Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 256
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PÁLL IMSLAND
mineral xenolith and especially the xenocrysts of the lavas revealed by the
analyses might indicate that this crystallization took place under rather
restricted conditions.
In the hydrous mineral xenolith the kaersutite occurs together with
plagioclase and biotite. These minerals seem to have formed simulateously
or nearly so. This indicates at least closely similar conditions of crystalliza-
tion for the plagioclase and the amphibole, which restricts the depth of
formation of the amphibole to less than ~40 km (lOkb; see discussion on
plagioclase crystallization), which is close to an agreement with the 30 km
depth range of Aoki & Kanisawa (op.cit.). The compositional range of the
plagioclase in the hydrous mineral xenolith is from An59_46. This is among
the least calcic plagioclases found in the basic rocks and among the most
calcic plagioclases of the evolved rocks of Jan Mayen. This fact probably
means that these feldspars and the amphiboles are formed at rather shallow
depths compared to the ~40 km total range of feldspar crystallization. A
similar depth restriction is indicated by the presence of the biotite in the
hydrous mineral xenolith. Earlier the biotite of the lavas was estimated to
have crystallized just prior to the crystallization of the alkali feldspar but
after that of plagioclases. These biotites are of identical composition to that
of the hydrous mineral xenolith. Qualitatively, therefore, the kaersutites
may be concluded to have crystallized at high crustal levels, probably as
high as the commonly supposed and occasionally demonstrated “magma
chamber depth” of 3 to 7 km (Einarsson, 1978). The range of the Mg/Fe
ratio of the hydrous mineral xenolith kaersutites might though be taken as
an indication of varying depth of crystallization, which would fit crystalliza-
tion in magma approching this “magma chamber depth” rather than within
it.
Merrill & Wyllie (1975) studied the stability of Kakanui kaersutite at
varying pressure and temperature with and without water in excess of that
contained by the natural mineral. Within the frame of the above estimated
depth restrictions the upper stability temperatures of the kaersutite of
natural water content are 1200°C at 10 kb, 1140°C at 5 kb, 1100°C at 2.5 kb,
and 1060°C at 1 kb. In the case ofexcess water the temperatures are lowered
by ~100°C. At the “magma chamber” depths, 3 to 7 km (1 to 2.5 kb), the
maximum temperature of the kaersutite would be from 1060 to 1100°C
which is not unreasonable compared to the temperature of the biotite
crystallization previously estimated to take place well above 1000°C and not
exceeding 1100°.
Furthermore these temperatures are in a qualitative agreement with the
status of the kaersutite xenocrysts in the basalts. A kaersutite, formed at
nearly 1100°C at the depths of ~7 km, taken up by most Jan Mayen basalt
magmas would be remelted in the magma because of the temperature
difference (helped further by the upward movement and eventual increase
in volatile content caused by crystallization of anhydrous minerals). Taken