Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 161
mineral chemistry and relationships
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ratio (mol.%) of 3.36—5.18 according to Roeder & Emslie (1970). A ratio of
MgO/FeO = 3.36 in a liquid is similar to the MgO/FeO ratio in the less
magnesian ankaramites, e.g. Jan 10 and 30, which have a MgO/FeO ratios
of 3.08 and 2.77 respectively. A liquid with a MgO/FeO ratio 5.18, on the
other hand, has a lower ratio than the most magnesian ankaramite, Jan 166,
which has a MgO/FeO ratio of 6.22. The liquids these early spinels of the
ankaramites seem to have formed in are thus of ankaramitic compositions as
regards MgO and FeO.
Fig. 94 shows a part of the Fe3+ free spinel prism base and shows the
compositions of the olivines that would be in equilibrium with the early
spinels of the ankaramites at 1400 and 1350°C. There is a general fall in Fo
content with decreasing Mg/(Mg+Fe2+) and a clear gap in the Fo composi-
tions between Fo89 8 and 91.7 while the Fo range is practically continuous on
both sides of this gap. Above the gap there is a continuous decrease in the Fo
content from Fo94.5_9I.7 as Cr/(Cr+Al) increases and Mg/(Mg+Fe2+)
decreases, while below the gap the relationships are not as clear. The Fo
contents above the gap are higher than for any olivine occurring in the rock
suite (in fact higher than most olivines of igneous rock), while the composi-
tions below the gap match those of the olivines occurring in the rocks.
Hill & Roeder (op.cit.) showed that at basaltic liquidus temperatures and
ÍOz below about 10-8 atm the crystallization ofearly spinel is interrupted by
the crystallization of clinopyroxene as was postulated by Irvine (1967). The
exact f02 of the ankaramitic magmas at temperatures as high as 1400°C is
difficult to attain. Judging from the buffer curves (after extrapolation) in the
fö2 versus T° diagram (e.g. Lindsley, 1976) the f02 at these conditions may
be assumed to be low, possibly less than 10“10 atm. The iron-titanium oxides
of the Jan Mayen basaltic rocks, as will be shown later, give a temperature
of 1000—1100°C and f02 round 10“10 atm. Fudali (1965) pointed out that
Í02 increases with increasing acidity of rocks. There is thus no obvious
reason to believe that the f02 of the ankaramitic magmas is higher than
those of the basalts. The f02 of the ankaramitic magmas at the time of early
crystallization was therefore presumably sufficiently low for the spinel
crystallization to be interrupted by the crystallization of the chromian
diopside.
The early spinels of the ankaramites, having high Mg/(Mg+Fe2+) ratios,
have therefore probably crystallized prior to the chromian diopside and the
olivine (~Fo90). These spinels show a trend of increasing Cr/(Cr+Al) and
slightly decreasing Fe3+/(Fe3++Cr+Al) with slightly decreasing Mg/(Mg+
Fe2+) ratio, as can be seen from Figs. 92 and 94. These changes were found
by Hill & Roeder (op.cit.) to occur at constant temperature and decreasing
f02 in their experiments. Such spinels may be concluded to have formed at
high temperatures (1350— 1400°C) and high pressures (>18 kb) and at low
f02 (<10“8 atm.).
The rest of the early spinels of the ankaramites, below the gap in the