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anhydrous minerals of the mantle constitute a
water reservoir quite comparable to the mass of
the oceans (e.g. Smyth, 1987; Bell & Rossman,
1992) and that some hydrous phases as well as
nominally anhydrous minerals are able to recycle
water deep into the mantle during subduction.
R upke et al . (2004) presented a model of water
recycling into the mantle based on the stability of
hydrous phases in subduction zones. They show
that serpentine in altered peridotite may not de-
hydrate if subduction follows a relatively cool
P,T path and it may therefore be able to recycle
water deep into the mantle. Estimated residence
times for water in mantle and ocean are in the
order of 1 Ga. The model of R upke, however, does
not consider water transport by nominally anhy-
drous minerals. Considering that both omphacite
in the eclogites of the basaltic layer as well as
aluminous orthopyroxenes in the ultrabasic layer
may dissolve more than 1000 ppm of water, sig-
nificantly shorter residence times could result
from the contribution of these minerals to water
recycling.
The large contrast of water solubility in the
wadsleyite and ringwoodite phases of the transi-
tion zone relative to both the upper and the lower
mantle makes it plausible that the transition zone
could be strongly enriched in water to such an
extent that it constitutes the main water reser-
voir in the Earth's interior. Enrichment of water
could already have occurred very early in Earth's
history (Kawamoto et al ., 1996), possibly by crys-
tallization of wadsleyite and ringwoodite from a
hydrous magma ocean. Archean komatiites have
sometimes been linked to a hydrated mantle,
but the evidence remains controversial (Marty &
Yokochi, 2006). Alternatively, accumulation of
water from subducting slabs may have hydrated
the transition zone over Earth's history. Such a
hydration requires a mechanism for efficient lat-
eral transport of water in the transition zone.
This could be achieved by hydrous silicate melts
ponding atop or on the bottom of the transition
zone. If water is sufficiently enriched in the tran-
sition zone, melting may occur upon upwelling
or downwelling of material once it leaves the sta-
bility field of wadsleyite and ringwoodite. This
does not necessarily require supersaturation of
water; rather, the change in water storage capacity
upon phase transition from wadsleyite to olivine
or from ringwoodite to perovskite and ferroperi-
clase will increase water fugacity in the system,
even when water contents remain constant. The
increase in water fugacity may be sufficient to
induce melting. Bercovici and Karato (2003) sug-
gested that such a continuous layer of melt at the
410 km discontinuity may act as a chemical filter
that strips ascending material from incompati-
ble trace elements and thereby helps to maintain
chemically distinct mantle reservoirs, even when
convection occurs over the entire mantle. Melts
formed at the bottom of the transition zone
may, depending on density contrast, either form
a layer at the 660 km discontinuity or sink to the
core-mantle boundary (Hirschmann, 2006).
The idea of an essentially dry lower mantle is
in some contrast to the view that the source of
the relatively volatile-rich ocean island basalts is
located somewhere in the deep mantle, deeper
than the MORB source. A possible solution could
be that volatiles and incompatible elements are
highly concentrated in a melt layer near the
core-mantle boundary and that some material
from there gets entrained in deep mantle plumes.
1.4
Carbon
1.4.1 Carbon solubility in mantle minerals
The solubility of carbon in the silicate minerals of
the Earth's mantle is generally very low (Keppler
et al ., 2003; Shcheka et al ., 2006). The solubility
of carbon in olivine in equilibrium with a car-
bonatite melt is less than 1 ppm below 4 GPa and
reaches about 12 ppm at 11 GPa (Shcheka et al .,
2006). The solubility is independent of oxygen
fugacity, suggesting that olivine dissolves carbon
as C 4 + , i.e. in the same oxidation state as it oc-
curs in carbonatite melt. Solubilities in enstatite,
diopside and pyrope are of similar magnitude as
for olivine, while transition zone and lower man-
tle minerals, such as wadsleyite, ringwoodite and
MgSiO 3 perovskite dissolve even less carbon.
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