A microprobe study of glass and minerals from the 1975-1984 Krafla Fires, NE Iceland: Implications for multiple magma reservoirs

Abstract

New petrologic and geochemical data from all nine basaltic fissure eruptions of the AD 1975-1984 Krafla Fires rifting episode in NE Iceland are presented in this thesis. Major element geochemistry of volcanic glasses and minerals as well as olivine-plagioclase-augite-melt and clinopyroxene-liquid thermobarometric calculations indicate that the eruptions tapped at least two distinct magma reservoirs: a primitive (MgO = 7.25 ± 0.74 wt.%) deep-crustal reservoir at ~20 km depth and a more evolved and compositionally uniform (MgO = 5.76 ± 0.21 wt.%) shallow-crustal reservoir at 6-10 km depth beneath the caldera. The primitive magma was erupted north of the caldera margin and the more evolved magma was erupted within the caldera itself. Existing geophysical studies of Krafla were reconciled with new geochemical insights to generate a multi-source model for magma supply that involves trans-crustal magma transport and some component of lateral flow. The widely accepted model that the 1975-1984 basaltic fissure eruptions were solely fed from and controlled by lateral flow from a well-mixed upper-crustal reservoir is not consistent with the data presented. This has important implications for volcano monitoring and interpretation of precursory geophysical signs of future eruptions, as well as placing additional constraints on fissure eruption dynamics and mechanisms of crustal accretion at mid-ocean ridges.