Magma storage and transport beneath the near-rift Fjallgarðar Volcanic Ridge, Northeast Iceland

Abstract

The bulk of the Icelandic crust is generated along the extensively studied neovolcanic zones. The subglacially formed Fjallgarðar Volcanic Ridge (FVR) in Northeast Iceland represents extensive near-rift volcanism and covers a large area east of the Northern Rift Zone, yet its magma plumbing system remains poorly understood. This study presents new geochemical data from basaltic pillow lavas and hyaloclastites sampled along the entire FVR, including major element compositions of whole-rock, groundmass glass, and minerals, trace element compositions of whole-rock and glass, and oxygen isotope data for whole-rocks and glasses. Our results suggest that two types of relatively evolved tholeiitic basalts (4.4 to 7.8 wt% MgO) are present along the entire, 200 km-long FVR: one group (Zr/Y = 2.5–4.3) is compositionally similar to basalts erupted in the Northern and Eastern Rift Zone, whereas a second group (Zr/Y = 4.7–5.4) is more enriched and overlaps compositionally with basalts from Kverkfjöll, a volcanic system adjacent to the southern tip of the FVR. These distinct magma types, which are generally similar to fissure basalts erupted in the rift axis, were stored at similar pressures and temperatures (2.2 ± 1.4 kbar, 1108 ± 45 °C), and experienced extensive crystal fractionation and some degree of crustal assimilation. We conclude that the crustal processes did not generate the compositional bimodality but instead, the two groups reflect the chemical variability of their parental mantle-derived melts. We propose that the high-Zr/Y magmas were injected in the FVR via long-distance (> 100 km) lateral dyke propagation from magma reservoir(s) from its southern distal end, and that the low-Zr/Y magmas were supplied from a series of more proximal magma reservoirs distributed along the FVR.