Jökull - 01.12.1986, Blaðsíða 37

(definition of Donaldson & Brown 1977) 1.5 cm
across. They are subequant, subspherical and com-
monly contain large (>100 pm) crystallized inclusions.
Occasionally, they also contain Cr-spinel and sub-
spherical and irregularly shaped inclusions of glass.
The clinopyroxene has a limited compositional range,
around Wo44 En49 Fs7 (Table 4), falling in the upper
right corner of the endiopside field of the pyroxene
quadrilateral as subdivided by Poldervaart & Hess
(1951) (Fig. 3). It contains an appreciable concen-
tration of Cr and A1 compared with other clinopyrox-
ene in the Hengill suite (Hardardóttir, 1983 ). There is
a general tendency for Cr to increase toward the rim
as A1 and Ca decrease. The round shape of the Al-
Cr-endiopside suggests that the megacrysts were not in
equilibrium with the enclosing melt at the time of
eruption.
Cr-spinel occurs as euhedral to subhedral (more
rarely skeletal) grains included in or attached to
group-II olivine and, more rarely, in pyroxene crystals
or as discrete grains in the groundmass. They are typi-
cally 20-70 |im in diameter, reaching a maximum
width of 350 pm. Reddish brown is the dominant
color, but a minor population of yellowish brown
spinels is also present; both types may occur in the
same olivine crystal. The crystals are usually with
what could be attributed to an overgrowth rim or
zonation; they contain glassy or partly crystallized
silicate-melt inclusions. Only four Cr-spinel grains
were analyzed, their composition (Table 5) can be
termed chromian hercynite or chromian spinel (Deer
et al. 1966). Because of the sparse data, it is not
possible to say whether or not the spinel is zoned;
however Mg and Cr contents decrease toward the rim
in two of the analyzed grains. The morphology of the
Cr-spinel included in group-II olivine indicates they
are primary inclusions, and their paragenesis indicates
that they may have been the earliest phase to
crystallize from the Maelifell basaltic melts.
The FeO/MgO ratio of the melt in equilibrium with
a given olivine composition can be estimated from the
equation (1):
K(D) = (FeO/MgO)0| / (MgO/FeO)liq (1)
defined by Roeder & Emslie (1970). A value of 0.34
for the distribution coefficent K(D) for olivine was ac-
cepted as representative of equilibrium conditions
(Hermes & Schilling 1976, Wilkinson 1982). The K(D)
value defined by Roeder & Emslie (1970) is expressed
on the basis of mole % of the oxides (molecular weight
Table 3. Composition of one plagioclase grain in
group-I olivine from Maelifell picrite basalt. (Taken
from Hardardóttir 1983).
anal.no 11800 11801 11802
rim center core
sío2 47.07 46.79 48.13
A1203 32.42 32.78 32.15
Fe203* 0.59 0.60 0.59
CaO 17.22 17.12 16.61
Na20 1.35 1.37 1.69
k2o 0.00 0.01 0.00
Total 98.65 98.67 99.17
An% 87.6 87.3 84.5
Table 4. Composition of two clinopyroxene macro-
phenocrysts in Maelifell picrite basalt (Taken from
Hardardóttir 1983).
anal.no 11304 11301 11312 11308
core rim core rim
sío2 51.15 49.88 51.17 51.31
tío2 0.19 0.27 0.26 0.32
ai2o3 4.97 4.97 4.17 4.04
Cr203 1.25 1.33 0.85 1.08
MgO 16.66 16.84 17.08 16.74
FeO* 4.32 4.25 4.26 4.42
MnO 0.07 0.09 0.14 0.06
CaO 20.79 21.02 21.13 21.15
Na20 0.20 0.14 0.22 0.23
Total 99.60 98.79 99.28 99.35
Wo% 43.9 44.0 43.8 44.2
Fe% 48.9 49.0 49.3 48.6
Fs% 7.1 6.9 6.9 7.2
Table 5. Composition of spinel inclusions in olivine
and of microphenocrysts in Maelifell picrite basalt
(Taken from Hardardóttir 1983).
anal. no 11500 11501 11502 11503
core core core core
Si02 0.09 0.11 0.17 0.08
A1203 28.83 29.85 28.07 32.40
Cr203 35.10 34.25 33.34 31.11
MgO 16.32 15.38 14.55 16.55
FeO* 18.75 19.52 22.32 19.34
NiO 0.16 0.15 0.13 0.23
CaO 0.01 0.07 0.24 0.05
Total 99.26 99.33 98.82 99.76
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