Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 255

MINERAL CHEMISTRY AND RELATIONSHIPS
251
hydroxy-halogen space. The same figures for the basaltic hornblendes are
0.6 to 1.5 per cent. The maximum C1 occupancy in the basaltic hornblende
structure is thus identical to the minimum F occupancy in the kaersutite
structure.
H20, as calculated, is shown in Fig. 122 plotted against the Mg/Fe ratio,
as are the directly determined halogens. According to this calculation
procedure, OFI is the major component of the hydroxy-halogen group in
both amphiboles. In the kaersutite it occupies over 95 per cent of the
hydroxy-halogen positions but ~50 to 60 per cent in the case of the basaltic
hornblendes. In both amphiboles the H20 concentration range is rather
narrow: 0.92 to 1.21 wt. per cent in the basaltic hornblendes and 1.90 to 2.08
per cent in the kaersutites. The averages are 1.03 and 1.99 per cent
respectively. Basaltic hornblendes are characteristically low in OH according to
Deer et al. (1963). It is generally believed that this is the result ofoxidation
and O replacement of the OH“, but this does not consider the halogen
content. The high analysed F content of the basaltic hornblendes reported
here and the resulting identical or only slightly higher calculated OH
content of these basaltic hornblendes and those reported by Deer et al.
(op.cit.) might indicate that such O replacement is quite variable in
basaltic hornblendes. On the other hand, the basaltic hornblendes ofthejan
Mayen rocks all occur as late crystallization products of evolved liquids,
which are usually in a relatively oxidized state. The HzO content reported
here in the basaltic hornblendes is therefore just as likely to be too high and
the mineral likely to contain some O in the hydroxy-halogen position
instead.
III. Crystallization relationships of the amphiboles
Kaersutite is a common minor phase in various igneous rocks and may
reach the status of an essential or major phase in relatively uncommon rock
varieties of igneous origin, occurring in close association with alkaline rock
suite members. Megacrysts and cumulus kaersutite in alkali basalt and
basanite magmas, for instance are thought to crystallize in the depth range
of 0 to 30 km (Aoki & Kanisawa, 1979), which clearly shows their wide
possibilities of formation.
The hydrous mineral xenolith occurring among the Jan Mayen rocks
contains 25 per cent of kaersutite, apparently crystallized contempor-
aneously with the plagioclase and biotite. The rock has a gabbroic type
texture. In the case of the euhedral microphenocrysts in the trachyte, there
is no doubt as to the igneous origin. The Jan Mayen magma system thus
obviously needs to crystallize a kaersutitic amphibole at some stage (or
stages).
At what depths and at what temperatures this happens is not as obvious.
The relatively homogeneous composition of the kaersutites of the hydrous