Earth’s Mantle Melting in the Presence of C–O–H–Bearing Fluid - Physics and Chemistry of the Deep Earth - page 47

abundance and because of the poor stability of
quench products during polishing and electron
probe microanalysis (Dasgupta & Hirschmann,
2006; Ghosh et al ., 2009). Mass-balance calcula-
tions of samples obtained below apparent solidi
in these studies show clear deficits of alkalis sug-
gesting the presence of minor alkali-rich liquid
or solid carbonate phases. Recently, we reported
new results on melting and subsolidus phase rela-
tions in alkali carbonatite systems (Litasov et al .,
2013a). We used two starting compositions of Na-
and K-rich (Na 2 O

(upto1wt%),andMgO(upto8wt%)inthe
Na-carbonatite system. Since it can store most
Na 2 O in the system, only minor K-Na-bearing
carbonates were observed in Na-carbonatite in
subsolidus runs. The maximum temperature sta-
bility of Na-aragonite is near 1400 ◦ Cat16GPa.
The solidus temperature is defined by the sta-
bility of double carbonate phases. The slope of
the solidus is relatively steep at pressures below
8-10GPa (Figure 2.8). Above 10GPa the solidus
becomes flat at a temperature near 1150 ◦ C.
Several K- and Na-bearing double carbonates
were observed in both the Na-carbonatite and K-
carbonatite systems. The major phases observed
in the experiments are (K,Na) 2 Mg(CO 3 ) 2 (K- Mg )
and (K,Na) 2 Ca 4 (CO 3 ) 5 (K-Ca) (Figure 2.9). The
high-temperature stability limit of the K-Ca phase
is different in the Na- and K-carbonatite systems.
The Na-bearing phase in the Na-carbonatite
has a slightly negative PT-slope of the upper
phase boundary above 15GPa, whereas the
K-bearing phase in the K-carbonatite is stable up
to 1250 ◦ C at 21GPa (Figure 2.9). Other subsolidus

2wt%
and vice versa) Mg-Ca carbonatite with minor
SiO 2 and FeO and studied their phase relations
at pressures from 3 to 21GPa. The experimen-
tal results and phase compositions are shown in
Figures 2.8 and 2.9. Major carbonate phases in
both the Na-carbonatite and K-carbonatite sys-
tems are aragonite and magnesite. Magnesite was
a liquidus phase together with silicates and was
found among run products in all experiments
up to 1400-1600 ◦ C. Aragonite contains signif-
icant amounts of Na 2 O (up to 7 wt %), K 2 O

= 7wt%andK 2 O =

Fig. 2.8 The solidi of Na- and K-bearing carbonatite from 3 to 21 GPa and the stability of alkali carbonate phases
(Litasov et al ., 2013a) (see Figure 2.10 for compositions). GS-11 shows the solidus of anhydrous carbonated pelite
after Grassi and Schmidt (2011). PT-profiles are after Figure 2.3. Abbreviations for double carbonates are as follows:
Na 2 Mg 2 =

Na 2 Mg 2 (CO 3 ) 3 ,Na 2 Ca = Na 2 Ca(CO 3 ) 2 ;Na 2 Ca 2 = Na 2 Ca 2 (CO 3 ) 3 ;(K,Na) 2 Ca 4 = (K, Na) 2 Ca 4 (Co 3 ) 5 ; and

K 2 Mg = K 2 Mg(CO 3 ) 2 .