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.

You do not currently have access to this article.