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NUMBER 19 111
FIGURE 7.—Graphite flakes in silicate area of inclusion 3, reflected
light. (Width of image = 280 microns.)
inclusions, and the spread within any one inclusion
is larger than for the olivines. Compared to the
mafic minerals, the plagioclase is very constant in
composition at about 3.0% CaO, except for inclusion
6 with 3.4% CaO and inclusion 8, which shows
the largest spread in An content of the feldspar
(Table 5).
The outer rim of the mafic minerals is lower in
FeO content than the central part. Table 6 gives
FeO values for several olivine centers and rims of
inclusion 8. In Figure 9 the results of microprobe
step-scans across several olivine and pyroxene grains
are plotted. The large amounts of Fe just outside
the grains are due either to a thin rim of iron oxide
(formed by terrestrial weathering) or to troilite
adjacent to the silicate grain. The drop in iron
measured at the grain boundary, despite the steep
rise just next to it, shows that this is a real effect,
since any overlapping effect of the electron beam
at the grain boundary should have raised the
iron reading, not lowered it. Mg is plotted together
with iron to show that the measured points
are still in the silicate grain. The rise in MgO
content compensating the drop in FeO (Table 6)
is not visible here, because it is too small relative
to the Mg level. In orthopyroxene Ca shows higher
values in the grain centers and lower ones towards
the rim. There is also a general tendency in the
orthopyroxene for high Fe grains to also have high
Ca contents, as shown in the CaO/FeO plot in
TABLE 2.—Composition of silicate minerals from inclusion 8, determined by electron microprobe
(numbers in parentheses give number of grains analyzed)
Constituent
SiO2 . . .
AI2O3 • •
MgO . . .
FeO . . .
CaO . . .
TiO2 . .
CT2O3 • •
MnO . . .
Na2O . .
K2O . . .
Total
Orthopyroxene
(65)
58.9
0.25
35.2
4.37
0.79
0.20
0.37
0.39
100.4
Olivine
(63)
42.00
53.8
4.00
0.04
0.02
0.33
100.2
Diopside
(33)
55.2
0.74
17.9
1.81
21.6
0.60
1.24
0.34
0.83
100.3
Plagioclase
(53)
64.9
22.4
0.15
3.24
10.0
0.57
101.2
K-Feldspar
(2)
64.0
19.8
0.52
1.17
1.0
14.7
101.2