Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 135

minkralchemistry andrelationships
131
In experimental investigations of the crystallization relations of some
natural rocks under varying pressures and temperatures, Thompson (1974a,
1974b & 1975) followed the pattern of compositional changes of the
clinopyroxenes formed. These experiments do not give totally unanimous
results regarding the response of individual elements of the clinopyroxenes
to the changing pressure and temperature. The primary complicating
factors are the different co-precipitating minerals and the unlike composi-
tions of the crystallizing liquids. From these experiments it nevertheless
emerges that: 1) At constant pressure and decreasing temperature the Mg/
Fe ratio (and Mg/(Mg+Fe) value) and the Cr and Mg contents decrease,
while the Na, Ti, and A1 contents increase. 2) At constant temperature and
decreasing pressure, on the other hand, Al, Ti, and Na decrease. 3)
Decreasing the pressure and the temperature together results in decreased
A1 and Na and increased Ti, and A1 in tetrahedral positions relative to A1 in
octahedral positions (A1IV/A1VI), while Si and Ca contents and the Mg/
(Mg+Fe) values change irregularily or are nearly constant. Some of the
changes, as e.g. in Si and the Mg/(Mg+Fe) ratio, are probably not reliable
for comparison with natural occurrences, as the experiments do not allow
any fractionation to occur — a factor of potential importance in most
naturally crystallizing magma systems.
Wass (1979) found Cr-diopside, two textural types of high pressure Al-
augite, and a low pressure clinopyroxene to occur together in the same
alkaline lavas from both Australia and France. She concludes that the Cr-
diopsides and the high pressure Al-augites, showing metamorphic imprints,
are accidentally included mantle xenoliths. The Al-augite, showing no signs
of metamorphism, is on the other hand a high pressure crystallization
product of the alkali basaltic melts. From these melts, the low pressure
clinopyroxene has crystallized as well. Wass relates the composition of the
clinopyroxenes to the dept of their formation. In this relationship the ratio of
the A1IV/A1VI and the Ti content increase and the Si content and the Mg/
(Mg+Fe) ratio decreases as the dept of formation decreases. Decrease of
depth of formation in this case results not only in lowered crystallization
pressure but in lowered temperature as well. When these relationships are
compared to those of the experiments of Thompson (op.cit.) several similar-
ities appear. Of primary importance is the fact that the changes in the Ti
content and the A1IV/A1VI ratio are identical. Furthermore the rather
irregular behaviour of the Si content and the Mg/(Mg+Fe) ratio in the
experiments turn out to be direct trends in nature.
The Cr-diopsides and Al-augites studied by Wass (op.cit.) are composi-
tionally identical to the Jan Mayen chromian diopsides and titan-salites
respectively. The low pressure clinopyroxene is in addition quite similar to
the late overgrowth on the titan-salites and to the groundmass clino-
pyroxene of the Jan Mayen lavas. The metamorphic nature discussed by
Wass (op.cit.) is not found in the case of the Jan Mayen clinopyroxenes