It is well known that volatiles such as water has significant influence on the properties of silicate melts. Carbon dioxide ($C{O}_2$) is also an abundant volatile in deep Earth, however the effect of $C{O}_2$ on the properties of polymerized melts, particularly the transport properties, are poorly understood. This is crucial for better understanding of the generation and migration of carbon bearing silicate magma in deep crustal and mantle settings. In this study, we explore the structure and properties of carbon bearing aluminosilicate melt up to a pressure of ~25 GPa and temperature range of 2500–4000 K using first principles molecular dynamics (FPMD) simulation. Our results show that $C{O}_2$ in the aluminosilicate melts dissolves as molecular $C{O}_2$ and carbonate ($C{O}_3^{2-}$) at lower pressures (~0–3 GPa). However, at higher pressures (>3 GPa) relevant to most of the upper mantle, $C{O}_3^{2-}$ is predominant carbon species along with carbon in 4-fold coordination ($C{O}_4$). Fraction of $C{O}_3^{2-}$ increases with decreasing temperature and increasing pressure. We find that at the reference isotherm (2500 K), the density of the aluminosilicate melt is reduced by addition of $C{O}_2$ (in wt.%) with $\frac{d\rho }{d{X}_{C{O}_2}}$ = −0.0214. Effect of water on the density of melt is more pronounced with $\frac{d\rho }{d{X}_{H_{2}O}}$ = −0.0422. Thus, the gravity-driven buoyancy of volatile rich magma will be greater than that of the magma without volatile components. The compressibility of the aluminosilicate melt is also affected by volatiles. For instance, both bulk modulus ($K_{T0}$) and its pressure derivative ($K_{T0}^{\text{'}}$) for volatile bearing melts are respectively lower and higher than that of dry melt. Aluminosilicate melts are highly polymerized and show an anomalous pressure dependence of melt viscosity. Pressure dependence of viscosity at low pressure regime (P < 5 GPa) is significant with $\frac{d\mathrm{l}\mathrm{o}\mathrm{g}(\eta )}{\mathit{dP}}=-2.06$. Melt viscosity is further reduced by the addition of volatiles. Effect of volatiles on viscosity is more pronounced at low pressures and it minimizes at pressures where the melt viscosity exhibits minima. Due to the lower viscosity and larger buoyancy for both the $H_{2}O$ and $C{O}_2$ bearing aluminosilicate melts at crustal and upper mantle depths, magma mobility ($\frac{\mathrm{\Delta }\rho }{\eta }$) is greater for volatile bearing melts. Thus, the residence time of such melts are expected to be shorter than volatile free melts in those settings. Like the mobility, electrical conductivity of aluminosilicate melts also increases when volatile is present in the silicate melt. We find that that as low as <1 vol.% volatile bearing aluminosilicate melt mixed with mantle matrix could be sufficient to explain the observed electrical conductivity anomalies in the upper mantle.

众所周知，水等挥发物对硅酸盐熔体的性质有显着影响。二氧化碳 （$C{哦}_2$) 在地球深处也是一种丰富的挥发物，但是 $C{哦}_2$关于聚合熔体的特性，特别是传输特性，我们知之甚少。这对于更好地了解深部地壳和地幔环境中含碳硅酸盐岩浆的生成和迁移至关重要。在这项研究中，我们使用第一性原理分子动力学 (FPMD) 模拟探索了压力高达 ~25 GPa 和温度范围为 2500-4000 K 的含碳铝硅酸盐熔体的结构和性质。我们的结果表明$C{哦}_2$ 在铝硅酸盐熔体中溶解为分子 $C{哦}_2$ 和碳酸盐（$C{哦}_3^{2——}$) 在较低压力下 (~0–3 GPa)。然而，在与大部分上地幔相关的更高压力（> 3 GPa）下，$C{哦}_3^{2——}$ 是主要的碳物种以及 4 倍配位的碳（$C{哦}_4$）。的分数$C{哦}_3^{2——}$随着温度的降低和压力的增加而增加。我们发现在参考等温线 (2500 K) 处，铝硅酸盐熔体的密度通过添加$C{哦}_2$ (wt.%) 与 $\frac{d\rho }{d{X}_{C{哦}_2}}$ = -0.0214。水对熔体密度的影响随着$\frac{d\rho }{d{X}_{H_2哦}}$ = -0.0422。因此，富含挥发性岩浆的重力驱动浮力将大于不含挥发性成分的岩浆。铝硅酸盐熔体的可压缩性也受挥发物的影响。例如，体积模量 (${钾}_{吨0}$) 及其压力导数 (${钾}_{吨0}^{\text{'}}$) 挥发性轴承熔体分别低于和高于干熔体。铝硅酸盐熔体高度聚合并且显示出熔体粘度的异常压力依赖性。低压状态下粘度的压力依赖性 (P < 5 GPa) 与$\frac{d\mathrm{升}\mathrm{○}\mathrm{G}(\eta )}{\mathit{分压}}=——2.06$. 通过添加挥发物进一步降低熔体粘度。挥发物对粘度的影响在低压下更为显着，在熔体粘度最低的压力下影响最小。由于两者都具有较低的粘度和较大的浮力$H_2哦$ 和 $C{哦}_2$ 地壳和上地幔深处的含铝硅酸盐熔体，岩浆流动性（$\frac{\mathrm{\Delta }\rho }{\eta }$) 对于挥发性轴承熔体更大。因此，在这些设置中，这种熔体的停留时间预计比无挥发性的熔体短。与流动性一样，当硅酸盐熔体中存在挥发物时，铝硅酸盐熔体的电导率也会增加。我们发现，与地幔基质混合的低至 <1 vol.% 的挥发性含铝硅酸盐熔体足以解释在上地幔中观察到的电导率异常。