Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 140

136
PÁI.I. IMSl.AND
near to being in equilibrium with the whole rock composition, while the
olivines of the more magnesium rich ankaramites are far from being in
equilibrium with the whole rock composition. The distribution of the
groundmass-, microphenocryst- and phenocryst crystal types in the various
samples in relation to the equilibrium line, on the other hand, indicates
diíferent crystallization histories for the various samples. If we consider the
simplest case first; Jan 66, the phenocrysts are in equilibrium with a liquid
of higher MgO/FeO ratio than that of the whole rock, while the groundmass
olivines are in equilibrium with a liquid of lower MgO/F’eO ratio. An
average of these olivines is thus in equilibrium with the whole rock composi-
tion and the olivines of this sample could have crystallized in a system
without olivine transport (closed system). Jan 52, on the other hand,
contains olivine phenocrysts, which arc in equilibrium with a liquid of
considerably lower MgO/FeO ratio than that of the whole rock and others
in equilibrium at much higher MgO/FeO ratio. 'Fhis can hardly be a closed
system crystallization phenomenon. Jan 15 contains only olivines, which are
in equilibrium with a liquid of lower MgO/FeO ratio than that of the whole
rock and Jan 20 contains only olivines in equilibrium with a liquid of higher
MgO/FeO ratio. Both these samples are nevertheless rather close to the
equilibrium conditions. The magnesium richer ankaramites contain olivine
phenocrysts, which all have higher MgO/FeO ratio than corresponds to
equilibrium with a liquid of the whole rock composition and have thus
crystallized out of a liquid relatively richer in iron. This is the case for most
of the olivines of the magnesium poorer ankaramites as well, but these
contain some olivines, which must have crystallized out of a liquid relatively
richer in magnesium than the whole rock. This indicates that the olivines of
the ankaramites mostly precipitated from a liquid more basaltic in composi-
tion than are the ankaramites themselves and that the liquid crystallizing
the most forsterite rich olivines, i.e. the most primitive liquid, was of the
composition presented by the magnesium poorer ankaramites (e.g. Jan 10
and 30).
Fig. 85 shows that the majority of the clinopyroxenes of the basalts and
the more evolved rocks are in equilibrium with a liquid of higher MgO/FeO
ratio than that of the whole rock composition (except the greenish clino-
pyroxene ofjan 66). The titan-salites of the ankaramites are in equilibrium
with a liquid oflower MgO/FeO ratio than corresponds to the whole rock
compostion. The chromian diopsides of the ankaramites seem to be mostly
in equilibrium with a liquid of higher MgO/FeO ratio and partly of lower
MgO/FeO ratio than corresponds to the whole rock compositions. If the
equilibrium line of Duke is relevant, then they are only in equilibrium with
this liquid.
The titan-salites, titan poor salites and ferroaugites of the basalts and
more evolved rocks seem to have crystallized out of a liquid slightly to
considerably richer in magnesium relative to iron than corresponds to the