Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 132

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PÁLLIMSLANÐ
onal one. This may sometimes be the reason for totally contrasting data, of
which two examples will be given: 1) Hytönen & Shairer (1961) showed by
experimental work on a pure system, that A1 in diopside increases with
decreasing temperature, while the A1 content of the augite series of the
tholeiitic rocks of the Skaergaard intrusion decreases with decreasing
temperature according to Brown (1967). 2) In a study of the clinopyroxenes
of the alkaline rocks of Japan, Aoki (1964) found ultramafic and mafic
nodule clinopyroxenes to contain 14—15 per cent CaAl2Si06 (Ca-Tscher-
mak’s component), supposed to be dependent on pressure. The clino-
pyroxenes of the ultramafic nodules from Jan Mayen studied here contain
only 0.5—2.4 per cent CaAl2Si06.
Kushiro (1960) has shown that A1 in Ca rich pyroxenes increases as the
rocks become more undersaturated and that along with fractionation of
rocks the A1 content of the clinopyroxenes decreases. Le Bas (1962) came to
the same conclusions except that along with fractionation he found A1 to
increase in the groundmass clinopyroxenes of alkaline rocks, whereas in
tholeiitic rocks it decreases. There is a correlation between Si and A1 in the
Jan Mayen clinopyroxenes, showing increasing A1 with decreasing Si.
Plotted as number of cations on the basis of 6 oxygens (Fig. 81), the total of
these cations is mostly above 2, which means that in most cases all the A1 is
not needed to fill the tetrahedral position along with Si. Those relatively few
pyroxenes plotting below the line of total = 2 need all the A1 and some other
cation in the tetrahedral position. This extra cation is most probably Fe3+ as
suggested by Kuno (1955) and several later authors. It is worth mentioning
that the clinopyroxenes in the Jan Mayen rocks thus containing Fe3+ in
tetrahedral positions mostly occur in the intermediate rocks (see Fig. 81),
which are the rocks with lowest total iron and highest Fe2Os/FeO ratios.
The diopside of both the wehrlite and ankaramites contains relatively
great amounts ofits A1 in octahedral positions (Fig. 82). On the other hand
the titan-salites, titan poor salites and ferroaugites, which occur in plutonic
xenoliths (gabbros, hydrous mineral- and syenitic xenoliths) all contain
relatively great amounts of their A1 in octahedral positions, while these same
minerals in the lavas do not (Fig. 82). When the clinopyroxenes of the
xenoliths and the lavas are otherwise compared they do not show systematic
chemical differences. The xenolith pyroxenes are generally lower in certain
elements in some cases or generally higher in other cases, or show no
difference at all. Le Bas (1962) considers clinopyroxenes crystallized under
solid-solid equilibrium conditions to favour A1 in octahedral positions, and
increasingly so with increasing load pressure, but the role of load pressure in
the case of liquid-solid crystallization to be of little consequence. The
distribution of A1 between tetrahedral and octahedral positions might thus
indicate a common origin of the chromian diopsides of the ankaramites and
the wehrlite, but its meaning in the case of the other xenoliths and lavas is
uncertain.