Jökull - 01.01.2017, Blaðsíða 30

Holocene lavas in Bárðardalur, NE-Iceland
La Jolla value of 143Nd/144Nd = 0.511858 (Weis et
al., 2005 and references therein) after correction for
Sm and Ce interference. Following correction for Lu
and Yb interference on 176Hf, measured 176Hf/177Hf
were normalized to a JMC-475 value of 176Hf/177Hf
of 0.282160 (e.g., Weis et al., 2005). Instrumental
mass fractionation for Pb isotopes were corrected us-
ing a 205Ti/203Ti spike, assuming an exponential law
and the constant ratio of 205Ti/203Ti = 2.3889 (White
et al., 2000; Baker et al., 2004). All Pb-isotopic ra-
tios were normalized to the SRM981 values of Baker
et al. (2004): 206Pb/204Pb = 16.9416, 207Pb/204Pb =
15.4998, and 208Pb/204Pb = 36.7249. Values obtained
for both internal and external standards during the
course of this study are given in Table 2. Notably,
multiple measurements (n = 3 to 5) of the interna-
tional rock standard JB-3 are in good agreement with
published values (e.g., Fourny et al., 2016).
RESULTS
Whole Rock Chemistry
Bárðarbunga and eruptive units north of Vatnajökull
Major, minor and trace element compositions of
whole rock samples from the Bárðarbunga basement,
Dyngjuháls, Urðarháls, Gígöldur and Hrímöldur were
compared to published data from Bárðarbunga and
the NRZ (Jakobsson, 1979; Nicholson et al., 1991;
Stracke et al., 2003; Halldórsson et al., 2008; Óla-
dóttir, 2009; Hartley, 2012; Manning and Thirlwall,
2014; Halldórsson, unpublished). Some of the vol-
canic units north of Vatnajökull show large variabil-
ity in representative major, minor and trace elements,
such as K2O vs. MgO and Zr vs. Y (e.g., Bárðarbunga
basement rocks and Gígöldur crater row) (Figures 4a
and c). However, majority of the units sampled near
the Bárðarbunga central volcano range between 5.28–
8.58 wt% in MgO and 0.08–0.19 wt% in K2O, simi-
lar to reported Bárðarbunga-Veiðivötn basalts (Jakob-
sson, 1979; Halldórsson et al., 2008; Óladóttir, 2009;
Manning and Thirlwall, 2014).
The samples collected from the Holocene lava
flows at Dyngjuháls overlap in composition with
the Bárðarbunga basement rocks. Interestingly, five
Dyngjuháls samples are generally more evolved than
the others, having elevated K2O, Zr and Y con-
tents (Figures 4a and c). We also note that sam-
ples from Urðarháls and a single sample available
from Hrímalda fall within the main major- and trace-
element chemical range of Bárðarbunga and Dyngju-
háls. However, two of the Urðarháls samples have
>9 wt% MgO. Urðarháls is the only sampled eruptive
unit of pre-Holocene age.
The Bárðardalur region
Major, minor and trace element compositions of
whole rock and groundmass samples from the Bárð-
ardalur region, were compared to published data from
Bárðarbunga and the NRZ (Figures 4b and d). We
note the primitive nature of the lava flows, with the
groundmass analyses of Kinnarhraun lava display-
ing K2O contents of 0.08–0.09 wt% and MgO con-
tent around 8.5 wt%. It is also noteworthy that four
groundmass samples collected from the Bárðardals-
hraun lava (B15–12, B15–13, B15–14, and B15–15)
are more evolved than other Bárðardalshraun lava
samples, with MgO < 8 wt% and K2O content > 0.1
wt%. A single whole rock sample from the Kvía-
hraun lava has a similar MgO value as the more
evolved Bárðardalshraun samples. On the other hand,
the whole rock samples from the Frambruni and Út-
bruni lavas are among the most evolved rocks col-
lected from the Bárðardalur valley, both having higher
K2O as well as elevated Zr and Y values, but lower
MgO than other whole rock samples. A difference in
the chemical characteristics of groundmass and whole
rock samples from the Kinnarhraun and Bárðardals-
hraun lavas is evident in Figures 4b and d, where
the whole rock analyses clearly have lower MgO, Zr
and Y values compared to the groundmass samples.
Lower values of the whole rock samples compared
to groundmass samples are due to plagioclase accu-
mulation, a characteristic feature of these lava units.
As the Kinnarhraun and Bárðardalshraun lavas con-
tain variable amounts of plagioclase macrocrysts, we
adopt groundmass analyses in further chemical com-
parison instead of whole rock analyses. This approach
will narrow the compositional range of these lavas
in most chemical plots, representing a better candi-
date for the composition of the erupted melt, facil-
itating the use of these analyses as tools to assess
their source provenance.
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