[S-IT17] Mass and energy transport in the crust and mantle: from properties to processes

[SIT17-P01] Experimental study on the stability of andradite (Ca₃Fe₂Si₃O₁₂): insights into fO₂ heterogeneity, ferric iron and diamond distributions in the deep mantle

*Jintao Zhu^1,2,3, Renbiao Tao³, Jinze He², Takayuki Ishii², Yunxiu Li⁴, Lifei Zhang¹ (1.Peking University, 2.Okayama University, 3.HPSTAR, 4.Second Institute Of Oceanography)

Keywords:Carbonate, andradite, ferric iron, redox reactions, phase relation, high presure

Redox state (i.e. oxygen fugacity) and redox reactions affect the physical and chemical reactions between the subducting slabs and the deep mantle. Among various subducting materials, carbonate especially CaCO₃ was proposed as an efficient oxidation and metasomatism agent, leading to the chemical and mineralogical heterogeneity of the Earth’s interior [1]. Andradite (Ca₃Fe³⁺₂Si₃O₁₂), as an important endmember in the Fe³⁺-rich garnet for the host of ferric iron, was found to be related to metasomatism of calcium carbonate [1-2] and they are favored to be formed with increasing pressure in oxythermobarometry equilibrium applied for the mantle silicate minerals [3]. However, the stability of andradite under high pressure is not well constrained, only one study reported andradite transforms from garnet to ferric iron-bearing perovskite (Ca,Fe³⁺)(Si,Fe³⁺)O₃ at 21 GPa [4]. Their study just conducted one experiment in diamond anvil cell and didn’t give a precise transformation pressure as well as detailed investigations on the phase relations, which limits our knowledge towards the products from carbonate redox reactions, ferric iron distribution in the deep mantle.
In this study, we firstly simulated the redox reactions between CaCO₃+SiO₂ and FeNi alloy under 6-22 GPa and 1000-1400 °C using a multi-anvil press. We found andradite, wustite and graphite/diamond were formed in the reaction zones between CaCO₃+SiO₂ and FeNi alloy up to 16 GPa. With increasing pressure, substitution of Ca²⁺ + Si⁴⁺ = 2 Fe³⁺ proceeded in andradite. Above 18 GPa, andradite was absent in the products, replaced by wustite coexisting with iron-free davemaoite. Diamonds were also presented at high temperature conditions due to excess FeNi alloy in the systems. We then synthesized pure andradite to investigate the phase relation using in-situ X-ray diffraction combined with quench high-pressure experiments in multi-anvil press. The andradite started to decompose at 19 GPa and 1400 °C, producing davemaoite containing no iron and hematite (Fe₂O₃) in Au capsule or maohokite (MgFe³⁺₂O₄) in MgO capsule. The compressibility of the davemaovite shows consistence with pure CaSiO₃. Our results suggested andradite could be formed during CaCO₃-SiO₂-FeNi reactions, CaCO₃ is efficient to generate ferric iron, causing oxygen fugacity heterogeneity in the reduced mantle. The andradite would release significant amounts of ferric iron in the mantle transition zone, which may account for the diminishing account of diamonds collected at the corresponding depth due to their exhaustion by ferric iron.

[1] Tao and Fei. Commun. Earth Environ. 2021. 2.
[2] Kiseeva et al., Nat. Geosci. 2018.11:144-147.
[3] Stagno et al., Nature, 2013, 493(7430): 84-88.
[4] Wang and Yagi, Mineral. Mag. 1998, 62: 719-923.