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To clarify the effect of water on carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments in the system KAlSi 3 O 8 -CaMgSi 2 O 6 -NaAlSi 2 O 6 -CaMg(CO 3 ) 2  under nominally dry and hydrous conditions by adding 1.5 wt% H 2 O at a pressure of 6 GPa and temperatures of 1000 to 1500 °C. Both systems start to melt at 1050–1100 °C. Under anhydrous condition the melting occurs via the following reaction: 6KAlSi 3 O 8  (K-feldspar) + 6CaMg(CO 3 ) 2  (dolomite) = 2(Ca n ,Mg 1-n ) 3 Al 2 Si 3 O 12  (garnet) + Al 2 SiO 5  (kyanite) + 11SiO 2  (coesite) + 3 K 2 (Ca 1-n , Mg n ) 2 (CO 3 ) 3  (carbonatitic melt) + 3CO 2  (fluid and/or liquid), where n ~ 0.3–0.4. The carbonatitic melt has the following composition 38(K 0.92 Na 0.08 ) 2 CO 3 ⋅62Ca 0.62 Mg 0.38 CO 3 . A second immiscible silicic melt containing (in wt%, volatile free) SiO 2  = 68.8, Al 2 O 3  = 12.6, CaO = 3.7, MgO = 2.4, Na 2 O = 1.1, and K 2 O = 11.3 appears at 1250 °C. Both melts remain stable up to 1500 °C and coexist with the clinopyroxene ± garnet ± coesite residue. In the presence of water stored away in phengite, the melting begins with silicic melt, which contains (in wt%, volatile free) SiO 2  = 61.4, Al 2 O 3  = 15.3, CaO = 4.8, MgO = 3.0, Na 2 O = 2.2, and K 2 O = 13.3, and coexists with phengite, dolomite, clinopyroxene, and coesite. The phengite + dolomite assemblage remains to 1100 °C and disappears at 1200 °C producing two immiscible melts carbonatitic with approximate composition, 19(K 0.89 Na 0.11 ) 2 CO 3 ⋅81Ca 0.57 Mg 0.43 CO 3 , and silicic containing (in wt%, volatile free) SiO 2  = 63.3, Al 2 O 3  = 15.6, CaO = 4.5, MgO = 3.0, Na 2 O = 2.0, K 2 O = 11.6. The present results imply that partial melting of continental material subducted to a depth of 200 km can yield simultaneous formation of two immiscible melts, K-dolomitic and K-aluminosilicate. Under dry conditions, carbonatitic melt appears earlier (at a lower temperature). Given the low density and high mobility of this melt, it must rapidly percolate upward, leaving a refractory eclogite-like residue and leaving no chance for the formation of a second aluminosilicate melt. However, under hydrous conditions silicate melt appears earlier than carbonatitic melt, leaving a phengite- and dolomite-bearing residue, which finally yields the formation of two immiscible silicic and carbonatitic melts. The compositions of these melts fall in the compositional range of carbonatitic and silicic high-density fluids (HDFs) in diamonds worldwide. Thus, we suggest that the presence of water is a necessary requirement for the formation of immiscible HDFs inclusions in diamonds, and this suggestion is strongly supported by natural data from HDFs.

american mineralogist

Effect of water on carbonate-silicate liquid immiscibility in the system KAlSi 3 O 8 - CaMgSi 2 O 6 -NaAlSi 2 O 6 -CaMg(CO 3 ) 2  at 6 GPa: Implications for diamond-forming melts