Earth’s Mantle Melting in the Presence of C–O–H–Bearing Fluid - Physics and Chemistry of the Deep Earth - page 49

Peridotite

Eclogite

(a) (b)

Fig. 2.10 Solidi in the systems peridotite-C-O-H fluid (A) and eclogite-C-O-H fluid (B) at fO 2 buffered by MMO
(Mo-MoO 2 ) and IW (Fe-FeO). TG, Solidus in the system peridotite 1 (Taylor &

Green, 1988) (Figure 2.5). For comparison a dash-and-dot line in B indicates the peridotite solidus buffered at IW
from A. Gray lines indicate the main phase transitions, volatile-free solidi, and mantle geotherms from Figure 2.3.
Ol, Olivine; Gt, garnet; Op, orthopyroxene; Cp, clinopyroxene; Wd, wadsleyite; Rw, ringwoodite; Fl, fluid.
Reproduced with permission of Nature.

+ C-O-H fluid at fO2 = IW +

compositions (Ghosh et al ., 2009; Kiseeva et al .,
2013). The factors determining the position of the
solidi at pressures above 6GPa in systems with
H 2 OandCO 2 reveal fundamental differences. In
the H 2 O-bearing systems, the solidus depends on
the H 2 O solubility in nominally anhydrous sil-
icates. The solidi in the system peridotite-H 2 O
within the transition zone can lie 300-400 ◦ C
above those of H 2 O-bearing eclogite because of
the high H 2 O solubility in wadsleyite and ring-
woodite (Figure 2.3). The position of the solidus in
CO 2 -containing systems depends on the presence
of alkalis and H 2 O. A small amount of K 2 O can re-
duce the solidus temperature of carbonate-bearing
eclogite or peridotite by 400-500 ◦ Cat20GPa.

The CO 2 content of the system itself should not
have significant influence on the solidus. The sta-
bility of carbonates (magnesite, aragonite) is not
significantly dependent on the alkali content, but
decreases dramatically if H 2 O is added to the sys-
tem (Figure 2.7). In peridotite and eclogite systems
with a hypothetical C-O-H fluid, the solidi are
at higher temperature than for systems with CO 2
and H 2 O. Nevertheless, they are still substan-
tially (300-400 ◦ C) lower than the ''dry'' solidi at
16GPa (Figure 2.10). Under reduced conditions,
prevalent in the most mantle, the first melt for
PT-profiles between the average adiabat and sub-
duction would be a metallic liquid in the Fe-S-C
system (Morard et al ., 2007; Nakajima et al .,