Jökull - 01.01.2005, Síða 64

Bergrún Arna Óladóttir et al.
Figure 5. Major-elements plotted against MgO. The compositional fields of basalt from Hekla (shaded dark
grey), Grímsvötn (light grey) and Veiðivötn (medium grey) volcanic systems are shown for comparison. Data
is from Jakobsson (1979), Steinþórsson (1977), Vilmundardóttir (1977), Óskarsson et al., (1982), Grönvold
and Jóhannesson (1984), Steinþórsson et al., (1985), Sigmarsson et al., (1992) and Thordarson et al., (1996,
2001, 2003). Katla data points from this study are shown as diamonds and the error bars are indicated when
the bar is larger than size of the data points. – Graf sem sýnir aðalefni forsögulegra og sögulegra Kötlugjósku-
laga (bláir tíglar) sem fall af magnesíumoxíði. Til samanburðar er sýnd efnasamsetning basalts frá Heklukerfi
(dökkgrátt svæði), Grímsvatnakerfi (ljósgrátt) og Veiðivatnakerfi (milligrátt), samkvæmt efnagreiningum Sveins
Jakobssonar (1979), Sigurðar Steinþórssonar (1977), Elsu Vilmundardóttur (1977), Níelsar Óskarssonar og
fl. (1982), Karls Grönvolds og Hauks Jóhannessonar (1984), Sigurðar Steinþórssonar og fl. (1985), Olgeirs
Sigmarssonar og fl. (1992) og Þorvaldar Þórðarsonar og fl. (1996, 2001, 2003).
Major-element composition
Several geochemical studies have been conducted on
Katla lava and tephra over the last four decades (Rob-
son and Spector, 1962; Sigvaldason and Óskarsson,
1976; Jakobsson, 1979; Einarsson et al., 1980; Larsen
et al., 2001; Lacasse et al., 1995; Thordarson et al.,
2001, 2003). These studies present both whole-rock
and glass analyses using different methods (i.e. wet
chemistry, XRF and electron microprobe) in a range
of laboratories, yielding major-element composition
of rather homogeneous and evolved basalts rich in Fe
and Ti. In Figures 5 and 6 are displayed the pub-
lished compositional variability of basalts from the
Eastern Volcanic Zone together with the results of this
study. The comparison of our results with the pub-
lished Katla data confirms that 111 out of 126 ana-
lysed layers originated from the Katla volcanic sys-
tem.
In this study, SiO2 in mafic Katla magma ranges
from 46.3 to 51.3 wt% and TiO2 is notably high
in Katla magmas, ranging from 3.71 to 5.16 wt%.
Concentrations of FeO, MgO, K2O and P2O5 fall
in the range 2.6–15.3, 3.87–5.19, 0.63–1.22 and
0.45–0.84 wt%, respectively. Also shown on Fig-
64 JÖKULL No. 55