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The reaction between clinopyroxene and Mg-carbonate is supposed to define the solidus of carbonated lherzolite at pressures exceeding 5 GPa. To investigate the effect of alkalis on this reaction, subsolidus and melting phase relations in the following systems have been examined at 6 GPa: CaMgSi 2 O 6 +2MgCO 3  (Di+2Mgs); CaMgSi 2 O 6 +NaAlSi 2 O 6 +2MgCO 3  (Di+Jd+2Mgs); CaMgSi 2 O 6 +Na 2 Mg(CO 3 ) 2  (Di+Na 2 Mg); and CaMgSi 2 O 6 +K 2 Mg(CO 3 ) 2  (Di+K 2 Mg). The Di+2Mgs system begins to melt at 1400 °C via the approximate reaction CaMgSi 2 O 6  (clinopyroxene) + 2MgCO 3  (magnesite) = CaMg(CO 3 ) 2  (liquid) + Mg 2 Si 2 O 6  (orthopyroxene), which leads to an essentially carbonate liquid (L) with composition Ca 0.56 Mg 0.44 CO 3  + 3.5 mol% SiO 2 . The initial melting of the Di+Jd+2Mgs system occurs at 1350 °C via the reaction 2CaMgSi 2 O 6  (clinopyroxene) + 2NaAlSi 2 O 6  (clinopyroxene) + 8MgCO 3  (magnesite) = Mg 3 Al 2 Si 3 O 12  (garnet) + 5MgSiO 3  (clinopyroxene) + 2CaMg(CO 3 ) 2  (liquid) + Na 2 CO 3  (liquid) + 3CO 2  (liquid and/or fluid), which yields the carbonate liquid with approximate composition of 10Na 2 CO 3  ·90Ca 0.5 Mg 0.5 CO 3  + 2 mol% SiO 2 . The systems Di+Na 2 Mg and Di+K 2 Mg start to melt at 1100 and 1050 °C, respectively, via the reaction CaMgSi 2 O 6  (clinopyroxene) + 2(Na or K) 2 Mg(CO 3 ) 2  (solid) = Mg 2 Si 2 O 6  (orthopyroxene) + (Na or K) 4 CaMg(CO 3 ) 4  (liquid). The resulting melts have the alkali-rich carbonate compositions Na 2 Ca 0.4 Mg 0.6 (CO 3 ) 2  + 0.4 mol% SiO 2  and 43 K 2 CO 3  ·57Ca 0.4 Mg 0.6 CO 3  + 0.6 mol% SiO 2 . These melts do not undergo significant changes as temperature rises to 1400 °C, retaining their calcium number and a high Na 2 O, K 2 O, and low SiO 2 . We suggest that the clinopyroxene–Mg-carbonate reaction controlling the solidus of carbonated lherzolite is very sensitive to the carbonate composition and shifts from 1400 to 1050 °C at 6 GPa, which yields K-rich carbonate melt if the subsolidus assemblage contains the K 2 Mg(CO 3 ) 2  compound. Such a decrease in solidus temperature has been previously observed in the K-rich carbonated lherzolite system. Although a presence of eitelite, Na 2 Mg(CO 3 ) 2 , has a similar effect, this mineral cannot be considered as a potential host of Na in carbonated lherzolite, because the whole Na added into the system dissolves as jadeite component in clinopyroxene if bulk Al/Na ≥ 1. The presence of jadeite component in clinopyroxene has little impact on the temperature of the solidus reaction decreasing it to 1350 °C at 6 GPa.

Effect of alkalis on the reaction of clinopyroxene with Mg-carbonate at 6 GPa: Implications for partial melting of carbonated lherzolite