Acta naturalia Islandica - 01.07.1964, Blaðsíða 16

14
SIGURDUR STEINTHORSSON
The black olivines are more forsteritic than normal olivine lower down
in the same rock. (Black olivine n = 1.663 giving 93% Fo as opposed to n =
1.682 giving 85% Fo for the normal olivine).
Bartrum (1942) has described olivines similar to these from New Zea-
land. He contributes the ore exolutions to a reaction between early formed
crystals and liquid. In the process the olivine was converted to a more for-
steritic variety; the conversion took place at a very late stage in a voloanic
cycle.
We may note the following facts in connection with the black olivines of
Hvammsmúli and their environment: They occur in the uppermost zone of a
thin sill (or lava flow), the thickness of which is about 10 m from the contact
at the base to the top of the exposure. The top is very vesicular, so that one
may assume that the top of the exposure is in fact close to the original top of the
sill. There is an apparently continuous gradation from the medium-grained
granular base, upwards through a fine-grained zone with norm'al olivines, into
the top zone, which is vesicular and very fine-grained. The top zone is red col-
oured (hematite); there is a ca. 50 cm wide transition zone with irregular red
and grey patches between the vesicular red above, and the massive grey below.
The base too is marked by a vesicular zone 50—70 cm wide, with the vesicles
drawn out as a result of flow. The central portion of the sill is massive. All the
olivines of the red rock are black; immediately below the transition zone the
olivines are normal. Otherwise similar rock in Pöst has normal olivines.
The main difference between the red and the grey types of groundmass
is the distribution of iron ore. In the gray rock magnetite occurs in small but
discrete grains; in the red it is disseminated all over the mass, mainly as hema-
tite. On Pl. IV a the red area is seen to be considerably more vesicular than
the grey one. It is noted too, that the olivine crystal in the grey area is less
blackened than the other one. Even in the hematite (red) rock the olivine
exsolutions are mainly magnetite. Only an occasional crystal is red in reflected
light. This suggests greater amount of oxidation of the groundmass than of
the crystals.
The geological setting suggests that this feature of the olivines was ac-
quired more or less in situ, otherwise black olivines would be found all over
the rock, not merely at the top. The freshness of the olivines seems to exclude
the possibility of weathering.
It has been shown recently (Hamilton, Burnham and Osborn 1964)
that basic magmas contain considerably more volatiles than had previously
been thought. Furthermore, the great number of vesicles in the red zone is an
indication of the presence of much volatiles, presumably mostly water vapour.