Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 165

mineral chemistry and relationships
161
wt. per cent at the time of olivine crystallization. This means that about 35
per cent chromian diopside would have to have crystallized before the
olivine crystallization started, which is a fairly high value. To estimate the
amount of the spinel crystallization one must use some gross assumptions.
According to Irvine (1967), 4000 ft of dunite and peridotite in the Muskox
intrusion contain 1 — 3 per cent of chromite. This chromite contains about 50
wt. per cent Cr203 on the average. Taking 2 per cent as an average chromite
content, and a chromite of 50 per cent Cr203, we obtain 1 wt. per cent Cr203
in that liquid, neglecting Cr203 in other minerals. If this was the Cr203
content of the primary Jan Mayen liquid crystallizing the ankaramites, then
1 —1.3 per cent early spinel was crystallized out of this liquid before the
chromian diopside started to crystallize. This is not an unrealistic amount.
During olivine crystallization the amount of Cr in the liquid slightly
increases according to DCr=0.9. This may in part be responsible for the just
slightly higher Cr203 content (0.05—0.08 wt. per cent), obtained as the
amount ofCr203 in the liquid at the beginning of crystallization of the titan-
salite (using DCr=2.5) compared to that at initial olivine crystallization
(0.04—0.06 wt. per cent).
The early spinels of the basalts occur as rounded small phenocrysts or
euhedral inclusions in olivine. According to Jackson’s (op.cit.) equation
they are generally not in equilibrium with the olivines of the rocks at basaltic
magma temperatures. The olivines they would be in equilibrium with are of
higher Fo content except in the case of the Cr free spinel of Jan 87. A
temperature of 1250°C gives Fo92 2_76 as the range ofolivine composition in
equilibrium with these spinels. The early spinels show a trend of strongly
decreasing Cr/(Cr+Al) at slightly increasing Mg/(Mg+Fe2+) and Fe3+/
(Fe3++Cr+Al). Some relatively late spinels of the early spinels show
decreasing Mg/(Mg+Fe2+) at slightly decreasing Cr/(Cr+Al) as do the late
spinels. The relationships of the early spinels of the basalts to the clino-
pyroxenes are not clear. Chromian diopside is scarce in these rocks and the
spinels have probably all crystallized later than these. The typical clino-
pyroxene of these rocks is the titan-salite, which may have interrupted the
spinel crystallization, but the few analyses of the basaltic spinels are not
enough to reveal these relationships. The strong increase in A1 at the
expense of Cr is in agreement with the A1 enrichment of these rocks, relative
to the ankaramites. The correlation between A1203 in the spinels and the
whole rock is shown in Fig. 95. The figure shows features very similar to
those noted by Sigurdsson & Schilling (1976) for spinel from the Mid-
Atlantic Ridge. A slight increase of fö2 of the magma at this stage of
crystallization may be expected and its occurrence is indicated by the slight
increase of Fe3+/(Fe3++Cr+Al) as clearly shown in Fig. 92 (trend B). The
slight increase of Mg/(Mg+Fe2+) (0.6—0.7) of this early spinel trend is
puzzling. The opposite might be expected, as the compositional evolution of
the liquid at this stage is definitively towards increased iron. Changes in the