Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 257
mineral chemistry and relationships
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up by a trachyte liquid the same kaersutite would not be remelted. In some
cases it might have to readjust slightly, as in the case ofjan 36, where it has
formed a coarse-grained isolating corona. In other cases it might, in fact, be
an original equilibrium phenocryst as in Jan 77 and 79.
The effects of changed kaersutite composition on the stability conditions
are unknown but Aoki & Kanisawa (op.cit.) point out that in spite of the 0
to 30 km depth range of kaersutite formation, the F content is limited and F
only occupies from 2 to 4 per cent of the OH positions. This is identical to
the F content of the Jan Mayen kaersutites. The F content ofkaersutite is
according to these authors more likely controlled by the magma composition
than by the pressure at which the crystallization takes place.
Helz (1973) found in experimental studies of basalts that hornblendes
(formed on the FMQ buífer) range in composition from common green
hornblendes at low temperatures to kaersutitic hornblendes at higher
temperatures. Ti in these amphiboles increases steadily as the temperature
increases. Holloway & Burnham (1972), similarily in experimental studies
of a basalt, found Ti in amphiboles to increase with increasing pressure. A
hornblende crystallizing at 5 kb and 1025°C was a kaersutite at 8 kb and
1050°C crystallization. The hornblendes of the Jan Mayen lavas are
relatively Ti poor basaltic hornblendes which occur in the trachytes and
tristanites only and clearly have crystallized out of these evolved liquids.
The crystallization took place at a rather late stage, as shown by its
occurrence as an overgrowth on the clinopyroxene phenocrysts and as
groundmass patches. These groundmass patches are irregular in form,
optically continuous, and big compared to the groundmass grains. The
overgrowth zone on the clinopyroxenes may reach considerable thickness
relative to the size of the phenocryst. These features are taken to indicate
that this basaltic hornblende crystallization is not of the absolute late
groundmass category, but more likely a late event in “magma chamber” or
an en route product.
Accordingly the basaltic hornblende is formed under pressures varying
from probably ~2 kb to somewhat less than 1 kb (~7—2 km). The
temperature of these evolved liquids at the stage of groundmass crystalliza-
tion has been found to be ~850°C on the basis of two-feldspar thermometry.
The alkali feldspar of these samples, commonly occurring as a phenocryst
phase, has been found to start to crystallize at or slightly above 1000°C.
Crystallizing between these extremes, but closer to the phenocrysts, the
basaltic hornblende of the tristanites and trachytes most likely crystallized
at temperatures around and probably over 950°C but less than 1000°C.