Jökull - 01.12.1986, Page 36

Table 2.
Composition of olivine phenocrysts (microprobe analyses, groups I and II) in the Maelifell picrite basalt (taken
from Hardardóttir 1983).
anal. 1 la 2 2a 3 3a 4 4a 5 5a 6 6a 7 7a
no core rim core rim core rim core rim core rim core rim core rim
group i i i i n ii ii
sample 11076 11072 11096 11092 11104 11098 11139 11137 11174 11177 11012 11008 11193 11191
Si02 39.50 39.90 39.46 39.96 39.28 39.13 39.52 39.90 39.98 39.89 40.37 40.07 39.38 39.35
ai2o3 0.08 0.05 0.08 0.09 0.08 0.08 0.09 0.07 0.10 0.08 0.10 0.10 0.13 0.15
Cr203 0.04 0.04 0.04 0.07 0.02 0.05 0.02 0.05 0.08 0.07 0.06 0.10 0.08 0.12
MgO 47.13 47.66 46.25 48.45 46.70 47.65 47.45 48.08 51.02 48.83 49.64 49.58 50.00 50.18
FeO* 12.33 11.68 12.99 10.97 12.86 11.43 12.56 11.89 8.64 10.64 9.69 9.92 10.14 10.00
MnO 0.21 0.20 0.18 0.17 0.25 0.19 0.19 0.17 0.12 0.14 0.15 0.18 0.15 0.13
NiO 0.23 0.30 0.18 0.24 0.17 0.25 0.16 0.24 0.44 0.21 0.35 0.27 0.28 0.29
CaO 0.35 0.36 0.29 0.36 0.34 0.37 0.35 0.38 0.33 0.36 0.35 0.33 0.38 0.36
Total 99.87 100.19 99.47 100.31 99.70 99.15 100.34 100.78 100.71 100.22 100.71 100.55 100.54 100.58
Fo 87.2 87.9 86.4 88.7 86.6 88.1 87.1 87.8 91.3 89.1 90.1 89.9 89.8 89.9
section and is characterized by abundant Cr-spinel
and subspherical glassy silicate inclusions. Plagioclase
was never observed as inclusions in these grains. A
few grains of group-II olivine appear resorbed which
suggests that they were not in equilibrium with the en-
closing melt at the time of eruption. Table 2 shows 14
representative analyses of the two types of olivine in
the Maelifell picrite basalt, selected from the 196
given in Table A1 of Hardardóttir (1983). The re-
versely zoned olivines (group I) have an average core
composition of Fo86_87 and a rim composition of
Fo87_88 (Fig. 4). One of the reversely zoned olivines
analysed, has much lower magnesian core compo-
sition (Fo80, Fig. 4) is interpreted to be xenocryst do to
its low magnesian core composition. This group char-
acteristically has a low NiO content in the core (aver-
age 0.19 wt.%) and a higher NiO content in the rim
(average 0.25%: Table 2). Olivines of group II either
exhibit normal zoning or are unzoned, with a core
composition of Fo88_9l. The NiO content is charac-
teristically high in the core (average 0.33 wt.%); NiO
content decreases toward the rim (average 0.28%:
Fig. 4. Diagrams showing Fo content (mole %) and Table2).
NiO (wt.%) in grains of reversely zoned olivine (group The plagioclase included in the group-I olivine is
I) at Maelifell. (a) anal. no. 11 152—11159. (b) anal. bytownite, exhibiting complex compositional zoning
no. 11128—1 1136 and 11141. (c) anal. no. from An84 to An88 (Table 3). Its paragenesis indicates
11061 — 11066. (d) anal. no. 11098—11104. (a) — (d) an origin as a primary inclusion.
taken from Hardardóttir (1983). Clinopyroxene occurs as scarce macrophenocrysts
34