Abstract
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: CaMgSi2O6+2MgCO3 (Di+2Mgs); CaMgSi2O6+NaAlSi2O6+2MgCO3 (Di+Jd+2Mgs); CaMgSi2O6+Na2Mg(CO3)2 (Di+Na2Mg); and CaMgSi2O6+K2Mg(CO3)2 (Di+K2Mg). The Di+2Mgs system begins to melt at 1400 °C via the approximate reaction CaMgSi2O6 (clinopyroxene) + 2MgCO3 (magnesite) = CaMg(CO3)2 (liquid) + Mg2Si2O6 (orthopyroxene), which leads to an essentially carbonate liquid (L) with composition Ca0.56Mg0.44CO3 + 3.5 mol% SiO2. The initial melting of the Di+Jd+2Mgs system occurs at 1350 °C via the reaction 2CaMgSi2O6 (clinopyroxene) + 2NaAlSi2O6 (clinopyroxene) + 8MgCO3 (magnesite) = Mg3Al2Si3O12 (garnet) + 5MgSiO3 (clinopyroxene) + 2CaMg(CO3)2 (liquid) + Na2CO3 (liquid) + 3CO2 (liquid and/or fluid), which yields the carbonate liquid with approximate composition of 10Na2CO3 ·90Ca0.5Mg0.5CO3 + 2 mol% SiO2. The systems Di+Na2Mg and Di+K2Mg start to melt at 1100 and 1050 °C, respectively, via the reaction CaMgSi2O6 (clinopyroxene) + 2(Na or K)2Mg(CO3)2 (solid) = Mg2Si2O6 (orthopyroxene) + (Na or K)4CaMg(CO3)4 (liquid). The resulting melts have the alkali-rich carbonate compositions Na2Ca0.4Mg0.6(CO3)2 + 0.4 mol% SiO2 and 43 K2CO3 ·57Ca0.4Mg0.6CO3 + 0.6 mol% SiO2. These melts do not undergo significant changes as temperature rises to 1400 °C, retaining their calcium number and a high Na2O, K2O, and low SiO2. 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 K2Mg(CO3)2 compound. Such a decrease in solidus temperature has been previously observed in the K-rich carbonated lherzolite system. Although a presence of eitelite, Na2Mg(CO3)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.
Acknowledgments
We are very grateful to Robert W. Luth for a constructive review, Roland Stalder and Keith Putirka for editorial handling. This work is financially supported by Russian Science Foundation (project no. 14-17-00609) and performed under the program of Ministry of Education and Science of Russian Federation (project no. 14.B25.31.0032).
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