Jökull - 01.12.1977, Page 16

TABLE III.
Microprobe analyses of the glasses in the Bárdarbunga tephra layers. The sample numbers (Bb)
as in Table I. "N” refers to the number of grains analyzed in each sample, and “±” to the
maximum deviation from the mean. FeO* is total iron calculated as FeO.
Bb 28 N = 8 X ± Bb N = X 89 7 Hh Bb N X 115 = 3 Bb N = X 124 = 8 Bb N = X 125 = 3
sío2 49.84 1.96 47.81 1.68 49.98 .71 51.45 1.10 47.51 .42
A1203 12.87 .36 13.40 .40 13.55 .07 12.96 .49 13.22 .42
TiOo 2.89 .11 2.90 .15 3.02 .07 3.15 .13 4.48 .15
FeO* 15.47 .29 13.74 .46 13.89 .61 14.55 .32 14.61 .51
MnO .29 .04 .23 .05 .24 .02 .21 .06 .23 .03
MgO 4.29 .24 5.65 .18 5.22 .04 5.72 .15 4.86 .10
CaO 9.32 .15 10.62 .12 9.83 .73 9.21 .17 10.08 .23
Na20 2.87 .20 2.82 .07 2.79 .23 2.46 .21 2.95 .24
KoO .53 .06 .41 .04 .55 .02 .48 .02 .89 .02
P2O5 .32 .06 .30 .04 .35 .04 .33 .06 .81 .02
98.69 97.88 99.42 100.52 99.64
Bb 193 N = 9 JC ± Bb 196 N = 3 xT ± Bb 211 N = 4 X ± Bb 221 N = 4 xT ±
SiOo 50.65 1.98 53.69 .14 49.80 .85 49.95 .52
AI0O3 12.95 .35 12.99 .05 13.70 .17 13.33 .29
TiOo 2.98 .24 3.03 .01 2.78 .06 3.07 .23
FeO* 14.11 .46 14.22 .20 13.64 .13 14.63 .99
MnO .22 .04 .27 .06 .24 .03 .23 .04
MgO 5.29 .25 3.50 .04 5.32 .20 5.37 .19
CaO 10.30 .49 7.35 .04 10.60 .09 9.56 .27
NaoO 2.71 .11 2.53 .30 2.80 .16 2.87 .07
KoO .45 .02 .71 .01 .55 .03 .52 .04
P2O5 .30 .09 .59 .02 .32 .03 .37 .02
99.96 98.88 99.75 99.90
are distinct from one another in all major com-
ponents except alumina, and that, relative to
the differences between groups, the scatter with-
in each group is small. This will be returned
to in a separate publication.
It has been shown (Sigvaldason et nl. 1976)
that within each dike swarm in Iceland a range
of compositions obtains. Furthermore, that the
14 JÖKULL 27. ÁR
most “developed” basalts are encountered to-
wards tlie centre of the country (e.g. Sigvalda-
son é1 Steinthórsson 1974). It will be shown
elsewhere (in preparation) that a dike swarm
evolves physically and chemically with time
from olivine tholeiite through tholeiite and
quartz tholeiite to an eventual silicic centre.
In accordance with this it may be generally