Geoscience Reference
In-Depth Information
Fig. 2.7
Magnesite and aragonite stability in different carbonate-bearing systems. Dashed lines indicate magnesite
decarbonation and melting, according to Irving and Wyllie (1975); Katsura and Ito (1990). The magnesite-SiO
2
line
shows magnesite stability limits for systems containing a free silica phase and roughly corresponds to the reaction
MgCO
3
CO
2
-bearing melt (Dasgupta & Hirschmann, 2006; Litasov & Ohtani, 2009a, 2010).
The stability of magnesite in the peridotite and eclogite systems with H
2
O-CO
2
and the Dol
+
SiO
2
=
MgSiO
3
+
Arg line are
after Litasov (2011); Litasov
et al
. (2011a). A, level of decarbonation reactions near 2-2.5 GPa (see Figure 2.5). Gray
lines indicate average mantle and stagnant slab PT-profiles (Figure 2.3). The stability of aragonite for eclogitic
assemblages is shown after Kiseeva
et al
. (2013) (coincides with peridotite-H
2
O-CO
2
line) and Shirasaka and
Takahashi (2003) (dotted line). The decarbonation reaction of aragonite roughly coincides with the
magnesite
=
Mgs
+
+
SiO
2
line.
In most peridotite and eclogite systems,
magnesite is the only carbonate phase at pres-
sures above 6-7GPa. However, in some eclogite
systems with an elevated CaO content, the
stable phase is aragonite (Shirasaka & Takahashi,
2003; Kiseeva
et al
., 2013). The decarbonation
reaction CaCO
3
+
decarbonation is replaced by melting at pressures
above 6GPa with the formation of a carbonatite
melt or a carbonate-bearing silicate melt. This
boundary lies at
∼
10 GPa for aragonite.
2.6.3 Alkali carbonatite melting
CO
2
occurs
at temperatures close to slightly above the mantle
adiabat. In eclogite-CO
2
systems, aragonite is
stable at temperatures of 1200
◦
C and below (at
a pressure below 7GPa; Figure 2.7). Magnesite
SiO
2
=
CaSiO
3
+
Near-solidus melts of Na- and K-bearing car-
bonated peridotite and eclogite systems, show
strong enrichments in alkalis. However, the pre-
cise determination of the compositions of these
melts is difficult due to their very low modal