Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 163

minkral chemistry andrelationships
159
composition of equilibrium olivines, have lower Mg/(Mg+Fe2+) ratios,
show no pronounced increase in Ti or Fe3+, but plot along the late spinel
trend, at the higher Mg/(Mg+Fe2+) end. These early spinels occur in
association with olivine with composition Fogg-go, with which they are in
equilibrium at 1350°C according to Jackson’s (op.cit.) equation. These
spinels do not show any clear trend oí'changing Cr/(Cr+Al), which can be
correlated to other changes, but decreasing Mg/(Mg+Fe2+), on the other
hand, correlates with decreasing Fo content of equilibrium olivine at
constant temperature. As noted previously they plot along the late spinel
trend, which shows a general, though gentle, decrease of Cr/(Cr+Al) as
Mg/(Mg+Fe2+) decreases. These spinels mark the beginning of spinel
crystallization after the chromian diopside interruption. This later early
spinel crystallization was probably completed at the onset of the titan-salite
crystallization. The titan-salite is frequently Cr free and has already been
shown to have crystallized later than the olivines of ~Fo90 (see Fig. 86).
The crystallization of the spinels in the ankaramites was thus probably
interrupted twice by clinopyroxene crystallization. First the liquidus spinel
crystallization was interrupted by chromian diopside crystallization and
then spinel crystallized again for some time, still at high temperatures
(~1350°C) in some samples at least. This spinel crystallization was prob-
ably interrupted by the titan-salite crystallization. This titan-salite shows an
enrichment of Cr at its extreme edge. After the titan-salite crystallization,
final crystallization of the late spinel took place, showing enrichment of Ti
and Fe3+ and bridging the gap between the early spinels and the late iron-
titanium oxides.
The Fe^Os/FeO ratio of spinels is much higher than that of basic magmas
and their crystallizing silicates. The crystallization of spinels alone is thus
accompanied by decreasing f02 of the remaining liquid and the spinels
themselves attain a successively lower Fe3+/(Fe3++Cr+Al) ratio. This trend
is clearly shown by the pre-silicate early spinels of the ankaramites (A in
Fig. 92).
The trend shown by the early spinels of the ankaramites resembles the
evolutionary trend of Kimberlite spinels as it is described by Haggerty
(1976b), in showing an early enrichment in Cr and a later enrichment in Al.
The most usually observed pattern of chromium behaviour during crys-
tallization is a decrease in the residual liquid. The reason for the increase in
Cr/(Cr+Al) of the pre-silicate trend of the early spinels of the ankaramites is
thus not obvious. Relative to A1 the remaining liquid is much more depleted
in Cr during the crystallization of the early spinels of the ankaramites. A
possible explanation of this pattern may be connected with relative changes
in the amount of tetrahedral complexes of Cr3+ and Al3+ in the liquid. In a
silicate liquid the Si4+ cation (ionic potential=9.52) usually forms tetra-
hedral complexes or chains of these (polymers). According to Ringwood
(1955) other cations may do this as well and the higher their ionic potential