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 Ca0.56Mg0.44CO3 + 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 10Na2CO3·90Ca0.5Mg0.5CO3 + 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 alkali-rich carbonate compositions of Na2Ca0.4Mg0.6(CO3)2 + 0.4 mol% SiO2 and 43K2CO3·57Ca0.4Mg0.6CO3 + 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.