Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 141

mineral chemistry and relationships
137
whole rock compositions, while the titan-salites of the ankaramites have
clearly crystallized out of a liquid, magnesium poorer relative to iron, than
the ankaramite whole rock. The chromian diopsides of the ankaramites have
most likely crystallized out of a liquid similar in composition to the less
magnesian ankaramites.
This chromian diopside is identical in composition to the chromian
diopside of the wehrlite xenolith. The chromian diopside and kink-banded
olivine (~Fo9o) make up about 95 per cent of the wehrlite xenolith minerals
where they are fairly close to being in equilibrium with each other according
to the clinopyroxene-olivine equilibrium conditions presented by Duke
(op.cit.) and Mákipáá (op.cit.), as shown by Fig. 86. When the ankaramite
clinopyroxenes are plotted in this íigure against the olivines of composition
Fo92_88, the most common and mafic olivines of the ankaramites, they fall
close to the equilibrium line in the case of the chromian diopsides but far
from the line in the case of the titan-salites. Thus the chromian diopsides
and the most forsterite rich olivines of the wehrlite and the ankaramites are
identical pairs of minerals generally in equilibrium with each other and may
be concluded to be of common origin. Furthermore these mineral pairs
appear to have crystallized more or less contemporaneously. In the case of
Jan 12 and 166, Fig. 86 would though indicate that the chromian diopsides
of these samples were in equilibrium with a slightly more magnesian olivine
than the one occurring in the rocks.
The titan-salites of the rock suite are matched by more fayalitic olivines,
which frequently occur in the rocks in equilibrium with the titan-salites.
In Fig. 87 the iron-magnesium relationships of the olivines and clino-
pyroxenes of various lavas have been drawn. This is done such that for each
sample the most magnesian (relative to iron) clinopyroxene is plotted
against the most magnesian olivine and the least magnesian clinopyroxene
against the least magnesian olivine, these two points then being connected
by a line. If successive intermediate pairs of these minerals are plotted in
this way they fall fairly close to the tie line of the end points, as shown by Jan
12 (as an example on the figure). This results in a number of lines falling
parallel or subparallel to the equilibrium lines of Duke (op.cit.) and
Mákipáá (op.cit.). This procedure assumes that the first and the last crystal-
lizing clinopyroxenes precipitated contemporaneously with the first and the
last olivines respectively. This assumption is correct only if these minerals
are in equilibrium with each other, i.e. if the points fall close to the
equilibrium lines. By manipulating a little with the first and last crystal-
lizing minerals, giving an early clinopyroxene start and a longer period of
olivine crystallization or vice versa, the individual lines may be moved such
that they fall closer to the equilibrium lines and show a better parallelism.
This shows that during the crystallization of the olivine and clinopyroxene
phenocrysts of the ankaramites and most of the basalts, these minerals
formed contemporaneous pairs changing composition successively as the