Jökull - 01.01.2017, Blaðsíða 22

Reviewed research article
Geochemistry and petrology of Holocene lavas in the
Bárðardalur region, N-Iceland.
Part I: Geochemical constraints on source provenance
Sigríður Inga Svavarsdóttir, Sæmundur Ari Halldórsson? and Guðmundur Heiðar Guðfinnsson
NordVulk, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland
?Corresponding author, saemiah@hi.is
Abstract — Because of extensive volcanic production in Iceland during the Holocene, many of the early post-
glacial large-volume fissure lavas cannot be unambiguously traced to their eruptive craters solely by obser-
vations in the field. For example, the Bárðarbunga volcanic system has been suggested as a likely source of
the large Holocene lava flows found in Bárðardalur valley, but this idea mainly relies on petrographic obser-
vations. We conducted a chemical and isotopic study of the lavas in Bárðardalur. For comparative purposes,
we also targeted basement rocks of the Bárðarbunga central volcano, as well as several eruptive units in the
region north of Vatnajökull. Based on a comparison of chemical and radiogenic isotope data of lavas from the
Bárðardalur region and the eruptive units north of Vatnajökull and the Bárðarbunga central volcano, it appears
most likely that the lavas of Bárðardalur valley belong to the Bárðarbunga volcanic system. These new data,
and a compiled dataset for other selected volcanic systems of the NRZ, shed light on possible limitations when
assigning erupted material to its source volcano by means of chemical composition. Furthermore, this study
demonstrates that our understanding of the relative importance of the different processes at play during the
petrogenesis of Icelandic basalts is likely to be greatly improved by multi-parameter datasets for geologically
well-characterized eruptive units.
INTRODUCTION
Field observations indicate that eruption rates within
the neovolcanic zones in Iceland may have been up
to 30 times greater during periods of rapid deglacia-
tion in early postglacial times relative to the current
ones (e.g., Maclennan et al., 2002; Sinton et al., 2005;
Sims et al., 2013). This enhanced magmatic activ-
ity has been explained by (1) increased decompres-
sion melting in the mantle associated with the disap-
pearance of the ice load (Sigvaldason et al., 1992;
Jull and McKenzie, 1996) (2) extensive tapping of
crustally-stored magma due to changes in the stress
field as a result of rapid uplift (Guðmundsson, 1986).
Early postglacial lavas are volumetrically dominant
features in many volcanic regions of Iceland, particu-
larly within the Bárðarbunga volcanic system and the
Northern Rift Zone (NRZ) (Vilmundardóttir, 1977;
Jakobsson, 1979; Hjartarson, 2006; Thordarson and
Larsen, 2007). During the period between ∼9000–
6000 years BP, eruption rates in the region of the
Bárðarbunga-Veiðivötn fissure swarm were excep-
tionally high, resulting in the production of some of
the largest Holocene lava flows in Iceland (Figure 1),
including the ∼8600 years BP Þjórsárhraun lava (see
box 2 in Figure 1) (e.g., Hansen and Grönvold, 2000;
Hjartarson, 2006; Halldórsson et al., 2008), with an
estimated minimum volume of 25 km3 (Hjartarson,
2011). Another large lava field, which might repre-
sent an early postglacial volcanic pulse from the Bárð-
arbunga volcanic system, can be found in Bárðardalur
valley in North Iceland (see box 1 in Figure 1) (Sig-
bjarnarson, 1988; Vilmundardóttir and Kaldal, 1991;
Hjartarson and Kaldal, 2004; Hjartarson, 2004).
JÖKULL No. 67, 2017 17