Jökull - 01.01.2015, Blaðsíða 21

Delimiting Bárðarbunga and Askja volcanic systems with Sr- and Nd-isotope ratios
86Sr/88Sr = 0.1194 with an exponential law to cor-
rect for mass fractionation. Rubidium-87 and 86Kr
interferences were closely monitored and corrected
for following the correction scheme of Konter and
Strom (2014). Over the analytical period the NIST
SRM-987 standard reproduced with a 87Sr/86Sr ratio
of 0.710244 ± 56 (2SD, n = 5) and values reported
here are normalized to a NIST SRM-987 87Sr/86Sr ra-
tio of 0.710248. Neodymium isotope analyses were
corrected for instrumental mass fractionation to the
natural 146Nd/144Nd = 0.7219. For Nd isotope anal-
ysis, the La Jolla Nd isotope standard was used to
bracket each sample. Over the analytical period, it
gave a 143Nd/144Nd ratio of 0.511853±13 (2SD, n
= 5) and all data reported here have been normal-
ized to a 143Nd/144Nd ratio of 0.511858. The mea-
sured 143Nd/144Nd ratio for the La Jolla standard in
Reykjavík, corresponds to a JNd-1 value of 0.512110
(Tanaka et al., 2000).
RESULTS AND COMPARISON WITH
PUBLISHED DATA
The new results (Table 1, Figures 2 and 3) were com-
pared to (i) recent high-precision thermal-ionization
mass spectrometric (TIMS) results of Kokfelt et al.
(2006), Kuritani et al. (2011) and Manning and Thirl-
wall (2014), and (ii) Sr isotope ratios of Halldórs-
son et al. (2008) measured with multi-collection in-
ductively coupled plasma mass spectrometry (MC-
ICP-MS). Encompassing a relatively narrow range
of 87Sr/86Sr (0.70307–0.70324) and 143Nd/144Nd
(0.513024–0.513074) ratios, our results are in good
agreement with published Sr and Nd isotope data
from the ERZ and the NRZ (e.g. Kokfelt et al., 2006,
Halldórsson et al., 2008, Kuritani et al., 2011; Man-
ning and Thirlwall, 2014 and references therein; Fig-
ure 1). However, in order to define Sr- and Nd- isotope
composition characteristic for lavas and tephras from
the Bárðarbunga and Askja volcanic systems, we will
critically evaluate published results in the following
sections. We first note that publications without stan-
dard values are of little value for this study and have
thus been omitted in our compilation.
Bárðarbunga range
In their thorough study of Veiðivötn basalts, Man-
ning and Thirlwall (2014) excluded historical basalts
(Vatnaöldur and Veiðivötn) analysed by Zellmer et al.
(2008), considering that they represent mixed mag-
mas with Torfajökull rhyolites. The results from Zell-
mer et al. (2008) have higher 87Sr/86Sr than those of
Sigmarsson et al. (2000) and Halldórsson et al. (2008)
for the same eruptions and are thus eliminated from
the compilation of Veiðivötn basalts. Indeed, on the
basis of Pb isotopes, Halldórsson et al. (2008) argued
that although most samples from these single fissure
eruptions, revealed values identical to Bárðarbunga,
samples in close proximity to the Torfajökull vol-
canic system have radiogenic Pb isotope ratios, which
represent mixing of magmas of the Veiðivötn fissure
swarm with those of the Torfajökull volcanic system.
Table 1. Analytical results and sample locations. – Mæliniðurstöður samsætuhlutfalla og staðsetning sýna.
Sample # Longitude (W) Latitude (N) 87Sr/86Sr ±2 SE 143Nd/144Nd ±2 SE
Fl-1 16.850 64.873 0.703118 7 0.513059 5
Fl-2 16.851 64.864 0.703112 6 0.513069 4
Fl-3 16.804 64.885 0.703115 6 0.513068 6
JPR030914-4 16.840 64.840 0.703122 15 0.513072 9
HRW-04 16.831 64.872 0.703105 15 0.513074 15
SAL-312 17.538 64.692 0.703070 16 0.513074 15
NAL-664 17.087 64.271 0.703089 14 0.513058 10
Urd-1 17.081 64.818 0.703220 7 0.513027 3
Gig-1 16.954 64.858 0.703246 7 0.513024 4
Internal errors are given on the last significant digits.
JÖKULL No. 65, 2015 21