Jökull - 01.11.1998, Síða 7

show that their glass composition closely matches
that of the strombolian tephra from the 1783-84 Laki
eruption (Table 1).
The prehistoric soil in the Leiðólfsfell area is ~3.5
m thick, and includes the well known silicic Hekla
tephra layers, the Selsund Pumice (~3500 BP), and
H4 (3800-4000 BP) (Larsen and Þórarinsson, 1977;
Jónsson, 1978) and the so called "Needle" layers
which were produced by eruptions from Mýrdals-
jökull around 2700 BP and 3200 BP, respectively
(Larsen, 1994). The prehistoric tephra layers have no
bearing on the age of the Leiðólfsfell scoria deposit
and are therefore not considered further. Figure 4
shows the historic tephra sequence at locations 2,7, 8,
and 9 on Figure 1 and the chronology is constructed
on the basis of known marker tephras in southern Ice-
land. These markers are the rhyolitic tephras from
Öræfajökull 1362 AD and Hekla 1104 AD, the "olive
gray layer" from first part of the 13 Century, and the
"Settlement" layer from the ~870 AD Vatnaöldur
eruption (Þórarinsson, 1958, 1967; Larsen, 1978,
1979, 1984). Other layers useful for correlation in the
Síða highlands are the tephras of the -1480 AD
Veiðivötn eruption in -1480 AD, the Hekla eruptions
in 1389 and 1845, and the ~935 AD Eldgjá eruption
(Fig. 4). The identification of these marker layers was
confirmed by chemical analysis and their composition
is listed in Table 2.
Two tephra layers are included in the thin soil
above the Leiðólfsfell scoria; the upper from the 1918
Katla eruption and the lower from the 1845 Hekla
eruption (Fig. 5). The first three black tephra layers
below the Leiðólfsfell scoria have a FeTi basalt com-
position (Table 2) and their thicknesses are consistent
with the known dispersal of fall deposit from the 1755
AD, 1660 AD and 1625 AD eruptions at the Katla
volcano (Larsen, 1978). The 1625 layer usually con-
sists of three fall units, and therefore readily distin-
guished in profiles within the study area (Fig. 4). The
next two layers are dark basaltic tephras and their po-
sition above the Veiðivötn marker tephra, along with
the established eruption history of the region (Þórar-
insson, 1974; 1975; Larsen, 1978; 1979), indicate that
they are from the eruptions at Katla in 1612 AD and
Grímsvötn in 1598 AD, respectively (Fig. 5). Be-
tween the Veiðivötn and Öræfajökull marker tephras
are three dark basaltic ash layers and a thin dark
brown (andesitic) tephra layer (Fig. 4). The brown an-
desitic layer is from the 1389 AD Hekla eruption
(Larsen, 1978). The two black basaltic tephras are al-
Table 1. Chemical composition of Leiðólfsfell and Laki tephra deposits in the study area.
Whole-rock composition of the Leiðólfsfell scoria and the 1783 Laki lava flow
Si02 Ti02 ALOr FeO MnO MgO CaO Na 20 K20 P2O, Sum N
Lei-core' 49,82 2,69 13,38 13,66 0,23 5,91 10,55 2,71 0,53 0,43 99,86 2
std. 0,33 0,04 0,16 0,46 0,01 0,04 0,19 0,14 0,02 0,02 0,01
Lei-armor2 49,42 2,73 13,40 13,88 0,23 5,95 10,64 2,69 0,51 0,41 99,86 3
std. 0,30 0,07 0,11 0,20 0,01 0,12 0,21 0,05 0,02 0,01 0,01
Laki lava3 49,69 2,68 13,91 13,40 0,21 5,70 10,38 2,70 0,40 0,32 99,26 30
0,64 0,13 0,28 0,32 0,01 0,20 0,18 0,16 0,04 0,06 1,05
Groundmass glass composition of 1783 Laki tephra at Leiðólfsfell
Si02 Ti02 ALO, FeO MnO MgO CaO Na20 K20 P205 Sum N
Laki at Leiðólfsfell4 50,06 3,07 13,20 14,34 0,21 5,44 9,81 2,82 0,45 0,32 99,72 23
std. 0,21 0,06 0,09 0,24 0,04 0,08 0,10 0,07 0,02 0,05 0,25
Laki at vents5 49,91 3,04 13,19 14,16 0,21 5,42 9,84 2,76 0,45 0,30 99,40 77
std. 0,32 0,06 0,10 0,29 0,03 0,07 0,10 0,13 0,02 0,05 0,50
'N, number of analyses; std., standard deviation (2-sigma).
' Average composition of the core from two armored bombs (samples Lei-04, Lei-12). Source: Þórðarson (1995).
“Average compositon of the glassy skin from three armored bombs (samples Leirc-02, Leirc-03A, Leirc-I2). Source: Þórðarson (1995).
3 Averagc whole-rock composition of the Laki lava flow. Source: Þórðarson (1995).
4Laki tephra at location 2. Source: Þórðarson et al. (1996).
5 Average composition of strombolian tephra from outcrops proximal to the Laki fissures. Source: Þórðarson et al. (1996).
JÖKULL, No. 46, 1998
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