碳酸盐熔体中稀有气体溶解度和表面张力的分子动力学模拟
Noble gases in carbonate melts: constraints on the solubility and the surface tension by molecular dynamics simulation
摘要：
虽然稀有气体是地幔中的稀有元素，但由于其质量变化剧烈，与物理行为形成鲜明对比，使其成为重要的地球化学示踪剂。当在深度处发生部分熔融时，NG在相之间的分配由分配系数控制，该分配系数可以由NG在每个相中的溶解度确定。本文报道了基于分子动力学模拟的He、Ne、Ar和Ar在碳酸盐熔体中溶解度的定量计算。NG溶解度首先在K2 CO 3-CaCO 3混合物中在1巴下计算，并且有利地与迄今为止唯一可用的实验数据进行比较。然后我们研究压力（高达6 GPa）的影响，重点关注两种熔体成分：白云质成分和钠碳酸岩质成分（模拟Ol Doinyo Lengai的熔岩）。溶解度随熔体中碱土金属阳离子的量和惰性气体的大小而减小。在钠碳酸岩熔体中，亨利定律在低压（高达~ 0.1 GPa）下得到满足。在较高的压力下，溶解度趋于平稳，甚至开始平稳地减小。与此相反，在熔融白云石的压力的影响是可以忽略不计的研究P范围（3-6 GPa）。在地幔的压力下，稀有气体在碳酸盐熔体中的溶解度仍然与熔融硅酸盐中的溶解度具有相同的数量级（约1-10摩尔%）。这表明，碳酸盐熔体在深度不是惰性气体的优先载体，即使与熔体组合物的依赖性是不可忽略的，必须逐案进行评估。最后计算了碳酸盐熔体与惰性气体界面的表面张力及其随压力的变化。无论熔体和NG相的组成如何，当P从0增加到6 GPa时，表面张力增加。
英文摘要：
Although they are rare elements in the Earth's mantle, noble gases (NG) owe to their strongly varying masses contrasting physical behaviors making them important geochemical tracers. When partial melting occurs at depth, the partitioning of NGs between phases is controlled by a distribution coefficient that can be determined from the solubility of the NGs in each phase. Here we report quantitative calculations of the solubility of He, Ne, Ar and Xe in carbonate melts based on molecular dynamics simulations. The NG solubilities are first calculated in K2CO3-CaCO3 mixtures at 1 bar and favorably compared to the only experimental data available to date. Then we investigate the effect of pressure (up to 6 GPa), focusing on two melt compositions: a dolomitic one and a natrocarbonatitic one (modeling the lava of Ol Doinyo Lengai). The solubility decreases with the amount of alkaline-earth cation in the melt and with the size of the noble gas. In the natrocarbonatitic melt, Henry's law is fulfilled at low pressures (up to ~ 0.1 GPa). At higher pressures the solubility levels off or even starts to diminish smoothly. In contrast, in molten dolomite the effect of pressure is negligible on the studied P range (3-6 GPa). At the pressures of the Earth's mantle, the solubilities of noble gases in carbonate melts are still of the same order of magnitude as the ones in molten silicates (~ 1-10 mol%). This suggests that carbonatitic melts at depth are not preferential carriers of noble gases, even if the dependence with the melt composition is not negligible and has to be evaluated on a case-by-case basis. Finally we evaluate the surface tension at the interface between carbonate melts and noble gases and its evolution with pressure. Whatever the composition of the melt and of the NG phase, the surface tension increases when P increases from 0 to 6 GPa.
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