal structure). A sub-glacial unit (Vatnafell) on Snaefellsnes was chosen for study in detail, because it contains porphyritic samples (CPX, OL, & PLAG), and fresh hyaloclastite glass. These allowed for detailed reconstruction of magma ascent paths and magma chamber depths using mineral based geothermobarometry (CPX) and pressure estimates of glass in equilibrium with CPX, OL, and PLAG. The resulting calculations revealed a crystallization depth of ~25 km which corresponds with the base of the crust in this region of Iceland. A lack of shallow magma reservoirs suggests an incorporation of a crystalline cargo at the base of the crust, followed by a rapid ascent and subsequent sub-glacial eruption.
Copyright 2015 David Burney