[S-IT27] [EE] Carbon in Planetary Interiors

[SIT27-P04] Effect of alkalis on the reaction of clinopyroxene with Mg-carbonates at 6 GPa: Implications for partial melting of carbonated lherzolite

Shatskiy Anton^1,2, Ivan Podborodnikov^1,2, *Konstantin Litasov^1,2, Artem Chanishev^1,2, Anton Arefiev^1,2, Igor Sharygin¹, Nikolay Karmanov¹, Eiji Ohtani^1,3 (1.V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Science, Siberian Branch, , 2.Novosibirsk State University, 3.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University)

Keywords:Alkalis, Clinopyroxene, Mg-carbonates, High pressure and temperature, solidus, phase relations

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 systems CaMgSi₂O₆ + 2MgCO₃ (Di + 2Mgs), CaMgSi₂O₆ + NaAlSi₂O₆ + 2MgCO₃ (Di + Jd + 2Mgs), CaMgSi₂O₆ + Na₂Mg(CO₃)₂(Di + Na₂Mg), and CaMgSi₂O₆ + K₂Mg(CO₃)₂(Di + K₂Mg) have been examined at 6 GPa. The results are summarized in Fig. 1. The Di + 2Mgs system begins to melt at 1400 °C via the approximate reaction CaMgSi₂O₆ (clinopyroxene) + 2MgCO₃ (magnesite) = CaMg(CO₃)₂ (liquid) + Mg₂Si₂O₆ (orthopyroxene) leading to essentially carbonate liquid (L) with composition of Ca_0.56Mg_0.44CO₃ + 3.5 mol% SiO₂. The initial melting in 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) yielding the carbonate liquid with approximate composition of 10Na₂CO₃·90Ca_0.5Mg_0.5CO₃ + 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 alkali-rich carbonate compositions of Na₂Ca_0.4Mg_0.6(CO₃)₂ + 0.4 mol% SiO₂ and 43K₂CO₃·57Ca_0.4Mg_0.6CO₃ + 0.6 mol% SiO₂. These melts do not undergo significant changes as temperature increases to 1400 °C retaining their calcium number, 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 °C to 1050 °C at 6 GPa yielding K-rich carbonate melt if subsolidus assemblage contains K₂Mg(CO₃)₂ compound. Such a decrease in solidus temperature has been observed previously 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 as far as 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.
This work was supported by Russian Science Foundation (project No 14-17-00609) and performed under the program of Ministry of education and science of Russian Federation (No 14.B25.31.0032).

Fig. 1. Modal abundances of phases present as a function of temperature in the systems CaMgSi₂O₆ + 2MgCO₃ (a), CaMgSi₂O₆ + NaAlSi₂O₆ + 2MgCO₃ (b), CaMgSi₂O₆ + Na₂Mg(CO₃)₂ (c), and CaMgSi₂O₆ + K₂Mg(CO₃)₂ (d) at 6.0 GPa. Modes are in mol% were determined from the bulk compositions of starting mixtures and compositions of phases measured by electron microprobe.