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Fig. 2.6 Solidi of peridotite with different H 2 O contents in the system. The solid line indicates the stability of
dense hydrous magnesium silicate phases and the storage capacity (in wt % H 2 O) at the solidus of the system.
Dashed lines indicate the solidi in systems with lower H 2 O contents (wt %). Gray lines indicate the major phase
transitions and mantle PT-profiles from Figure 2.3. Ol, Olivine; Wd, wadsleyite; Rw, ringwoodite; Mpv,
Mg-perovskite; Fp, ferropericlase; Amp - amphibole; Serp - serpentine. (Constrained using data from Ohtani et al .,
2000; Litasov et al ., 2003, 2011b; Litasov & Ohtani, 2007, 2008).
CO 2 -bearing fluid/melt with low H 2 O-activity
olivine and wadsleyite may contain up to 10
times less H 2 OthaninthesystemwithH 2 O
only (Litasov et al ., 2009; Sokol et al ., 2010).
The parameterization and systematic study of
these dependencies are particularly important
for accurate modeling of H 2 O-bearing solidi and
melting in the deep mantle (see also Keppler,
Chapter 1, this volume). It should be noted that
under supercritical conditions, there is no solidus
in the system. Under these conditions, a fluid
always coexists with solid silicates and the com-
position of the fluid becomes more silicate-rich
with temperature, until it reaches a composition
similar to a silicate melt. They may, however, be
a ''practical solidus,'' if the silicate content of the
fluid changes from a ''fluid-like'' to a ''melt-like''
regime over a narrow temperature interval.
According to Figure 2.6, the transition zone
accumulates H 2 O even if it is only a minor or
trace component in the pyrolite mantle. At the
410 km and 660 km discontinuities, the solidus
temperature in H 2 O-bearing systems decreases
dramatically, and this may cause melting
(Litasov & Ohtani, 2002). If peridotite contains
0.1 wt % H 2 O (which is reasonable for the
transition zone), the solidus temperature at the
410 km discontinuity drops by 200-250 Cin
the olivine stability field and at the 660 km drops
by approximately 600 C in the Mg-perovskite
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