| Реферат: | eng: 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 degrees C. Both systems start to melt at 1050-1100 degrees C. Under anhydrous condition the melting occurs via the following reaction: 6KAlSi(3)O(8) (K-feldspar) + 6CaMg(CO3)(2) (dolomite) = 2(Ca-n, Mg1-n)(3)Al2Si3O12 (garnet) + Al2SiO5 (kyanite) + 11SiO(2) (coesite) + 3 K-2(Ca1-n, MgO2(CO3)(3) (carbonatitic melt) + 3CO(2) (fluid and/or liquid), where n similar to 0.3-0.4. The carbonatitic melt has the following composition 38(K0.92Na0.08)(2)CO3 center dot 62Ca(0.62)Mg(0.38)CO(3). 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 degrees C. Both melts remain stable up to 1500 degrees 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 degrees C and disappears at 1200 degrees C producing two immiscible melts carbonatitic with approximate composition, 19(K0.89Na0.11)(2)CO3 center dot 81Ca(0.57)Mg(0.43)CO(3), 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.
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