Effect of water on carbonate-silicate liquid immiscibility in the system KAlSi3O8-CaMgSi2O6-NaAlSi2O6-CaMg(CO3)2 at 6 GPa: Implications for diamond-forming melts

Abstract

Abstract To clarify the effect of water on carbonate-silicate liquid immiscibility in the diamond stability field, we performed experiments in the system KAlSi3O8-CaMgSi2O6-NaAlSi2O6-CaMg(CO3)2 under nominally dry and hydrous conditions by adding 1.5 wt% H2O 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: 6KAlSi3O8 (K-feldspar) + 6CaMg(CO3)2 (dolomite) = 2(Can,Mg1-n)3Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11SiO2 (coesite) + 3 K2(Ca1-n, Mgn)2(CO3)3 (carbonatitic melt) + 3CO2 (fluid and/or liquid), whereñ 0.3–0.4. The carbonatitic melt has the following composition 38(K0.92Na0.08)2CO3⋅62Ca0.62Mg0.38CO3. A second immiscible silicic melt containing (in wt%, volatile free) SiO2 = 68.8, Al2O3 = 12.6, CaO = 3.7, MgO = 2.4, Na2O = 1.1, and K2O = 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) SiO2 = 61.4, Al2O3 = 15.3, CaO = 4.8, MgO = 3.0, Na2O = 2.2, and K2O = 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(K0.89Na0.11)2CO3⋅81Ca0.57Mg0.43CO3, and silicic containing (in wt%, volatile free) SiO2 = 63.3, Al2O3 = 15.6, CaO = 4.5, MgO = 3.0, Na2O = 2.0, K2O = 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.