Jökull - 01.01.2015, Blaðsíða 24

Sigmarsson and Halldórsson
Table 2. Statistics. – Tölfræðileg gögn.
Volcanic system Bárðarbunga Askja
Range in 87Sr/86Sr 0.70307–0.70315 0.70316–0.70326
n 37 23
Median 0.70311 0.70322
Mean 0.70311 0.70322
Standard Deviation 2.22 x 10−5 2.23 x 10−5
Range in 143Nd/144Nd 0.51306–0.51307 0.51301–0.51304
n 15 25
Median 0.51307 0.51303
Mean 0.51307 0.51303
Standard Deviation 1.59 x 10−6 7.74 x 10−6
n denotes number of analyses from this study and published results of Kokfelt et al. (2006), Kuritani et al.
(2011) and Manning and Thirlwall (2014).
DISCUSSION
The irregular geographical variations of 87Sr/86Sr and
143Nd/144Nd in basalts at the northern edge of Vatna-
jökull glacier illustrate the complex overlapping of
eruptive fissures belonging to Bárðarbunga and Askja
volcanic systems. However, a critical evaluation of
published results for Sr- and Nd-isotope composition
in lavas from Bárðarbunga-Veiðivötn and Askja vol-
canic systems reveals marked difference between iso-
tope ratios in products from these volcanic systems.
This allows determining the provenance of both re-
cent and older lava fields in this complex region as
well as discussing the delimitation of these volcanic
systems.
Source provenance of the Holuhraun lava fields
The four 2014–2015 Holuhraun samples and a sam-
ple of the older Holuhraun unit fall within the ranges
of Sr- and Nd-isotope ratios observed in Bárðar-
bunga lavas, strongly suggesting an affiliation with the
magma system of Bárðarbunga. The results confirm
the limited spread of both Sr- and Nd-isotope ratios
in Bárðarbunga-Veiðivötn volcanics. This is further
supported by similar high-field-strength element con-
centrations and ratios of the older Holuhraun unit and
Veiðivötn basalts (Hartley and Thordarson, 2013).
Source provenance of the Krepputunguhraun and
Fjallsendahraun lava fields
The provenance of Krepputunguhraun has been a mat-
ter of debate. Based on the presence of large pla-
gioclase phenocrysts Sigbjarnarson (1988, 1995) pro-
posed an origin from the Bárðarbunga volcanic sys-
tem, possibly from the Gígöldur crater row, whereas
Guðmundsson et al. (2013) preferred an origin at the
Kverkfjöll volcanic system. Kokfelt et al. (2006)
measured its composition in a sample collected at
Hvannalindir. Its Sr- and Nd-ratios are very differ-
ent from those of Gígöldur and Kverkfjöll (Figures
2 and 3; Hémond et al., 1993), but they are well
within the range measured for Bárðarbunga-Veiðivötn
products thus confirming its provenance. In addi-
tion, we note that the large Krepputunguhraun (∼500
km2, Sigbjarnarson, 1988; >7 km3, Thordarson and
Self, 1998), which is approximately 8000 years old,
is partly overlain by the younger Trölladyngja shield
volcano and both these large formations have near-
identical Sr- and Nd-isotope ratios (Kokfelt et al.,
2006).
A single sample from Fjallsendahraun (Fram-
bruni), analysed for Nd isotope ratio by Koornneef et
al. (2012), is similar to those of Askja basalts. How-
ever, our Hf-Pb isotope measurements of this lava
(Halldórsson, unpublished) are indistinguishable from
24 JÖKULL No. 65, 2015