Jökull - 01.01.2015, Síða 17

Reviewed research article
Delimiting Bárðarbunga and Askja volcanic systems with
Sr- and Nd-isotope ratios
Olgeir Sigmarsson1,2 and Sæmundur Ari Halldórsson1
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
2Laboratoire Magmas et Volcans, CNRS – Université Blaise Pascal, 63670 Clermont-Ferrand, France
olgeir@hi.is, saemiah@hi.is
Abstract — Volcanic systems represent a fundamental component of the neovolcanic zones in Iceland. They
are composed of a central volcano and a fissure swarm, or a combination of the two. The 2014–2015 rifting
event at the Bárðarbunga volcanic system produced basaltic lava approximately 40 km to the north of the
central volcano, within a fissure swarm commonly attributed to the Askja volcanic system, highlighting the
complex tectonic structure of a region, directly above the Iceland mantle plume. New analyses of Sr- and
Nd-isotope ratios from the new lava (Holuhraun), and the underlying older Holuhraun lava, show that they
have identical values to those of the Bárðarbunga-Veiðivötn lavas and tephra erupted during the Holocene.
Moreover, comparison with published high-precision radiogenic isotope data, reveals that Holocene lavas and
tephra from the Bárðarbunga and Askja systems are characterized by contrasting Sr- and Nd-isotope ratios,
with the notable exception of the Þjórsárhraun lava and two early Holocene lavas from the extreme west and
east of the Veiðivötn fissure swarm. The 87Sr/86Sr and 143Nd/144Nd isotope ratios can thus be utilized to
define the provenance of lava flows north of the Vatnajökull ice cap, ascertaining that the large lava fields
of Krepputunguhraun and Fjallsendahraun (Frambruni) must also have originated within the Bárðarbunga
volcanic system.
INTRODUCTION
The neovolcanic zones in Iceland are composed of
volcanic systems that, in turn, are composed of a
fissure swarm, a central volcano or both (e.g. Sæ-
mundsson, 1978). Examples are the Reykjanes fis-
sure swarms without central volcanoes, Eyjafjalla-
jökull central volcano without an associated fissure
swarm and finally, the Krafla central volcano and its
fissure swarm (e.g. Jóhannesson and Sæmundsson,
1998). Associating any particular eruption unit to
a given volcanic system is of an importance when
discussing eruption frequency and magma production
rates for volcanic systems. It is also central when
forecasting volcanic activity from real-time measure-
ments. However, it is not always straight-forward to
associate a lava flow or a tephra layer to its point of
origin (e.g. Óladóttir et al., 2011), especially when a
large portion of the volcanic system is covered by a
glacier (e.g. Einarsson and Björnsson, 1990). Addi-
tionally, volcanic systems in Iceland have been de-
fined differently over the last decades. The exten-
sive mapping of the Neogene volcanic pile in eastern
Iceland revealed complex architecture of the central
volcanoes with associated dike swarms that together
formed a unity named volcanic system (summarized
in Walker, 1974). Tectonic criteria and fissure map-
ping were applied by Sæmundsson (1978) when dis-
cussing active analogues for central volcanoes and fis-
sure swarms within the neovolcanic zones of Iceland.
A different approach based on major element compo-
sition of basalts and magma suites of each volcano
were utilized by Jakobsson (1979) when defining the
JÖKULL No. 65, 2015 17