Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 157

MINKRAL CHEMISTRY AND RELATIONSHIPS
153
volcanic rocks, known to the author, where spinels are analysed in any great
numbers. Bearing in mind the few and often poorly documented studies of
spinels available for volcanic rocks, it is therefore to be expected that
continuous spinel suites, from Fe3+ poor spinels to titanomagnetites, may be
rather common.
Except for Ti, published data on the minor and trace element content of
spinels in volcanic rock suites is rather scarce and incomplete. The most
frequently reported minor element in published spinel analyses is Mn. Si
and Ca are occasionally reported, but Zn, Ni and V determinations seem to
be very scarce. The concentrations (Fig. 93) and variations of these elements
in the Jan Mayen spinel suite will now be briefly described.
Si02 content in the spinels is very low, ranging from undetectable to 0.23
wt. per cent, with an average of 0.10. The early spinels of the ankaramites
have a slightly narrower range (0.00—0.18) and lower average (0.09) than
the early spinels of the basalts (r=0.00—0.23, av=0.14). The late spinels of
the ankaramites have the same range and average as the early spinels of the
ankaramites. Si in the late spinels of the basalts was not determined. The
early spinels do not show a clear trend of Si increase or decrease with
changes in the spinel prism ratios, but the few late spinels of the ankaramites
show a decrease in Si as these ratios decrease.
Si is detected in almost all the spinels and shows a fairly even distribution.
It is probably unrelated to changes in major element concentrations and
physical properties of the liquid.
V203 ranges from not detectable to 0.75 wt. per cent, with an average of
0.18. The early spinels are poor in V relative to the late spinels. The range of
the early spinels is 0.00—0.23 and the average 0.16, while the range of the
late spinels is 0.19—0.75 and the average 0.41. The range and averages of
the early spinels of the ankaramites and the basalts are practically identical.
Fhe early spinels show a very slight increase in V as both Mg/(Mg+Fe2+)
and Cr/(Cr+Al) decrease and the late spinels a strong increase in both
cases. This shows that V is primarily taken up by late spinels. Evans &
Wright (1972) show V in the liquidus spinels ofthe 1959 Kilauea Iki basalt
to increase slightly with decreasing Cr, which is the same pattern as
observed here.
V is detected in practically all the spinels but especially in the late spinels.
MnO ranges from 0.14 to 1.32 wt. per cent, with an average of0.34. The
early spinels are poor in Mn relative to the late spinels. Practically no
diíference is found between the early spinels of the ankaramites (r=0.14—
0.56, av=0.29) and those of the basalts (r=0.15—0.47, av=0.28). The late
spinels of the ankaramites, on the other hand, are poorer in MnO (r=0.30—
0.61, av=0.46) than the late spinels of the basalts (r=0.61 — 1.32, av=0.98).
There is a general increase in Mn in the spinels as Mg/(Mg+Fe2+)
decreases. This increase is slight at high Mg/(Mg+Fe2+) values (early
spinels) and strong at low Mg/(Mg+Fe2+) values (late spinels). The early