Jökull - 01.12.1977, Page 17

TáBLE 111 (continued)
Bb 259 N = 5 xT ± Bb 264 N = 3 X ± Bb 304 N = 8 X~ ± Bb 419 N = 9 xf ± Bb 430 N = 5 xT ±
SÍ02 49.75 .68 50.47 .54 49.61 .82 49.27 1.42 49.01 .35
A1003 13.40 .45 12.44 .19 12.30 .64 12.56 .70 13.41 .35
TiOo 2.88 .36 3.01 .11 2.87 .12 3.10 .23 2.72 .10
FeO* 12.93 .51 13.46 .32 12.92 .61 14.22 1.13 12.88 .23
MnO .25 .05 .25 .04 .24 .07 .25 .06 .20 .06
MgO 5.39 .41 5.08 .17 5.34 .29 5.28 .42 5.94 .07
CaO 10.27 .45 10.46 .74 10.62 1.37 10.57 .35 10.53 .16
Na^O 2.51 .18 2.36 .45 2.48 .38 2.14 .31 2.56 .18
KoO .46 .07 .43 .04 .40 .04 .46 .08 .44 .03
1’oOg .32 .07 .34 .04 .31 .06 .33 .05 .30 .06
98.16 98.30 97.09 98.18 97.99
Bb 436 N = 4 X ± Bb N = xf 436 = 6 Bb 438 N = 2 X ± Bb N = 440 = 5 Bb 449 N = 8 xT ±
SiO 49.08 1.05 49.82 1.71 59.69 .51 49.33 .78 48.31 .65
AI2O3 14.53 .25 12.62 .42 14.85 1.12 13.54 .49 12.80 .64
Ti02 1.78 .12 2.97 .38 1.31 .21 1.74 .04 4.72 .26
FeO* 12.13 .23 14.03 1.50 10.80 .22 11.66 .10 15.09 .68
MnO .23 .03 .26 .08 .24 .01 .20 .03 .25 .06
MgO 8.06 .27 5.90 .44 1.27 .01 6.76 .29 5.36 .73
CaO 11.52 .32 9.90 1.05 4.98 .32 11.72 .34 9.45 1.05
Na->0 2.57 .14 2.35 .48 4.39 .23 2.30 .37 2.57 .37
k2o .20 .03 .42 .07 1.64 .01 .22 .04 .67 .32
P2O5 .16 .04 .30 .10 .72 .15 .16 .04 .70 .04
100.25 98.57 99.30 97.63 99.92
stated that the three groups of basalt encounter-
ed in the Bárdarbunga core may be correlated
to volcanic type:
Katla is distinguished by higli TiOo — the two
samples in tlie core characterized by tliis com-
position turned out to be K 1918 and K 1755,
both of which were carried to the east and
northeast (Thorarinsson 1975). The third sample
is heterogeneous, containing glass of two distinct
basaltic types (Bb 496).
Basalt from central volcanoes in the tholeiitic
rift zone. such as calderas, are distinguished by
their tholeiitic composition (i.e. intermediate
TÍO2 values in Fig. 6). In this group fall tephras
from Grímsvötn and Askja (Dyngjufjöll). A
notable exception is the Kverkfjöll (? 1477)
tephra (Table II, 110. 12). Furthermore, the
lava flow of Lakagígar 1783 falls within this
category in accordance with the notion men-
tioned before that Grímsvötn and Lakagígar are
related.
Into the third category, that of olivine tho-
leiite, fall basalts from minor fissure eruptions,
i.e. unrelated to central volcanoes. These tephras
JÖKULL27. ÁR 15