Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 158
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PÁI.I. IMSI.AND
spinels of the basalts show a slight decrease in Mn with decreasing Cr/(Cr+
Al) while the late spinels show a strong increase in Mn with decreasing Cr/
(Cr+Al). The early spinels of the ankaramites do not reveal a clear trend of
changing Mn plotted against Cr/(Cr+Al).
Mn is detected in all the analysed spinels. It shows a clear correlation
with Mg/(Mg+Fe2+) and occurs mainly in the late spinels. The pattern of
Mn described here seems to be the same as for other spinel suites of volcanic
rocks reported in the literature. As in most other minerals Mn in the spinels
seems to follow Fe closely.
NiO ranges from undetectable to 0.26 wt. per cent, (av=0.11 per cent).
No systematic difference is observed between the early and late spinels of
ankaramitic and basaltic rocks, nor is there any systematic variation in Ni
content with changes in the spinel prism ratios. In about 25 per cent of the
spinel analyses Ni was not detected, (i.e. <0.005 per cent).
CaO in these spinels is extremely low. It ranges from undetectable to 0.1
wt. per cent (av=0.014 per cent). About 70 per cent of the analyses reveal no
Ca. No significant differences can be observed between early and late
spinels. Compared to the ankaramite spinels, relatively few of the basalt
spinels are Ca free but the significance of this is not known. No systematic
variations are observed in Ca as the spinel prism ratios change.
ZnO was only determined in 8 samples and ranges from undetectable to
0.34 wt. per cent (av=0.12 per cent). Five of the determinations are of
spinels from the wehrlite xenolith and show the entire range and an average
ofO. 15. This might indicate that Zn is not evenly distributed in the spinel
structure.
b. Crystallization relationships of the spinels
Early spinels are the first or among the first minerals to crystallize out of
basic magmas. Thus the early spinel trends may indicate the successive
compositional changes from one uncrystallized liquid of a rock suite (primi-
tive or not) to another. Late spinels, on the other hand, crystallize after
variable amounts of other minerals have crystallized from the liquid, and
may thus partly reflect the successive changes in the composition of each
liquid as its crystallization continues.
It can prove difficult to recognize the direction of these trends because of
the complex spinel compositions. Intermineral relationships, seen under the
microscope, may fa.il to give the exact crystallization order of the spinels in
the sample studied, and will usually not tell much about the relative
crystallization relationships of spinels from various samples. Compositional
changes in the spinels and the whole rock may in such cases be of some help.
Controlled crystallization of fused samples and other experimental studies
are of much more value, as well as determinations of crystallization
temperatures. Once again, because of the complex compositions of the
spinels, experimental investigations relevant to spinel studies are difficult to