Jökull

Issue

Jökull - 01.01.2019, Page 84

Jökull - 01.01.2019, Page 84
Mid-crustal storage and crystallization of Eyjafjallajökull ankaramites, South Iceland Paavo Nikkola1,2,∗, Enikő Bali2,3, Maren Kahl4, Quinten H. A. van der Meer2, O. Tapani Rämö1, Guðmundur H. Guðfinnsson2, and Thorvaldur Thordarson3 1Department of Geosciences and Geography, Geology and Geophysics Research Group, P.O. Box 64, FI-00014 University of Helsinki, Finland 2Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland 3Faculty of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland 4Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany ∗Correspondence: paavo.nikkola@helsinki.fi https://doi.org/10.33799/jokull2019.69.083 Abstract — Our understanding of the long-term intrusive and eruptive behaviour of volcanic systems is ham- pered by a relatively short period of direct observation. To probe the conditions of crustal magma storage below South Iceland, we have analysed compositions of minerals, mineral zoning patterns, and melt inclusions from two Eyjafjallajökull ankaramites located at Brattaskjól and Hvammsmúli. These two units are rich in composi- tionally diverse macrocrysts, including the most magnesian olivine (Fo88−90) and clinopyroxene (Mg#cpx 89.8) known from Eyjafjallajökull. Olivine-hosted spinel inclusions have high Cr#spl (52–80) and TiO2 (1–3 wt%) and low Al2O3 (8–22 wt%) compared to typical Icelandic chromian spinel. The spinel-olivine oxybarometer implies a moderate oxygen fugacity of ∆logFMQ 0–0.5 at the time of crystallization, and clinopyroxene-liquid thermobarometry crystallization at mid-crustal pressures (1.7–4.2 kbar, 3.0±1.4 kbar on average) at 1120– 1195◦C. Liquid-only thermometry for melt inclusions with Mg#melt 56.1–68.5 and olivine-liquid thermometry for olivine macrocrysts with Fo80.7−88.9 yield crystallization temperatures of 1155–1222◦C and 1136–1213◦C, respectively. Diffusion modelling of compositional zonations in the Brattaskjól olivine grains imply that the Brattaskjól macrocrysts were mobilized and transported to the surface from their mid-crustal storage within a few weeks (at most in 9–37 days). Trends in clinopyroxene macrocryst compositions and the scarcity of plagio- clase indicate that the mid-crustal cotectic assemblage was olivine and clinopyroxene, with plagioclase joining the fractionating mineral assemblage later. In all, the crystal cargoes in the Brattaskjól and Hvammsmúli ankaramites are composed of agitated wehrlitic or plagioclase wehrlitic crystal mushes that crystallized over a large temperature interval at mid-crustal depths. INTRODUCTION In South Iceland, at the southern tip of the Eastern Volcanic Zone (SEVZ), magmatism occurs outside the main zone of plate spreading in three volcanic systems: Eyjafjallajökull, Katla, and Vestmannaeyjar. The SEVZ is the most recently activated volcanic zone in Iceland (younger than 3 Ma; Martin et al., 2011), where mantle-derived magmas intrude rela- tively cold oceanic crust (Flóvenz and Saemunds- son, 1993). Magma batches fractionate comparatively fast under these conditions, as indicated by U-series disequilibria (Sigmarsson, 1996) and the absence of equilibrium phenocryst assemblages (Mattsson and Oskarsson, 2005) in erupted lavas. In Vestmanna- eyjar, the mantle-derived melts have been envisioned to evolve in the crust in isolated, small magma reser- voirs over a large depth range (Furman et al., 1991; Mattsson and Oskarsson, 2005). Seismic, geodetic and petrogenetic studies of the Eyjafjallajökull 2010 eruption have highlighted a multi-tier volcanic plumb- JÖKULL No. 69, 2019 83
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