Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 245

MINERAL CHEMISTRY AND RELATIONSHIPS
241
The biotites of the Jan Mayen rocks are phenocrysts and accordingly
early crystallization products of the liquid. On the other hand, the liquid is
of an evolved type (trachyte and tristanite compositions). One could thus
expect to find the biotites relatively F rich. The opposite is the case. The
biotites are OH rich and F poor. This indicates that the most evolved liquids
of the Jan Mayen magma system are, at early stages of crystallization, F
poor relative to the evolved liquids of other areas.
If the evolution of these liquids had taken place through fractional
crystallization of “dry minerals”, from more primitive liquids, one could
expect the liquid to be sufficiently enriched in F to allow the crystallization
of a F rich biotite. Only if a hydrous mineral, incorporating F, was
fractionating at a late stage of this process could it lead to an evolved liquid
depleted in F but still rich in OH. The only mineral of the Jan Mayen rock
suite answering this description is an apatite occurring as extremely scarce
microphenocrysts associated with iron-titanium oxides in the basaltic
tristanites and some of the tristanites. Accumulation of such an apatite, on
the other hand, has not been found in any of the rock types. Assuming the
rock suite, as represented by the samples studied, to represent all vital stages
of the magmatic evolution, it is therefore strongly suggested that the more
evolved rocks of the rock suite, the tristanites and primarily the trachytes,
are not the crystallized liquids produced by fractional crystallization of the
Jan Mayen basalt magmas. This is considered in relation to other data later.
The basic magma of Jan Mayen, when crystallizing a mica, crystallizes
phlogopite, i.e. micas richer in Mg relative to Fe than the biotites. The mode
of occurrence of this phlogopite in the lavas (allotriomorphic flakes in the
groundmass and more or less euhedral blades in vesicles) is a clear
indication of late crystallization. This phlogopite is rich in I' relative to OH
as is to be expected of a late crystallizing mineral. Furthermore, in each rock
sample the phlogopite shows a decrease in F and an increase in OH as the
Mg/Fe ratio decreases, thus envisaging the F depletion of the liquid caused
by the crystallization of the phlogopite itself and preferentially taking up F
to OH.
This compositional trend, the mode of occurrence in the rocks, and the
relatively common presence of this mineral in the Jan Mayen basalts seems
to exclude Flower’s (1969) conclusion that the phlogopite of the Jan Mayen
basalts is a minor upper mantle mineral “locally escaping assimilation by
basalt magmas ascending to the earth’s surface”. In contrast the features
mentioned above conform to the idea that this phlogopite is a minor phase of
thc late groundmass crystallization of the basic magma itself.
fhe phlogopite of the coarse-grained ultramafic wehrlite xenolith is richer
in Mg (relative to Fe) than the phlogopites of the lavas. It shows the same
kind of simultaneous F depletion and Mg/Fe ratio decrease as those of the
lavas and it shows the same signs of a minor phase late crystallization
expressed by the crystal form and mode of occurrence. The slightly higher