Jökull - 01.01.2015, Síða 23

Delimiting Bárðarbunga and Askja volcanic systems with Sr- and Nd-isotope ratios
Figure 3. Histogram of Nd isotope ra-
tios in same basalts as in Figure 2. –
Súlurit fyrir Nd samsætuhlutfall í sömu
basöltum og á 2. mynd
and tephra are given in Table 2, excluding the three
old lavas from the Veiðivötn area. Restricted spread
in isotope composition is evident with 87Sr/86Sr
and 143Nd/144Nd in the narrow ranges of 0.70307–
0.70315 and 0.51306–0.51307, respectively.
Askja range
When compiling published results for the Askja vol-
canic system, it becomes clear that three samples have
most likely been erroneously associated with Askja.
These are samples H25, collected along the track
from Hrossaborg to Herðubreiðarlindir, just south of
Ferjuás, and H24, collected east of Herðubreiðartögl
(Kokfelt et al., 2006). Both these are olivine tholei-
ites that display the highest 87Sr/86Sr and lowest
143Nd/144Nd of all the analyses examined here and
major element composition significantly less evolved
than the Askja basalts (Kokfelt et al., 2006). Their
origin remains unknown but we note that they have
near identical values to those reported from Kverk-
fjöll by Hémond et al. (1993). A single Mg-rich and
relatively old (>6000 a) basalt (sample# IA1709) col-
lected just south of Herðubreiðartögl by Kuritani et al.
(2011) is of unknown origin, displaying both low and
high 87Sr/86Sr and 143Nd/144Nd, respectively. Be-
fore associating these curious basalts to Askja, a more
thorough study of the Dyngjufjöll basalts is needed.
Alternatively, the Askja basalts exhibit a much larger
range in radiogenic isotope ratios than any other vol-
canic system studied so far in Iceland. Excluding
these three anomalous basalts, Table 2 gives the range,
median, mean and standard deviation for both Sr and
Nd isotope ratios for Askja. The respective ranges are
0.70316–0.70326 and 0.51301–0.51304, only slightly
but nevertheless significantly different from those ob-
served for Bárðarbunga volcanic system.
JÖKULL No. 65, 2015 23