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Cs
30
Qz
20
10
400
500 600
Tem pe r a tur e ( C )
0
Fig. 12.19 P-T diagram showing experimental results on an assemblage comprising K-
feldspar + phlogopite yielding a mixture composed of K-feldspar, phlogopite, phengite and
quartz [no H
2
O(circles) or a minimum of 3 wt% of H
2
O(boxes) had been added]. The symbols
represent experimental conditions including the estimated 2
uncertainty; closed = no decom-
position detected; half closed = partial disappearance of K-feldspar + phlogopite, in particular,
when less than 11 wt% of H
2
O had been added; open = total or nearly partial disappearance of this
assemblage in the presence of about 3.5 wt% of H
2
O. (after Massonne (1992)
σ
however, cannot be observed in natural MARIDs. Therefore, MARID-type rocks
do not represent the bulk composition from which they were formed and, hence
must be products of an open system crystallisation.
12.8.2 Investigation on High Pressure Stability of Phengite
Experiments were conducted by Massonne (1992) to study following reactions in
the system K
2
O
MgO
Al
2
O
3
-
SiO
2
-
H
2
O at temperatures between 400 and 700
°
C:
-
-
(1) Phengite
+
quartz + potassium and magnesium-rich siliceous
uid
feld-
→
spar + phlogopite + H
2
O, and
(2) phengite + talc + potassium- and magnesium-rich siliceous
uid
phlogo-
→
pite + quartz/coesite + H
2
O.
His experimental results are summarised in Fig.
12.19
, which suggests that
subduction of potassium-rich metasediments, constituting fraction of the oceanic
crust, could give rise to low-temperature ultrapotassic
fluid, at
temperatures
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