Jökull - 01.01.2017, Side 33

Svavarsdóttir et al.
within a narrow range of 0.70305–0.70315, except
one outlier sample from Dyngjuháls (NAL-486) with
87Sr/86Sr ratio of 0.70322. Similarly, three sam-
ples from the Gígöldur crater row also display high
Sr-isotopic ratios (0.70320). Basement rocks of the
Bárðarbunga central volcano display 143Nd/144Nd ra-
tios between 0.51305–0.51310 and samples from Gíg-
öldur and Dyngjuháls fall mostly within this range
(Figure 5a). However, a single sample from the
Dyngjuháls region (NAL–491) has somewhat higher
143Nd/144Nd ratio of 0.51313. This sample is unique
among the samples collected in this region in hav-
ing the highest MgO value of 8.17 wt%. In general,
published radiogenic isotope data from Bárðarbunga
and the NRZ follow a negative trend in 143Nd/144Nd–
87Sr/86Sr space (Figure 5a). Offsets from this general
trend, as evident in the case of the samples NAL–491,
NAL–495 and SAL–309, can be explained by sec-
ondary alteration processes following emplacement,
which may act to shift the ratios toward more radio-
genic Sr-isotopic values, which may be at least partly
influence by seawater (e.g., Hémond et al., 1993).
Such modifications can be difficult to avoid for Sr iso-
topes, but should not affect Nd isotopes, as Nd, being
relatively fluid-immobile, is largely unaffected by sec-
ondary alteration processes.
Hafnium isotopes have been measured in only
five samples from the Bárðarbunga volcanic system,
and published Hf isotope analyses from the NRZ
are mostly of Þeistareykir lavas (e.g., Stracke et al.,
2003). Hafnium isotopic ratios of the eruptive units
north of Vatnajökull fall within a narrow 176Hf/177Hf
range (0.28320–0.28323; Figure 5c and e). As Hf-
Nd isotopes generally follow a strong positive trend
in oceanic basalts (White, 2015), the slightly elevated
143Nd/144Nd ratios of samples NAL–491 and NAL–
495 become apparent (Figure 5e). We note, how-
ever, that such offsets have been reported previously
for Icelandic basalts, e.g., from Þeistareykir (Stracke
et al., 2003), Krafla (Koornneef et al., 2011; Sims
et al., 2013) and Bárðarbunga (Manning and Thirl-
wall, 2014), suggesting that analytical issues are an
unlikely cause for these offsets.
Lead isotopic ratios (208Pb/204Pb vs. 206Pb/204Pb)
of eruptive units north of Vatnajökull (Figure 6b and
Table 2) are also in good agreement with recently
published data from Bárðarbunga and the NRZ (See
captions for Figure 6 for references). The same ap-
plies to 207Pb/204Pb ratios (not shown here, see de-
tails in Svavarsdóttir, 2017) which have been found
to be highly variable in rocks from these regions. As
basalts from Þeistareykir are significantly less radio-
genic than remaining data from the NRZ, they are ex-
cluded in further comparison. We note that sample
NAL–486 has slightly elevated 208Pb/204Pb ratio of
38.207 (Figure 6b), and somewhat elevated Sr iso-
topic ratio (0.70322), but that these values are con-
sistent with published data.
The Bárðardalur region
Generally, the Sr-, Nd- and Hf-isotopic ratios of the
Bárðardalur lava flows overlap those of the published
data from Bárðarbunga (Figures 5b, d and f). In de-
tail, 87Sr/86Sr ratios of groundmass and whole rock
samples from the Kinnarhraun and Bárðardalshraun
lavas lie within a narrow range (0.70306-0.70309), ex-
cept for two groundmass samples, B15–02 and B15–
10 that display significantly higher 87Sr/86Sr ratios
(up to 0.703497; Figure 5d, note difference in x-axis
scale on Figure 5b and 5d). The fact that plagioclase
separates from these samples display 87Sr/86Sr ratios
which are within the range of other samples from the
Bárðardalur region and since sample B15–10 was an
unusually weathered sample, suggests that modifica-
tions of primary values have taken place (see a com-
panion paper and Svavarsdóttir, 2017). The single
sample analyzed from the Kvíahraun lava (B15–01)
displays similarly elevated Sr-isotopic ratios suggest-
ing a different source. The 87Sr/86Sr obtained for the
historical Frambruni lava (B15–05) is in good agree-
ment with 87Sr/86Sr ratio published for this lava (i.e.,
Sigmarsson and Halldórsson, 2015). A single sam-
ple reported from Útbruni lava is within the typical
87Sr/86Sr range reported for Kinnarhraun and Bárðar-
dalshraun lavas.
Neodymium isotopic ratios were measured in
three samples (B15–02, B15–19, B15–16) from the
Kinnarhraun and Bárðardalshraun lavas, and fall
within a narrow 143Nd/144Nd range of 0.51309–
0.51310. We note the rather large difference between
143Nd/144Nd ratios of samples from Frambruni deter-
28 JÖKULL No. 67, 2017