Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 136
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PÁLL IMSLAND
except for the granular texture of the wehrlite xenolith and the anhedral
habit of the chromian diopside. The general occurrence of the chromian
diopside in the ankaramites and its common occurrence in the basalts ofjan
Mayen contradicts an accidental relationship with the mantle providing
these pyroxenes.
A reliable clinopyroxene geobarometer has not yet appeared. Wass
(op.cit.) points out the possibilities of the distribution of A1 between
octahedral and tetrahedral positions in clinopyroxenes. Meanwhile indirect
ways must be used to approach the physical conditions of the environment
of the clinopyroxene formation. These may be qualitatively depicted as
below but will be considered further later.
The close similarities of the Jan Mayen clinopyroxenes and those studied
by Wass (op.cit.), both occurring in alkaline magmas, indicate a similar
evolution path. This path is, in the Jan Mayen case, characterized by
crystallization of Si rich, Al- and Ti poor clinopyroxenes of high Mg/(Mg+
Fe) ratio and a high Cr content (though depending on the Cr supply from
the crystallizing liquid) at high pressures and temperatures and characteris-
tically occurring in the mantle. Moving the evolving liquid towards lowered
pressures and temperatures and into the crust results in clinopyroxenes
showing a trend of decreasing Mg/(Mg+Fe) ratio, Si content and prac-
tically the disappearance of Cr but increase of both Ti and Al. All these
clinopyroxenes form phenocrysts in mafic lavas. Relatively scarce micro-
phenocrysts of clinopyroxene occur in the more salic lavas. These are partly
identical to the most evolved clinopyroxenes of the mafic lavas and partly Fe
enriched relative to the others and are without doubt of crustal origin. The
mafic lavas contain a groundmass clinopyroxene which is similar to the
titan-salite in composition but which shows the turning of the trend towards
the more primitive compositions again.
D. EQUILIBRIUM RELATIONSHIPS OF
OLIVINES AND CLINOPYROXENES AND THEIR
CRYSTALLIZATION
The relatively great compositional variations of both olivines and clino-
pyroxenes of the rocks indicate that these minerals might be the crystalliza-
tion product of a compositionally evolving magma or from diíferent mag-
mas. This again indicates that fractional crystallization and/or magma
renewal were taking place in the magma system of Jan Mayen during the
evolution of these rocks, if all the minerals are true phenocrysts. If some of
these minerals are, on the other hand, to be explained as residual minerals
from the magma generating source or otherwise originated xenocrysts, it
would be the kink-banded Fo rich olivine and the chromian diopside, which
occur in the wehrlite and characterize the ankaramites. A certain com-
positional variation is found in these minerals, which could then indicate a