Geoscience Reference
In-Depth Information
Fig. 2.4
Oxygen fugacity (
fO
2
; relative to the IW
buffer) calculated for garnet peridotite along the
T-profile of a cratonic geotherm as a function of
pressure. The curve for Fe-Ni alloys was calculated for
peridotite using equations in Frost and McCammon
(2008). The data for garnet peridotites from the
Kaapvaal Craton (South Africa) is after Woodland and
Koch (2003). The positions of the buffers FMQ (Fayalite
+
Fig. 2.5
Position of solidi in volatile-free and
volatile-bearing Hawaiian pyrolite after Green and
Falloon (1998). The decarbonation reaction
Fo
Dol is shown for the
CO
2
-bearing pyrolite. Reduced solidus is for the
Hawaiian pyrolite equilibrated with a CO
4
-H
2
O fluid
at
fO
2
=
+
Di
+
CO
2
=
En
+
O
2
=
Magnetite+Quartz) and EMOG/D
+
1. Spl, spinel; Grt, garnet; Parg,
pargasite; Fo, forsterite; Di, diopside; En, enstatite; Dol,
dolomite; Lz - lherzolite. Reproduced with permission
of Cambridge University Press.
IW
(En
+
Mst
=
Ol
+
Graphite
/
Diamond
+
O
2
), are after
Kadik (2003), Stagno and Frost (2010).
(Dasgupta
et al
., 2004; Dasgupta & Hirschmann,
2006; Dasgupta & Hirschmann, 2010) models
for eclogite and peridotite melting beneath
mid-ocean ridges were developed. Melting occurs
when small portions of carbonatite melt rise
from deep levels and cause further melting upon
ascent with the participation of H
2
O.
Systems with a reduced C-O-H fluid were
studied experimentally at pressures of up to
6-7GPa. A peridotite system with a C-O-H
fluid buffered at WCWO (WC-W-WO
2
=
3.6GPa (Figure 2.5). At the same time, Jakobsson
& Holloway (2008) studied peridotite with a
reduced C-O-H fluid buffered by IW at 5-12GPa
and observed melting at 1200-1250
◦
C, which
is substantially below the solidus of the system
peridotite-C-O-H fluid at IW
1 (Figure 2.5).
The fluid compositions measured in these
and other experiments (Taylor & Green, 1988;
Jakobsson & Oskarsson, 1990; Matveev
et al
.,
1997; Sokol
et al
., 2004; Sokol
et al
., 2009) some-
times agrees with the model fluid compositions
calculated from the equations of state for real
gases (Saxena & Fei, 1987; Zhang & Duan, 2009)
but sometimes not. For example, high hydrogen
and low CH
4
contents of quenching melt, which
do not agree with calculations, are reported by
Sokol
et al
. (2009).
+
1)
was studied by Taylor and Green (1988) at
0.9-3.6GPa. According to Taylor and Green
(1988), the CH
4
/
IW
+
(CH
4
+
H
2
O) ratio of the fluid
measured by mass spectrometry decreases from
0.8 to 0.3 at 1050-1250
◦
C and 1.5-3.6GPa.
The solidus temperature in a system with a
reduced fluid (CH
4
>
H
2
O) was 200-300
◦
C
higher than in systems with H
2
OandCO
2
at
