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Fig. 6.12 A schematic P-f
(O 2 ) diagram illustrating
increase in oxygen fugacity of
a lamproitic liquid as it
ascends from the mantle (after
Foley et al. 1986b)
-Log f(0 2
10
9
8
7
6
5
4
3
B
Temperature
1300 °
1
2
3
4
G
D
5
A
6
CW
NNO
WM
IW
(carbon
water) buffer, if not constrained by carbon saturation. They also think that
survival of diamond in an olivine lamproite would be related to sluggish diamond
breakdown reaction. Foley (1985) studied chrome spinels found in olivine
phenocrysts in lamproites and experimentally calibrated the ferric number [100 X
Fe 3+ /(Fe 3+ +Fe 2+ )] of spinel as oxygen fugacity sensors. Foley concluded that the
Gaussberg lamproites crystallized under f(O2)-T 2 )-T condition, which is close to 10 6.5
and 1,300
-
C, which corresponds to NNO buffer condition.
If the Gaussberg lamproitic melt originated at a depth > 52 km under C - H buffer
condition, then the primary melt probably originated under a much more reducing
condition. If the melt travelled along the line AB (at a constant f(O 2 )of10 6.5 to
reach B, then according to their estimate 0.09 wt%H 2 O must dissociate to maintain a
constant f(O 2 ) path from C
°
C.
The reaction, H 2 O <=> H 2 + 1/2O 2 , coupled with H 2 loss by diffusion was
suggested by Sato (1972) for oxidation mechanism in a magma. Foley et al. (1986b)
suggested that the intrinsic f(O 2 ) of the system may be maintained between the path
A
H to FMQ buffer condition near the surface at 1,300
°
-
B during the ascent by diffusion of H 2 loss via following reaction,
-
þ
=
¼
=
2H 2
FeO
1
2H 2 O
1
FeO 1 : 5
They thought that the Gaussberg composition has a median value for primary
lamproitic magma (FeO in Gaussberg lamproite is 6 wt% compared to a range of
4
8 % for primary lamproites, Barton and Hamilton 1978).
-
 
 
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