Geology Reference
In-Depth Information
15.11 Recycling of Potassium from Subducted
Oceanic Crust
It has been argued by Schmidt (1996) that metasomatism should occur directly and
continuously from subducted potassium-bearing crust from shallow levels to a
depth of 300 km. He therefore, conducted experiments on mid-ocean ridge basalts
(MORB) and andesite bulk compositions in order to explain the mechanism for
transport Metasomatic Fluid of K
2
O to mantle depths and to understand the gen-
eration of transport of K
2
O rich metasomatic
fluids into the mantle depth. His
results are summarized in Fig.
15.2
, which show that the principal K-bearing phase
observed at subsolidus condition was phengite, a white mica. This phase formed at
low pressures (<1.5 Gpa) and remain present until 9.5
C,
Fig.
15.2
). At pressures representing depths >110 km, omphacite (a sodium and
calcium-rich clinopyroxene) became an important host for potassium in addition to
phengite. K-clinopyroxene component in omphacite increases from <0.5 mol%
(2.5 GPa) to as much as 6 mol% at 10.5 GPa (900
10.0 Gpa (750
1,050
°
-
-
°
C); thus omphacite can
incorporate signi
cant amounts of K also, at great depths in subduction zones.
Schmidt (1996) observed that when the weight solidus of MORB is achieved
with increasing temperature
1050
°
C at 9.0 GPa, then phengite disappeared
*
rapidly resulting in the generation of 3
5 volume per cent of a K-rich liquid. Small
euhedral garnets were observed in the interstitial melt patches. He found that the
weight solidus for K-bearing MORB is located at temperatures 100
-
200
°
C lower
-
than the weight
solidus
for
synthetic K-free MORB (970
°
-
1,160
°
Cat
4.5
-
6.5 GPa). The experimental results of Schmidt (1996) suggest that the solidus
0
Temperature ( C)
400
600
800
1000
1200
50
2
75
100
4
150
6
200
warm
slab
8
250
phengite
K-hollandite
or
K-rich fluid
300
10
cold
slab
350
12
Fig. 15.2 Experimental studies on a MORB rock, showing the P-T
-
T stability of chlorite, lawsonite,
epidote and phengite. The solidi for K-free MORB and K-bearing MORB are also shown. The
breakdown curve of phengite to produce K-hollandite and fluid around 300 km is also shown (after
Schmidt 1996)
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