地幔条件下碳酸钙熔体的密度与压缩性

碳酸盐熔体在地球内部碳循环过程中扮演重要角色,但目前对碳酸盐熔体的研究局限于低温低压条件,且主要以富钾钠组分的碱性碳酸盐熔体为主,对于高压条件下碳酸盐熔体的研究还非常不足.通过大规模第一性原理分子动力学模拟,获得地幔条件下CaCO₃熔体的状态方程.研究表明,碳酸钙熔体表现出显著的可压缩性,其压缩因子大于文石及典型硅酸盐熔体,因此其密度随压力升高而快速增大;当压力超过10 GPa时,碳酸盐熔体的密度大于钠长石熔体的密度;当压力超过37 GPa时,其密度会大于金刚石的密度;地幔条件下碳酸钙熔体的密度始终小于晶体(文石及后文石)的密度,但其密度差随压力升高而显著降低.碳酸钙熔体的这一特性,对探讨其在地幔中的分布和探讨超深金刚石的形成等具有启示意义.

Densities and compressibilities of calcium-carbonate melts under the mantle condition

Carbonate melts play important roles in the carbon cycle process in the earth interiors. However, the previous studies were limited to the low temperature and low pressure conditions, and mainly focused on K⁺ and/or Na⁺ bearing carbonate melts. In this study we used extensive first-principle molecular dynamics simulation to obtain the equation of state of CaCO₃ melts under the mantle condition. The results are listed in the following. CaCO₃ melts have significantly greater compressibility than aragonite or typical silicate melts and hence the densities rapidly increase with pressure. When the pressure exceeds 10 GPa, densities of CaCO₃ melts are higher than that of albite melt and the densities are even higher than that of diamond (at the pressure above 37 GPa). Densities of CaCO₃ melts are lower than that of crystalline phase throughout mantle condition, but the density difference between them significantly decreases when the pressare increases. The high compressibility of CaCO₃ melts may have important implications for uncovering distributions of carbonate melts in the mantle and for the formation of ultra-deep diamonds.