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Off-Rift Subglacial Volcanism in Iceland\'s Western Volcanic Zone: Hvalfell Analysis.

Birta Blöndal

Abstract

Hvalfell, located in the Western Volcanic Zone (WVZ) of Iceland, formed during the Weichselian glaciation and provides a case study for subglacial volcanic processes. This research evaluates magma evolution, volatile contents, and environmental conditions during its formation through field mapping, geochemical analyses, and comparative studies with nearby formations, including Ármannsfell, Mosfell, and Sandfell í Kjós, to better understand interactions between volcanic and glacial processes. Field data showcases a transitions from subglacial pillow lavas to subaerial lava flows, suggesting a change in eruption dynamics. Observations indicate Hvalfell lies on an older unit of pillow basalts, with another nearby pillow basalt unit that may be geologically connected, separated by a fault system. Evidence of southwest-directed glacial flow is supported by deposits in the gully leading from Hvalfell. Electron microprobe (EMPA) analyses reveal variations between two distinct magmas. The older unit shows higher MgO (up to 9.0 wt%) and lower K₂O, indicating a more primitive melt, whereas Hvalfell's glass exhibits lower MgO (7.75 wt%) and higher K₂O, suggesting more evolved magma. Fourier transform infrared (FTIR) analyses of volcanic glasses provide additional insights into volatile contents, with data indicating melt compositions and degassing behavior consistent with subglacial eruptive environments. Comparative geochemical data reveal significant differences in mantle-derived melts and magma storage conditions. Rare earth element (REE) analyses show enrichment in light REEs and inclined patterns (La/LuN > 2.0), indicative of partial mantle melting with limited crustal assimilation. These findings align with models of off-rift magmatism influenced by localized mantle anomalies.