Crustal magma storage and fractionation of Eyjafjallajökull ankaramites, South Iceland

Abstract

Our understanding of the long-term intrusive and eruptive behaviour of volcanic systems is hampered 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 compositionally diverse macrocrysts, including the most magnesian olivine (Fo$_{88−90}$) and clinopyroxene (Mg\#$_{cpx}$ 89.8) known from Eyjafjallajökull. Olivine-hosted spinel inclusions have high Cr\#$_{spl}$ (52–80) and TiO$_2$ (1–3 wt\%) and low Al2O$_3$ (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 Fo$_{80.7−88.9}$ 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 plagioclase 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.