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

Anton Shatskiy, Ivan V. Podborodnikov, Anton V. Arefiev, Konstantin D. Litasov, Artem D. Chanyshev, Igor S. Sharygin, Nikolai S. Karmanov, Eiji Ohtani

Результат исследования: Научные публикации в периодических изданияхстатья

14 Цитирования (Scopus)

Аннотация

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.

Язык оригиналаанглийский
Страницы (с-по)1934-1946
Число страниц13
ЖурналAmerican Mineralogist
Том102
Номер выпуска9
DOI
СостояниеОпубликовано - 1 сен 2017

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