Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 195

mineral chemistry and relationships
191
coexisting titanomagnetite and ilmenite. The increasing Mn content of iron-
titanium oxides with decreasing temperature seems to be a general case and
data occur presented in several publications. It has e.g. been demonstrated
by Lipman (1971) for the oxides of zoned ash-flows in Nevada.
Lipman (op.cit.) also obtained a covariation between increasing tempera-
ture and increasing A1 content of magnetite phenocrysts. This covariation is
found in the Jan Mayen titanomagnetites as well. These observations are in
agreement with the findings of Turnock & Eugster (1962) who, in
experimental investigations, found A1 in magnetite solid solution to increase
with increasing temperature at buífered f02 below 900°C. In spite of much
lower A1 contents in the ilmenites than the titanomagnetites of the Jan
Mayen rocks there is apparently a similar covariation between the A1
content of the ilmenite and its temperature of crystallization.
Mg in both titanomagnetite and ilmenite shows a crude covariation with
temperature. This covariation is of increasing Mg content at increasing
temperature as in the case of Al. From Fig. 105 it may be seen that for each
temperature plotted the MgO content of the ilmenite is always slightly
higher than that of the titanomagnetite and that this difference of concentra-
tions increases with decreasing temperature. This pattern is in agreement
with the claim of Pinckney & Lindsley (1976) that Mg prefers the ilmenite
structure to that of the titanomagnetite and that this preference increases
with falling temperature. With decreasing temperature a crystallizing
magma usually becomes depleted in Mg as a result of olivine and clino-
pyroxene (and spinel) crystallization. The MgO-temperature covariation of
the Jan Mayen oxides may thus be caused by the decreasing Mg content of
the liquid. The similar pattern of Mg and A1 content of the titanomagne-
tites, when plotted against temperature, on the other hand, indicates that
the solubility of spinel in titanomagnetite may depend on temperature. This
might also be the cause of the lower Mg and A1 contents of the titanomagne-
tites at the lower temperatures.
In the Jan Mayen titanomagnetites V increases with increasing tempeia-
ture, but the sparse data do not reveal such a covariation in the case of
ilmenites. Lipman’s (op.cit.) data on the V contents of the ash-flow oxides
are scarce, but they show V to be constantly higher in the silica richer rocks
than the silica poorer ones. This holds for both ilmenites and magnetites. Si
in the rocks decreases with increasing temperature and there is thus a
covariation between increasing temperature and increasing V content of the
oxides. In the lower gabbros of the Skaergaard intrusion the iron-titanium
oxides are enriched in V relative to the upper gabbros according to Vincent
& Phillips (1954). Wager & Mitchell (1950) have shown that the earliest
magnetites depleted the Skaergaard magma effectively in V. The decreasing
V with decreasing temperature in the Jan Mayen titanomagnetites is thus
more likely a reflection of continuing titanomagnetite crystallization and
resulting depletion of V in the remaining magma than increased access of V