マグマ中での元素拡散の同位体効果 : シミュレーションと実験

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Isotopic effects on diffusion of elements in magma are examined by molecular dynamics (MD) simulation and diffusion experiments at high temperature and pressure in order to discuss kinetic isotopic mass fractionation due to diffusion in magmatic systems. Mass dependence of self-diffusion of oxygen. magnesium and silicon ions in MgO-SiO_2 melts has been determined by means of MD simulation with hypothetical masses. The diffusion coeffcients, D, are approximately proportional to the power of the atomic mass, m(D^∝m^β), where the coefficient of the power law, β, represents the degree of the isotopic effect. The values of β are about. -0.11〜-0.17, 0.00〜-0.06 and -0.05〜-0.10 for Mg, Si and O, respectively; the isotopic effect for Mg is larger than that for Si, and that for O is between those for Mg and Si. The isotopic effects in the melts are smaller than that in a gas phase (β=-0.5). The present results can be explained by structures of the silicate melts; Mg and Si are network modifier and network former, respectively. Diffusion experiments using a Mg_<0.5>K_2SiO_<5.6>-Ca_<0.5>K_2SiO_<5.6> pair were done at 15Kb, 1400℃ for 1, 2 and 4 hrs. The results are consistent with the results of the MD simulation although the experimental results are preliminary. The present results are also consistent with previous experiments and simulations on isotopic effects of diffusion in other materials, such as alkali silicate glasses, metals, alkali halides, and water. The present results were applied to isotopic mass fractionation in magmatic processes controlled by diffusion in silicate melts (mixing of magmas by diffusion and diffusion-controlled growth). The isotopic mass fractionation in magma is generally small except for a specific situation, such as diffusion of elements into a region where the element is initially at zero concentration. Isotopic fractionation for Mg (^<26>Mg/^<24>Mg), which is light network modifier, is usually less than a few ‰ and those for O and Si are usually less than 1‰ and would be comparable to thermodynamic fractionation.
特定非営利活動法人日本火山学会の論文
1997-04-25

マグマ中での元素拡散の同位体効果 : シミュレーションと実験

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