Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 144

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PÁLL IMSLAND
(titan-salite) started to crystallize. Its greenish iron rich clinopyroxenes,
mentioned before (not plotted in Fig. 87) are clearly xenocrysts in this rock.
Jan 87 is a case of early olivine-late clinopyroxene crystallization as well.
The absence of typical picrites from the rock suite of Jan Mayen and
dunites from the xenolith spectrum would indicate that olivine never
crystallized alone in huge quantities. The strong effect of the clinopyroxene
composition on the evolutionary trend of the ankaramites of Jan Mayen, as
shown by the Murata plot (Fig. 8), further emphasizes this and even
indicates that clinopyroxene might be the first phase to crystallize out of the
primitive magma in many cases. The slope of lines 12 and 166 on Fig. 87, for
example, indicate that the earliest clinopyroxene of these samples precipi-
tated before the earliest olivine. This is further indicated by the fact,
mentioned above, that the chromian diopsides of Jan 12 and 166 would
actually be in equilibrium with an olivine slightly more magnesian than
those which occur in these samples.
Arculus (1975) reports high pressure experiments on basanites, where
orthopyroxene is not a near liquidus phase, olivine is the primary liquidus
phase up to 18—20 kb only, and clinopyroxene is the primary phase above
18—20 kb and up to at least 35 kb. One of these basanites is quite similar in
composition to the least magnesian ankaramites of Jan Mayen, especially
regarding Mg, Ca, Na, Fe, Mn and P. The ankaramites are 3.5—4 wt. per
cent lower in A1203 but 0.15 wt. per cent higher in Cr203. Furthermore the
ankaramites are slightly higher in K and Si (see Table 20 for comparison).
Plagioclase is a very late mineral to crystallize in both these rocks and spinel
is the second to third phase to crystallize. The difference in the A1 concentra-
tions of these rocks is therefore probably of minor importance. The calcic
augite of the basanite might however be more aluminous than the chromian
diopside of the ankaramites. The increased Ti concentration of the ankara-
mites would stabilize clinopyroxene relative to olivine at high pressures
according to Arculus (op.cit.). The experiments on this basanite are thus
probably relevant for comparison in the case of the Jan Mayen ankaramites,
which have crystallized out olivine and chromian diopside as the first
minerals. This early crystallization would then have taken place at about 18
to 24 kb pressure (65—80 km) and probably around or slightly above
1350°C. In the cases where clinopyroxene was the first phase to crystallize
out of the Jan Mayen liquids, it was probably well above the 18 kb pressure
limit. Presence of water in the system would move the stability field of
olivine towards higher pressures according to Arculus (op.cit.) and the 18
kb limit of clinopyroxene as a first phase is thus probably the lowest pressure
possible. The absence of garnet from the Jan Mayen rocks would indicate
that the crystallization of the olivines (~Fo90) and chromian diopside, i.e.
the wehrlite crystallization, took place at less than 25 to 30 kb pressure
according to Arculus (op.cit.). The presence of An rich, zoned micropheno-
crysts of plagioclase, which occur in trace amounts in some of the ankara-