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composition of olivine phenocrysts, pyroxenites
and eclogites account for 10-30% of MORB,
up to 60% and more of OIB and continental
basalts, and 20-30% of komatiites. Such melting
may also take place under ''dry'' conditions, be-
cause the solidus temperature of ''dry'' eclogite is
also below that of peridotite. However, volatiles
play a substantial role in almost all models for
basaltic magma generation (for example, MORB
or OIB) even if their contents are low. Accord-
ing to recent reviews (Hirschmann
et al
., 2009;
Dasgupta & Hirschmann, 2010), the most de-
pleted MORB sources contain 30-120 ppm CO
2
and 50-150 ppm H
2
O, and OIB sources contain
120-1800 ppm CO
2
and 350-1000 ppm H
2
O.
Analysis of the melt compositions obtained by
partial melting of peridotite and eclogite at pres-
sures below 7GPa can be directly compared with
natural igneous rocks with the exception of reac-
tive kimberlite and other alkaline volcanics that
may have been modified upon ascent. Melting of
volatile-free mantle is possible at 20-50 km depth
under the mid-ocean ridges or by significant in-
crease of temperature, for example, in very hot
mantle plumes. The low-degree partial melts of
peridotite correspond to tholeiite basalts at the
depth levels to 100 km.
Partial melts obtained in experiments at pres-
sures above 6-7GPa cannot be directly compared
with natural magmas. Therefore, we can only
constrain hypothetic models for deep melting
and magmatism. There are several important is-
sues for melt compositions formed by melting
of volatile-bearing mantle lithologies. Melting
of H
2
O-bearing peridotite and eclogite produce
andesitic, basaltic, and komatiite-like melts at
20-40% melting. In the systems with CO
2
, low-
degree partial melts are carbonatitic in a wide
pressure range at least to 30GPa. Similar car-
bonatite melts can be formed from carbonated
peridotite or eclogite at H
2
O-undersaturated con-
ditions. We should emphasize that primary man-
tle carbonatites are extremely rare. Most of them
were formed either by liquid immiscibility or by
fractionation of alkali magma (Mitchell, 2005).
Under H
2
O-saturated conditions low-degree par-
tial melts of carbonated peridotite and eclogite
Composition of the C-O-H fluid at 1200
◦
C
at
fO
2
buffered at IW (Fe), Mo-MoO
2
, MMO (Mo), and
Ni-NiO, NNO (Ni); calculated from the equations of
state for real gases (Zhang & Duan, 2009). Only H
2
O
content is shown for NNO. Reproduced with
permission of Elsevier.
Fig. 2.11
2009). The melting occurs when the line IW
0is
crossed, because the system is no longer buffered
at IW. This boundary cannot be correlated with
a depth of 200-250 km as yet; it may certainly
lie above or below. On crossing the line IW
=
0,
the solidus temperature shifts gradually toward
increasingly oxidized systems as
fO
2
changes; it
also depends on the H
2
O solubility in silicates.
This process may cause redox melting and is
considered in detail in the subsequent section.
The eclogite solidus will always be below the
peridotite solidus, which implies the preferen-
tial melting of eclogite. The difference in the
solidi and carbonate-stability temperatures is up
to 100-200
◦
C at pressures above 6GPa. In the
presence of mantle heterogeneities and for melt-
ing caused by plume, eclogites will be the first
to melt, leading to an enrichment of eclogite
component in the melt. This conclusion is con-
sistent with modern models for mantle mag-
matism, where eclogites and hybrid pyroxenites
(produced by the reaction of basaltic melts with
mantle peridotites) play an important role in dif-
ferent mantle sources (Hofmann & White, 1982;
Sobolev
et al
., 2007). Judging by the trace-element
=
