Petrogenesis of the Kverkfjöll volcanic system

Kverkfjöll is a major, but understudied volcanic system located in the far highlands of Iceland, at the southern end of the Northern Rift Zone. It is defined by a 1943 m high central volcano partly covered by the Vatnajökull Ice Cap, and a connecting fissure swarm that extends 60 km to the NNE and has a 20 km subglacial limb to the SW. Located between the main rift axis and the off-rift Öræfajökull volcanic belt, Kverkfjöll is an example of rift flank volcanism. The volcanic system has been erupting basalts for probably a few hundred thousand years. The typical eruptions are relatively small fissure eruptions on the order of 0.1-0.5 km³ , but two large calderas below the ice, measuring roughly 3 x 8 km may indicate larger eruptions in the past. The last confirmed eruption took place about 1300 years ago forming the Lindahraun lava. Because of its remote location and small eruptions, the major hazard associated with Kverkfjöll are subglacial eruptions that have been shown to trigger major jökulhlaups (glacial outburst floods) along the Jökulsá á Fjöllum river all the way to the north coast.

Some unsolved problems are related to Kverkfjöll. A volcanological pecularity that is not found in many places on Earth is the occurrence of explosive fissure eruptions (usually fissure eruptions are effusive). A geodynamical and petrological question is related to the asymmetrical occurrence of volcanism around the Northern Rift Zone: Why is there almost no volcanism on the western flank of the rift, but on the eastern flank, a 200 km arc of Pleistocene volcanics stretches from Vatnajökull and Kverkfjöll to the north coast of Iceland?

However, for a geochemist, the most interesting aspect of Kverkfjöll are its unusually volatile-enriched basalts with a Pb isotopic signature that is unique in Iceland. This indicates that the the Kverkfjöll basalts are tapping a mantle component that is not seen in Icelandic rift volcanoes. Surprisingly, no prior petrological studies have been published on Kverkfjöll.

The aim of the project is to (1) define the geochemistry of Kverkfjöll, (2) understand the conditions of magma generation at a rift-flank setting, and (3) investigate a unique, volatile-rich mantle source that feeds the system.

Approach. We focused on a a sample set (n = 70) of subglacial pillow lavas and hyaloclastites. These have been collected during several field campaigns during the last 30 years, including our own in 2017. Our dataset includes major element data on glasses, plagioclase, clinopyroxene, olivine, spinel and magnetite (microprobe), glass trace elements (LA-ICP-MS) as well as glass radiogenic (Sr-Nd-Hf-Pb) (MC-ICP-MS) and oxygen isotopic data (LF-IRMS). We also analyzed olivine- and spinel-hosted melt inclusions. The data is used to (1) constrain the depth, temperature and oxygen fugacity of associated magma reservoirs, (2) construct a model of magma genesis and evolution, (3) determine the involved mantle sources.