Jökull - 01.01.2017, Page 41

Svavarsdóttir et al.
same, relatively narrow range. Taken together, this
confirms that the lavas found in Bárðardalur valley are
compositionally similar to those of the Bárðarbunga
central volcano.
Postglacial craters and lava flows from Dyngju-
háls display a narrower chemical range than the Bárð-
arbunga basement rocks (Figure 9c and 9d), at least
partly due to the almost aphyric character of the
Dyngjuháls lava units. Notably, analyses of Fram-
bruni lava fit well within the range defined by the
Dyngjuháls lavas, suggesting that Frambruni origi-
nates from the same source. The same applies to
the Kvíahraun lava, that reveals TiO2-MgO and Zr-Y
characteristics that closely resemble Dyngjuháls prod-
ucts.
Along with whole rock and groundmass analyses
of lavas in Bárðardalur, we plot whole rock analyses
of the volcanic units Urðarháls, Hrímalda, and Gíg-
öldur in Figures 9e and f. Most Urðarháls samples
fall within the more primitive range of the Dyngju-
háls samples and trend towards high MgO content.
Although the Urðarháls lavas have high MgO values,
similar to samples from Kinnarhraun and Bárðardals-
hraun lavas, they reveal distinct major and trace el-
ement trends. A single sample from Hrímalda has
nearly identical chemical characteristics as the Bárð-
arbunga lava products. Notably, a highly evolved sam-
ple from Gígöldur (NAL–451) falls within the major
and trace element range of the Askja volcanic sys-
tem (Hartley, 2012). This observation is in agree-
ment with the Nd and Sr isotopic analyses reported by
Sigmarsson and Halldórsson (2015), who suggested
an Askja connection for Gígöldur. However, we note
that the majority of samples from Gígöldur closely re-
semble products that we infer to be related to Bárð-
arbunga, including Frambruni and some Dyngjuháls
lavas. Thus, the Gígöldur crater field is a special case
in this region as its products appear to be related to the
magmatic systems of both Askja and Bárðarbunga.
Source provenance of Bárðardalur lavas: Isotopic
constrains
In the following chapter, the source area of the lavas
from Bárðardalur will be investigated by comparing
their isotopic signature to the isotopic signatures of
both Askja and Bárðarbunga. Published data from
these volcanic systems were evaluated systematically,
following various filtering criteria, and we have only
included data that we deem to be of high-quality and
from well characterized volcanic units and therefore
representative for these volcanic systems. This is par-
ticularly important in the case of Pb isotopes and we
maintain that by considering recent Pb-isotopic data
only, insights into the source province of the lavas
from Bárðardalur can be obtained (see details in Svav-
arsdóttir, 2017).
Askja
Most of the published isotopic analyses of samples
from the Askja volcanic system are of historical lavas
within the caldera that display limited variation. In
Figure 10a, it becomes evident the Sr and Nd isotopic
signature of Askja products is distinct from the sig-
nature of nearly all the volcanic units included here.
This difference becomes even more distinct when Pb-
isotope ratios are plotted (Figure 10d), but the lack of
Hf isotope data for Askja product precludes compar-
ison with Bárðarbunga products. We therefore con-
clude on the basis of major and trace element data,
but particularly on the basis of radiogenic isotopes,
that the Askja volcanic system can be excluded as a
likely eruptive source of the lavas in Bárðardalur.
Bárðarbunga
In Figure 10a-d, we plot the isotopic composition of
Bárðarbunga products versus the lavas of Bárðardalur
valley, Bárðarbunga basement rocks, and other vol-
canic units north of Vatnajökull. It is apparent that
most samples of this study plot within or near the iso-
topic space of the Bárðarbunga products. The field
of published Pb-isotopic data from Bárðarbunga pro-
vides a close fit to most of the Pb-isotopic range de-
fined by the lavas from Bárðardalur valley (Figure
10d). However, it is also evident that the Pb-isotopic
variability is larger in the dataset presented here rela-
tive to previously published data that are associated
with the Bárðarbunga volcanic system. In this re-
gard, it should be noted that prior to this study, pub-
lished Pb-isotopic ratios from eruptive units associ-
ated with the Bárðarbunga volcanic system were ex-
clusively from Holocene products from the Veiðivötn
region (e.g., Thirlwall et al., 2004; Halldórsson et al.,
36 JÖKULL No. 67, 2017