[S-IT22] [EE] 核-マントルの相互作用と共進化

[SIT22-37] Iron-carbonate interaction in the lower mantle and at the core-mantle boundary

★招待講演

*Konstantin Litasov¹、Naira Martirosyan¹、Hiroaki Ohfuji²、Sergey Lobanov³、Alexander Goncharov³ (1.V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia、2.Geodynamic Research Center, Ehime University, Matsuyama, Japan、3.Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA)

キーワード：Mg-carbonate, carbide, diamond, lower mantle, core-mantle boundary

The presence of carbonates in the deep Earth strongly depends on the oxygen fugacity, controlled by oxidation state of iron in minerals and melts. A large part of the lower mantle can be significantly reduced with detectable amount of Fe⁰. Therefore, subducted carbonates would interact with Fe⁰ dispersed in the ambient mantle. However, the mechanism of this interaction remains controversial. We investigated the MgCO₃–Fe⁰ system at 70–145 GPa and 800–2600K using in situ X-ray diffraction in a diamond anvil cell. MgCO₃ crystals and Fe foil (99,9%) were used as the starting materials. Formation of wustite (FeO), ferropericlase (Mg_0.6Fe_0.4)O, carbide (Fe₇C₃) and diamond was observed. Three different modifications of FeO were detected: B1 at T = 1100–2600K and 70–145 GPa, rB1 at T<1100 K and P<136GPa; and B8 at P = 143-145 GPa. Interestingly, we observed coexistence of wüstite and ferropericlase, which may suggest an existence of immiscibility gap in FeO-MgO system at P > 70 GPa. Mg-carbonate reduction can be presented by following reaction: 6MgCO₃ + 19Fe = 8FeO + 10(Mg_0.6Fe_0.4)O + Fe₇C₃ + 3C. The formation of diamond was confirmed by TEM study of run products at 100-145 GPa. The studied carbonate-iron reaction supports formation of the (Fe,Mg)O, carbide and diamond in the lower mantle and at the Earth’s core-mantle boundary indicating that subducted carbonates transported to the core-mantle boundary would be reduced to carbide or diamond. Similar reaction may occur in the Fe-CaMg-carbonate systems. Using these data we propose that core-mantle boundary is important to produce diamond and Fe-carbide.