JpGU-AGU Joint Meeting 2017

Presentation information

[EE] Poster

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

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

Tue. May 23, 2017 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall HALL7)

convener:Craig E Manning(University of California Los Angeles), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Hiroyuki Kagi(Geochemical Research Center, Graduate School of Science, University of Tokyo), Konstantin Litasov(V.S. Sobolev Institute of Geology and Mineralogy SB RAS)

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

Shatskiy Anton1,2, Ivan Podborodnikov1,2, *Konstantin Litasov1,2, Artem Chanishev1,2, Anton Arefiev1,2, Igor Sharygin1, Nikolay Karmanov1, Eiji Ohtani1,3 (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 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.