Jökull - 01.01.2019, Síða 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