Geology Reference
In-Depth Information
13.2 Investigation on a Synthetic Leucite Basanite
and Melilite-Nephiline Leucitite up to 2.5 Gpa
and Variable Temperatures
13.2.1 Investigation on a Natural Leucite Basanite
and a Tephrite
Experiments in the presence of water vapour [P(H 2 O) = P(Total)] were made
(between 0.1 and 0.5 Gpa) on two natural leucite-bearing rocks, collected from
Kunkskopf and Hochsimmer (East Eifel, West Germany, Table 13.2 ). The rock
from Kunkskopf contained phlogopite, plagioclase, nepheline, leucite, apatite, and
magnetite in the groundmass with clinopyroxene, and small amounts of olivine as
phenocrysts, thus it is a basanite. The rock from Hochsimmer is a tephrite, con-
sisting of clinopyroxene and a small amount of phlogopite as phenocrysts with rare
amounts of nepheline, plagioclase, and opaque minerals in the groundmass. At a
pressure of 0.5 Gpa and temperatures of 800 and 850
C, the powdered samples of
the rocks from both the areas produced the same assemblage of clinopyroxene,
amphibole, phlogopite, magnetite and liquid, although the relative proportions of
amphibole and phlogopite are less in the Hochsimmer sample. At 0.5 GPa below
750
°
C the tephrite from Hochsimmer produced an assemblage of phlogopite and
clinopyroxene with minor amounts of amphibole and magnetite. Disappearance of
leucite was noted in both cases slightly above 0.1 GPa from 600 to 800
°
°
C. At
1 GPa below 800
C the leucite- nepheline tephrite is represented by an assemblage
containing nepheline, amphibole, phlogopite, magnetite and pyroxene. Nepheline
disappears above 800
°
C and the resulting assemblage at 1 GPa corresponds to a
magnetite-amphibole-phlogopite pyroxenite. The amphibole is a potassium-
richterite.
Experimental investigation was made on a leucite basanite (F 20 Di 30 Lc 30 An 20 )
under different P-T
°
T conditions in presence of excess water up to 2.5 GPa by Gupta
et al. (1976). Their results are summarized in Fig. 13.2 , which shows that above
1 GPa the synthetic leucite basanite is converted to a garnet and phlogopite-bearing
pyroxenite. Appearance of garnet may be explained by the following reaction:
-
Mg 2 SiO 4 +Ca 2 Al 2 Si 2 O 8
(Mg 2 Ca)Al 2 Si 3 O 12
ð 13 : 1 Þ
forsterite
anorthite
ð
pyrope
grossularite
Þ ss
It should be pointed out that in their experiments on an assemblage comprising
forsterite and anorthite (present in 1:1 molar ratio), Kushiro and Yoder (1966) noted
that at 1.05 GPa and 1,000
°
C, the assemblage is converted to a garnet pyroxenite,
which has a wide P-T
T stability.
-
 
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