Geoscience Reference
In-Depth Information
Earth's Mantle Melting in the
Presence of C-O-H-Bearing Fluid
2
KONSTANTIN D. LITASOV,
1,2
ANTON SHATSKIY,
1,2
AND EIJI OHTANI
1
1
Department of Earth and Planetary Materials Science, Graduate School of Science,
Tohoku University, Sendai, Japan
2
V.S. Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk, Russia
Summary
with H
2
OandCO
2
at 15GPa. At the same time
they are still 300-400
◦
C lower than volatile-free
solidi. Thus, we provide a first calibration of the
dependence of mantle melting at constant pres-
sure on redox conditions. The stability boundary
of Fe-Ni alloy, which may coincide with the
lithosphere-asthenosphere boundary under cra-
tons (200-250 km), the 410 km discontinuity and
the transition zone itself, may be paramount to re-
dox and decarbonation-dehydration melting and
freezing. Subducted carbonates rather than water
may control melting in the ''big mantle wedge''
model for stagnant slabs. We proposed a phe-
nomenological model for the segregation of a
slab-derived carbonated melt in the transition
zone, which can move as a diapir through the
transition zone and upper mantle and initiate
magmatism at the surface.
Recent experimental data on phase transforma-
tions and melting in peridotite and eclogite sys-
tems with a C-O-H fluid at pressures up to about
30GPa are reviewed with special attention to the
effect of redox conditions. The fundamental dif-
ferences for partial melting in systems with H
2
O,
CO
2
and reduced C-O-H fluid (CH
4
-H
2
O-H
2
)
are outlined. Melting in systems with H
2
O de-
pends mainly on the total H
2
O content and is
controlled by H
2
O solubility in nominally anhy-
drous minerals. Partial melting occurs when the
total H
2
O content of the system exceeds the H
2
O
storage capacity in the minerals of the rock under
given
P
-
T
-
X
-
fO
2
conditions. Melting in systems
with CO
2
is determined by carbonate stability
and is strongly affected by the alkali (particu-
larly, K
2
O) content in the system. The onset of
melting is relatively insensitive to the total CO
2
content. Studies of peridotite and eclogite sys-
tems saturated with H
2
OandCO
2
show that
H
2
O -bearing phases, such as dense hydrous sil-
icates, superhydrous phase B and phase D, can
control initial melting and low degree partial
melts are silica-rich. Moreover, most solidi are
flattening out at pressures above 6-8GPa. The
solidi of peridotite and eclogite with coexisting
reducedC-O-Hfluid (presumably CH
4
+
2.1 Introduction
In a widely accepted pyrolitic model, the upper
mantle consists of peridotite with
50-60%
olivine, 20-30% pyroxenes, and 15-25% garnet
(e.g., Ringwood, 1979). Subducted oceanic plates
are the major source of heterogeneities in the
present-day mantle. They supply the mantle
with terrigenous sediments, altered basalts and
variably depleted peridotite. A significant part
of this material, especially incompatible and
∼
H
2
O, at
the oxygen fugacity near the Fe-FeO buffer) are lo-
cated 300-500
◦
C above the solidi of the systems
