Jökull - 01.01.2015, Side 25

Delimiting Bárðarbunga and Askja volcanic systems with Sr- and Nd-isotope ratios
Veiðivötn basalts (Manning and Thirlwall, 2014). In
addition, both Sr- and Nd-isotope of this lava (Table
1) fall within the range of Bárðarbunga-Veiðivötn vol-
canics. Further studies of this large lava are needed
but the Hf isotope data of Koornneef et al. (2012) to-
gether with our Sr and Nd isotope ratios agree with
the tectonic lineation that strongly suggest an origin
from the Bárðarbunga magma system.
Gígöldur and Urðarháls craters: eruptive fissures
associated with Askja?
The single sample of the large Gígöldur crater row
(sample Gig-1) has isotope ratios suggesting an affil-
iation within Askja volcanic system (Table 1, Figures
2 and 3). In addition, the neighbouring W-E trend-
ing eruptive fissure on the eastern side of interglacial
Urðarháls lava shield (sample URD-1) also produced
basalts with isotope ratios akin to Askja volcanic sys-
tem. Further work on these impressive and highly pe-
culiar crater rows is needed before detailed discus-
sion of their origin. In principle, however, this ei-
ther suggests an inter-fingering relationship between
the two volcanic systems, or that the two magmatic
systems at depth erupt through the same tectonic frac-
tures. The results presented here are only based on
some of the most recent products in the region north
of Vatnajökull that tend to support the former proposi-
tion. However, the latter explanation, which may op-
erate on a longer time scale cannot be eliminated yet.
Taken together with the complex tectonic settings in
this region, namely the intersection of the two dom-
inant tectonic lineaments of Iceland (e.g. Jónsson et
al., 1991), it is clear that further studies of older lavas
above the Iceland mantle plume centre would be valu-
able.
Bárðarbunga: the largest magma producer in Ice-
land?
Magma production at the Bárðarbunga volcanic sys-
tem is thought to be amongst the highest in Ice-
land. However, in their compilation of lava vol-
ume estimates over historical time, Thordarson and
Larsen (2007) concluded that Bárðarbunga was only
the fourth highest producer of magma. We note,
however, that when adding the volumes of the Great
Þjórsárhraun and the multiple lava fields of Tungna-
árhraun (e.g. Vilmundardóttir et al., 2000), together
with Krepputunguhraun and Fjallsendahraun to the
shield-volcanoes, such as Trölladyngja, Bárðarbunga
is likely to get the first place as the highest magma
production volcanic system during the Holocene in
Iceland. In addition, there are hints (from Sr- and
Nd isotope ratios; Carpentier and Sigmarsson, unpub-
lished results) that the large (∼5 km3) Bárðardalur
lava flow (Hjartarson, 2004), also belongs to this very
large volcanic system. Finally, improved volumetric
data and age determinations are needed for quantita-
tive treatment and comparison of the magma produc-
tivity between different volcanic systems above the
mantle plume.
CONCLUSIONS
Isotope ratios of Sr and Nd are relatively uniform
for a given volcanic system above the Iceland man-
tle plume and the delineation of these systems can be
readily achieved with radiogenic isotope ratios. The
large Krepputunguhraun and Fjallsendahraun (Fram-
bruni) lava fields, as well as the very recent Holu-
hraun and the older Holuhraun lava field, are shown to
have originated within the Bárðarbunga volcanic sys-
tem. These results amplify the magma productivity at
Bárðarbunga during the Holocene, but also emphasize
the complex inter-fingering of eruption fissures at the
junction of Askja and Bárðarbunga volcanic systems.
Acknowledgement
We thank our colleagues, including several post-docs
and graduate students at IES that aided with sample
collection during the 2014–2015 Holuhraun eruption.
Chantal Bosq and Delphine Auclair helped with the
Sr and Nd isotope analysis. Guðrún Larsen read over
first draft of the manuscript. Rósa Ólafsdóttir pre-
pared Figure 1. Margaret Hartley, Thomas F. Kok-
felt and Bryndís Brandsdóttir provided constructive
reviews and comments. All this is gratefully acknowl-
edged. SAH is especially grateful to Guðrún Sverris-
dóttir and Gylfi Sigurðsson for their hard work and
dedication to the isotope facilities at IES. Financial
support came from the Iceland Science Fund through
the project "Volcano Anatomy" and the Nordic Vol-
canological Centre (Nordvulk) at IES.
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