Jökull - 01.01.2015, Blaðsíða 20

Sigmarsson and Halldórsson
10 km3 of magma erupted during historical time (e.g.
Thordarson and Larsen, 2007). The southwestern part
of the Bárðarbunga volcanic system, referred to as the
Veiðivötn fissure segment, has been subject to a num-
ber of studies in the past (e.g. Halldórsson et al., 2008;
Zellmer et al., 2008; Manning and Thirlwall, 2014
and references therein). However, the northern part
of the Bárðarbunga volcanic system, at the edge of
Dyngjujökull, has been given less attention.
Holocene lava fields north of the Vatnajökull
glacier, have been either attributed to the Askja
or Bárðarbunga systems (e.g. Jóhannesson and Sæ-
mundsson, 1998; Figure 1). For example, the Holu-
hraun lava field has been associated with both Askja
(e.g. Sigurdsson and Sparks, 1978) and Bárðarbunga-
Veiðivötn (Hartley and Thordarson, 2013). The Fjalls-
endahraun (Frambruni) lava field, with an estimated
volume of approximately 4 km3 (Thordarson and
Larsen, 2007) has also been associated with the
Bárðarbunga volcanic system. Finally, a significantly
larger lava, Krepputunguhraun (>7 km3, Thordarson
and Self, 1998), mapped by Sigbjarnarson and col-
leagues (1988; 1995) has also been assigned to the
Bárðarbunga volcanic system whereas Thordarson et
al. (2013) prefer an origin from the Kverkfjöll vol-
canic system. Thus there is a need for an improvement
in determining the provenance of volcanic formations
north of Vatnajökull.
SAMPLES
For the purpose of delimiting lava flow fields at the
intersections of Bárðarbunga and Askja volcanic sys-
tems, we selected (i) a short crater row lying W-E
on the eastern flank of the interglacial lava shield,
Urðarháls (sample Urd-1; Figure 1; unit Urh in Sig-
bjarnarson, 1988), (ii) a large crater row further
east, extending S-SW from Dyngjufjöll forming large
craters named Gígöldur from which a pillow lava frag-
ment was collected (Gig-1; unit Gb in Sigbjarnarson,
1988), (iii) the older Holuhraun (HRW-04, see Hart-
ley and Thordarson, 2013) and (iv) the Fjallsenda-
hraun (or Frambruni) lava field (NAL-664) at Suður-
árbotnar. In addition, we included one sample from a
nunatak on the flank of the Bárðarbunga central vol-
cano (SAL-312). Finally, four samples from the first
three months of the 2014 eruption at Holuhraun (Fl-1,
-2, -3 and JPR030914-4) were included in this study.
All the lava samples are of basaltic to intermedi-
ate composition with a normative composition rang-
ing from 1.68 ol to 6.33 qz in the nunatak (SAL-312),
which is the only lava located within the Bárðarbunga
central volcano (Figure 1). Only a few plagioclase and
clinopyroxene crystals are visible in hand specimens.
The samples are fresh looking without any sign of al-
teration as judged from inspection under the binocular
microscope.
ANALYTICAL METHODS
Lava samples were crushed in stainless steel
(Clermont-Ferrand) or pure Mn-steel (Reykjavík) jaw
crushers before reduction to a fine powder in agate
mill. Five samples (Fl-1-3, Urd-1 and Gig-1) were
processed in Clermont-Ferrand and additional four
(JPR030914-4, HRW-04, NAL-664 and SAL-312) in
Reykjavik using the following methods:
Laboratoire Magmas et Volcans, Clermont-
Ferrand
The powder was dissolved and Sr and Nd isolated us-
ing established methods (Pin et al., 1994; Pin and
Zalduegui, 1997). Their isotope ratio was obtained
on a Triton mass spectrometer in a static mode and
corrected for mass fractionation by normalization to
88Sr/86Sr and 146Nd/144Nd of 0.1194 and 0.7219,
respectively. The international standards NIST-987
and JNd-1 gave 87Sr/86Sr and 143Nd/144Nd equal to
0.710240 ± 10 (2SD, n = 28) and 0.512105 ± 14
(2SD, n = 27), respectively, during the period of anal-
yses. These values are within error of the accepted
standard values.
Institute of Earth Sciences, Reykjavík
Methods adopted in Reykjavík, are similar to those
initially described by Halldórsson et al., (2008) with
modifications and improvements following Marske et
al. (2007) that will be detailed elsewhere. The Sr
and Nd isotopic compositions of samples were mea-
sured using a Nu Plasma multi-collector inductively-
coupled plasma mass-spectrometer (MC-ICP-MS).
Strontium isotope values were normalized to a
20 JÖKULL No. 65, 2015