High‐Pressure Na‐Ca Carbonates in the Deep Carbon Cycle

Abstract

Carbonates play a central role in the subduction transport of oxidized carbon from Earth's surface to the deep mantle. The melting of carbonated rocks also leads to the release of carbonate melts, regarded as important oxidizing and metasomatizing agents, from the subducting slab. Although at upper mantle conditions the chemistry of carbonates within the subducting slab is restricted to the thoroughly studied ternary CaCO 3 -MgCO 3 -FeCO 3 system, recent experimental and natural evidences strongly suggest that at conditions of the mantle transition zone, Na-Ca carbonates become the main host of oxidized carbon and define solidus temperatures and chemistry of deep melts. In this chapter, we discuss transport of carbonates to the mantle transition zone within the subducting slab, evidences of sodium migration from silicates into carbonates under high pressure, and crystal chemistry of currently known high-pressure Na-Ca carbonates. 13 13.

Crystal structure of nyerereite: A possible messenger from the deep Earth

“…The proposed hypotheses are consistent with recent studies on the HP behavior of Na-Ca carbonates (Grassi and Schmidt, 2011;Kiseeva et al, 2013;Litasov et al, 2013Borodina et al, 2018Vennari et al, 2018;Rashchenko et al, 2020) that have already suggested an important variety of Na-Ca double carbonates in the system Na2CO3-CaCO3 at HP-HT, linked each other by a sequence of decomposition reactions (Rashchenko et al, 2020), as well as the existence of high-pressure polymorphs of Na2Ca2(CO3)3-shortite. As a consequence, the stability/decomposition reactions occurring in the mentioned phases during decompression are a fundamental constraint for the CO2 release from mantle-derived magma, which can be expected to influence magma viscosity and eruption explosivity (Allison et al, 2021).…”

“…providing evidences for a likely sublithospheric mantle origin for alkaline ultramafic magmas and extrusive carbonatites (Humphreys et al, 2010). The solubility of Na in the structure of aragonite might be drastically increased through the formation of nano lamellae of HP polymorph of Na2Ca(CO3)2 giving rise to what has been recently called "Na-aragonite" (Rashchenko et al, 2020). Our hypothesis about formation of modular structures between the HP form of Na2CO3 and aragonite at HP is supported by the amount of dissolved Ca 2+ in the structure of Na2CO3 up to 15% (Podborodnikov et al, 2018), which cannot be explained with isomorphism as the authors suggested.…”

“…Our investigation helps to diagnose natural and synthetic alkaline and earth-alkaline carbonates by deciphering the structural characteristics of pure synthetic nyerereite with respect to natural samples. The accurate knowledge of the crystal structure of nyerereite allows us to speculate on its behavior at non ambient conditions, thus opening the possibility of a scenario where the Litasov et al, 2013;Thomson et al, 2016;Vennari et al, 2018;Rashchenko, 2020). (Shatskiy et al, 2015) and dashed red is thermonatrite (Jentzsch et al, 2013;Frezzotti et al, 2012).…”

“…In the present work we consider different synthesis conditions, namely hydrothermal synthesis and synthesis from the melt, the latter one employed to obtain a new type of nyerereite samples that resembles the natural samples observed in melt inclusions of kimberlites. The synthetic alkali-carbonates mixtures are studied by a multimethodological approach, namely, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), Raman spectroscopy and single crystal X-ray diffraction (SC-XRD) with a particular focus on the structure solution of nyerereite, showing that it can be centrosymmetric or not, depending on conditions of crystallization Despite the well-known high-pressure behavior of Ca-Mg-Fe carbonates(e.g., Zucchini et al, 2014Zucchini et al, , 2017Merlini et al, 2012Merlini et al, , 2016Cerantola et al, 2017), the Na-Ca phase stability at mantle/deep crust conditions is poorly known and experimental and theoretical studies have been limited to minerals other than nyerereite (Borodina et al, 2018;Vennari et al, 2018;Rashchenko et al, 2020). The only exception is the high-pressure (HP) Raman study of nyerereite made by Rashchenko et al (2017) where, however, the analyzed P range (up to 6.4…”

Ab initio исследования влияния давления на структуру, электронные и упругие свойства карбонатов щелочных--щелочно-земельных металлов

“…Исследования фазовых диаграмм двойных карбонатов при высоких давлениях позволяют выявить ряд новых фазовых состояний кристаллов [11], которые еще предстоит открыть [12]. Поэтому структурные свойства двойных кристаллов под давлением изучались с помощью синхротронного излучения в [13], где показано, что сжимаемость карбонатов калия (натрия)-магния ниже, чем у магнезита и доломита, а K 2 Mg(CO 3 ) 2 переходит в моноклинную полиморфную форму при 8.05 GPa.…”