Japan Geoscience Union Meeting 2018

Presentation information

[EE] Evening Poster

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

[S-IT25] Deep Carbon: Diamond formation and carbon speciation in Earth and planetary processes

Tue. May 22, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Konstantin Litasov(V.S. Sobolev Institute of Geology and Mineralogy SB RAS), Hiroyuki Kagi(東京大学大学院理学系研究科附属地殻化学実験施設, 共同), Craig E Manning (University of California Los Angeles)

[SIT25-P02] The T-X phase diagram Na2CO3–CaCO3 at 3 GPa

Ivan Podborodnikov1, Anton Shatskiy2, Anton Arefiev1, Artem Chanyshev2, *Konstantin Litasov2 (1.Novosibirsk State University, Novosibirsk, Russia, 2.V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia)

Keywords:carbonate, phase diagram, mantle metasomatism

Phase relations in the system Na2CO3–CaCO3 have been studied at 3 GPa and 800–1525 °C. The system has one intermediate compound, Na2Ca3(CO3)4, at 800 ºC, and two intermediate compounds, Na2Ca(CO3)2 and Na2Ca3(CO3)4, at 850 ºC (Fig. 1a). CaCO3 crystals recovered from experiments at 950 °C and 1000 °C are aragonite and calcite, respectively. Maximum solid solution of CaCO3 in Na2CO3 is 20 mol% at 850 ºC. The Na-carbonate–Na2Ca(CO3)2 eutectic locates near 860 °C and 56 mol% Na2CO3. Na2Ca(CO3)2 melts incongruently to Na2Ca3(CO3)4 and a liquid containing about 51 mol% Na2CO3 at ~ 880 °C. Na2Ca3(CO3)4 disappears above 1000 °C via incongruent melting to calcite and a liquid containing about 43 mol% Na2CO3. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 87 mol% Na2CO3. Na-carbonate remains solid up to 1150 °C and melts at 1200 °C (Fig. 1c). The Na2CO3 content in the liquid coexisting with calcite decreases to 15 mol% as temperature increases to 1300 °C. CaCO3 remains solid up to 1500 °C and melts at 1525 °C (Fig. 1d).
Considering the present and previous data, a range of the intermediate compounds on the CaCO3-Na2CO3 join changes as pressure increases in the following sequence: Na2Ca(CO3)2, Na2Ca2(CO3)3 (0.1 GPa) → Na2Ca(CO3)2, Na2Ca3(CO3)4 (3 GPa) → Na4Ca(CO3)3, Na2Ca3(CO3)4, Na2Ca4(CO3)5 (6 GPa) (Fig. 1b). Thus, the nyerereite stability field extends to the shallow mantle pressures, whereas the shortite stability field terminates somewhere between 0.1 and 3 GPa. Consequently, findings of nyerereite and shortite among daughter phases in the melt inclusions in olivine from the sheared garnet peridotites are consistent with their mantle origin.
This study was supported by Russian Foundation for Basic Research (No 17-05-00501).

Fig. 1. (a) The system Na2CO3–CaCO3 at 3 GPa. (b) Comparison with previous data at 0.1 GPa (Cooper et al. 1975) and 6 GPa (Shatskiy et al. 2013). (c) Na2CO3 melting. (d) CaCO3 melting. Arg = aragonite; Cal = calcite; Na2 = solid solution of CaCO3 in Na2CO3; Na4Ca = Na4Ca(CO3)3; Na2Ca = Na2Ca(CO3)2; Na2Ca3 = Na2Ca3(CO3)4; Na2Ca4 = Na2Ca4(CO3)5; L = liquid; F = CO2 fluid.