MORB脱气过程中的囊泡、气泡形成和惰性气体分馏
Vesicularity, bubble formation and noble gas fractionation during MORB degassing
摘要：
本研究的目的是使用分子动力学模拟（MD），以评估泡状和惰性气体分馏，并阐明在MORB脱气过程中气泡的形成。先前的模拟研究（Guillot和Sator（2011）GCA 75，1829-1857）已经表明，CO2在玄武岩熔体中的溶解度随着压力稳定增加，并且在高压下显著偏离亨利定律（例如，与亨利定律的2.5重量%相比，在50千巴下9.5重量%的CO2）。从CO2的溶解度曲线和状态方程的两个共存相（硅酸盐熔体和超临界CO2），推导出从MD模拟，我们已经评估了气泡的MORB熔体在深度作为其初始CO2含量的函数的演变。计算结果与在洋脊收集的MORB样品的数据非常吻合。此外，通过实施测试粒子方法（Guillot和Sator（2012）GCA 80，51-69），惰性气体在两个共存相（超临界CO2和CO2饱和熔体）中的溶解度、惰性气体在熔体和囊泡之间的分配和分馏作为压力的函数进行了评估。我们发现，熔体/CO2分配系数的稀有气体的压力显着增加，而在文献中报道的大分布的4 He/40 Ar * 比的解释，如果岩浆在上升过程中经历了一套囊泡和囊泡损失。通过对含挥发分熔体施加压降，MD模拟揭示了纳米尺度下气泡形成和惰性气体传输的主要步骤。一个关键的结果是，稀有气体的转移被发现是由CO2气泡成核，这一发现表明，在脱气熔体中的He和Ar之间的扩散率差几乎没有影响的4 He/40 Ar * 的比测量的囊泡。
英文摘要：
The objective of this study is to use molecular dynamics simulation (MD) to evaluate the vesicularity and noble gas fractionation, and to shed light on bubble formation during MORB degassing. A previous simulation study (Guillot and Sator (2011) GCA 75, 1829-1857) has shown that the solubility of CO2 in basaltic melts increases steadily with the pressure and deviates significantly from Henry's law at high pressures (e.g. 9.5 wt% CO2 at 50 kbar as compared with 2.5 wt% from Henry's law). From the CO2 solubility curve and the equations of state of the two coexisting phases (silicate melt and supercritical CO2), deduced from the MD simulation, we have evaluated the evolution of the vesicularity of a MORB melt at depth as function of its initial CO2 contents. An excellent agreement is obtained between calculations and data on MORB samples collected at oceanic ridges. Moreover, by implementing the test particle method (Guillot and Sator (2012) GCA 80, 51-69), the solubility of noble gases in the two coexisting phases (supercritical CO2 and CO2-saturated melt), the partitioning and the fractionation of noble gases between melt and vesicles have been evaluated as function of the pressure. We show that the melt/CO2 partition coefficients of noble gases increase significantly with the pressure whereas the large distribution of the 4He/40Ar* ratio reported in the literature is explained if the magma experiences a suite of vesiculation and vesicle loss during ascent. By applying a pressure drop to a volatile bearing melt, the MD simulation reveals the main steps of bubble formation and noble gas transfer at the nanometric scale. A key result is that the transfer of noble gases is found to be driven by CO2 bubble nucleation, a finding which suggests that the diffusivity difference between He and Ar in the degassing melt has virtually no effect on the 4He/40Ar* ratio measured in the vesicles.
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