Jökull - 01.12.1977, Side 46

Fig. 24. Veined pieces o£
pumice on the boundary
between the brown and
greyish-pink units of H4
at Ofærugil.
Mynd 24. Tvílitir vikur-
molar úr H3 við Ófaru-
gil.
Photo Ævar Jóhannesson.
variation against time in one eruption and the
variation of some main elements against silica.
These preliminary results indicate a constant
composition in the lowermost 50 cm of H3 (top
of the magma column) followed by a gradual
change towards the boundary between the white
and greyish-pink unit. The lower part of the
greyish-pink unit seems to be of fairly constant
composition, but after that changes are more
or less gradual towards the top of the layer
(bottom of the magma colurnn). The sample
from the brown unit had well defined white
veins and both components were analysed. The
acid component is almost identical to the
samples frorn the lowermost part of the white
unit.
So far our data from the field ancl the chemi-
cal analyses favour a process of mixing of silicic
and basaltic andesitic end-members. The main
argument against mixing of such end-members,
Fig. 25. Colour changes
and corresponding change
of SÍO2 content of H3 in
a profile at the Ofærugil
canyon, 7.5 km NW of
Hekla’s summit. Only one
small sample from each
of the 16 horizons was
analysed. No attempt was
made to include the vein-
ed pumice clasts except
in the sample from tlte
brown unit.
Mynd 25. Litarbreyting-
ar samfara breytingum á
kisilsýrumagni H3 i sniði
norðan i Ófœrugili, NV
af Heklu.
44 JÖKULL 27. ÁR