Effect of carbon speciation on the properties and isotope

In this section, we further discuss the structural origins of the pressure-induced changes in carbon solubility in silicate melts at high pressure above 4 GPa and geochemical implications. Recent study

suggested that the composition of partial melt formed from the subducting

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oceanic crust and the pelitic sediments can be rhyolitic at 2–3 GPa (Duncan and Dasgupta, 2017). The recent field study showed that composition of melt inclusion in the subducted carbonate rock in Kokchetav massif (with the estimated peak pressure of 4.5–6.0 GPa) is reported to be granitic (Korsakov and Hermann, 2006). Furthermore, the high-pressure melting experiment of mineral assemblage relevant to basalts, together with clay minerals and CaCO3, confirmed the formation of rhyolitic melts containing up to 10% of CO2 at 2.5–5.0 GPa (Thomsen and Schmidt, 2008). Whereas the experimental confirmation at higher pressure may be necessary, the melt composition of the subducting oceanic crust containing CaCO3 and other clay minerals could be rhyolitic at the extended pressure condition. As the albite melts serve the model system for rhyolitic melts, the current results for the carbon-bearing albite melts formed at 6 GPa would, therefore, be useful to infer the carbon speciation and the carbon carrying capacity of rhyolitic melts formed by the partial melting of the oceanic crust with the politic sediments.

Earlier studies at low pressure showed that CO2 dissolves as bridging carbonate units like Si(CO3)Si and Al(CO3)Si into silicate network in depolymerized glasses and as molecular CO2 bound near the bridging oxygen, such as Si-O-Si, in polymerized melts (e.g., Eggler and Kadik, 1979;

Fine and Stolper, 1985; Holloway and Blank, 1994; King and Holloway, 2002; Morizet et al., 2002; Morizet et al., 2014b; Mysen, 2012; Mysen et al., 1975; Mysen et al., 1976; Ni and Keppler, 2013). These observations are generally consistent with our high-pressure results. Unexpected differences include a decrease in peak intensity of free-carbonates in the NS3 melts at 6 GPa. Note again that the reduction in solubility has also been inferred from

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the formation of void in the Pt capsule of carbon-bearing NS3 glasses quenched from melt at 6 GPa (section 3.2.1). The origin of the observed void may be from the excess molecular carbon species, such as gas phase CO2. The presence of a void inside the Pt capsule is evident only in the carbon- bearing NS3 melts at 6 GPa. The reduction in peak intensity may result from the discontinuous drop in the solubility of carbon in NS3 melts with

increasing pressure and the formation of bridging carbonates at 6 GPa. The solubility of carbon in some depolymerized silicate melts may change non- linearly with increasing pressure and show a potential drop in the

solubility. This non-linear change in the carbon solubility needs to be taken into consideration in order to understand fluid-induced melting relations in mantle peridotite deep below mid-ocean ridge.

The observed changes in the carbon species imply the complex changes in melt viscosity of fluid-bearing silicate melts at high pressure. As the melt viscosity largely depends on the degree of polymerization of silicate melts with the formation of Si(CO3)Si, it is important to identify the formation of bridging carbonates and its effect on the polymerization of silicate melts at high pressure. The extent of polymerization increases either by forming bridging Si-O-Si bond or by reducing the fraction of NBO species: as for the former, the formation of Si(CO3)Si may not be regarded as polymerization process because its formation of Si(CO3)Si bond in silicate melts does not form a Si-O-Si bond. On the other hand, the formation of Si(CO3)Si reduces NBO fraction, contributing to an overall increase in the network polymerization. Depending on pressure and composition, the fractions of diverse bridging carbonates may control the melt viscosity.

Although the bond strength of the bridging carbonates (Si-O-C bonds) is

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expected to be weaker than that of the Si-O-Si or Si-O-Al bonds, the fraction of Si-O-C bonds above a certain threshold value may lead to the increase in viscosity. Therefore, the formation of bridging carbonates in silicate melts can be used as a structural proxy for the increase in polymerization of silicate melts and the viscosity at an elevated pressure conditions.

The estimated speciation of carbon in silicate melts in the current study allows us to infer the carbon isotope composition in the Earth interior.

Recent experimental studies on the isotope fractionation of carbon between silicate melts and fluids have shown that the δ¹³C in CH4 and CO32- in the silicate melts is higher than that in the coexisting C-O-H fluids, and the differences in δ¹³C between reduced silicate melts and fluids decrease with increasing temperature (Mysen, 2016, 2017). These studies addressed the importance of carbon speciation in carbon isotope composition of melts, fluids, and thus mantle rocks. The electronic structures around an isotope of interest play a dominant role in determining the equilibrium isotope

fractionation and thus the isotope composition of the melts, fluids, and minerals (e.g., Deines, 2004; Schauble et al., 2006; Seo et al., 2007). Each carbon species in the carbon-bearing silicate melts has unique atomic structures and configurations around carbon. For instance, the C-O bond lengths in bridging carbonates Si(CO3)Si, (C-O bond in Si-O-C), and free carbonates is 1.313 Å and 1.265 Å, respectively (Tossell, 1995), and bridging carbonates have two more covalence bonds with network forming cations.

As also demonstrated in crystalline calcite, pressure-induced changes in atomic structures around C atom in the carbonate units result in a significant carbon isotope fractionation (Deines, 2004). Therefore, while further theoretical confirmation is necessary, the formation of bridging

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carbonates and its strong interaction with silicate network at high pressure above 6 GPa may further contribute to the pressure-induced changes in isotope fractionation factors in the melts in the upper mantle.