As a geochemical tracer in mantle studies, lithium isotopes play an important role in diffusion and fractionation in major mantle minerals. Olivine and its high-pressure phases, wadsleyite and ringwoodite, are considered to predominate in the upper mantle and transition zone on the earth. We carried out simulations of lithium isotopes’ diffusion and fractionation in Mg end member olivine and its high-pressure phases to learn the details of the signatures of lithium isotopes preserved in mantle materials. In this work, the diffusion and fractionation mechanisms between different lattice sites of lithium isotopes in forsterite (Mg2SiO4), wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4) at the atomic level are studied using empirical atomistic simulation techniques. It is found that Li can pass through the high-pressure phases (wadsleyite and ringwoodite) energetically much easier due to the low migration energy barriers via either substitutional or interstitial mechanism. The activation energy is high for Li diffusion along the interstitial path in the forsterite and decreases drastically with the assist of Mg vacancies. The temperature-dependent fractionation for two Li isotopes between two different lattice sites is calculated at the temperatures from 300 to 2500 K. This behavior generates the lighter Li storage in the near-surface mantle-derived rocks and provides insights into zoning and composition of lithium isotopes in olivine and its high-pressure phases.

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镁橄榄石及其高压相中锂扩散分馏的理论研究

作为地幔研究中的地球化学示踪剂，锂同位素在主要地幔矿物的扩散和分馏中起着重要作用。橄榄石和它的高压相，瓦兹利铁矿和菱镁矿，被认为在地球上地幔和过渡带中占主导地位。我们对镁端元橄榄石及其高压相中锂同位素的扩散和分馏进行了模拟，以了解地幔材料中保存的锂同位素特征的详细信息。在这项工作中，使用经验原子模拟技术研究了镁橄榄石（Mg2SiO4）、瓦德斯利石（β-Mg2SiO4）和菱镁矿（γ-Mg2SiO4）中锂同位素不同晶格位点在原子水平上的扩散和分馏机制。研究发现，由于通过置换或间隙机制的低迁移能垒，Li 可以更容易地通过高压相（瓦兹利石和菱形伍德石）。Li沿着镁橄榄石中的间隙路径扩散的活化能很高，并且在Mg空位的帮助下急剧降低。在 300 到 2500 K 的温度下计算了两个不同晶格位点之间的两种锂同位素的温度依赖性分馏。这种行为在近地表地幔衍生岩石中​​产生了较轻的锂储存，并提供了对锂的分带和组成的深入了解橄榄石及其高压相中的同位素。Li沿着镁橄榄石中的间隙路径扩散的活化能很高，并且在Mg空位的帮助下急剧降低。在 300 到 2500 K 的温度下计算了两个不同晶格位点之间的两种锂同位素的温度依赖性分馏。这种行为在近地表地幔衍生的岩石中产生了较轻的锂储存，并提供了对锂的分带和组成的深入了解橄榄石及其高压相中的同位素。Li沿着镁橄榄石中的间隙路径扩散的活化能很高，并且在Mg空位的帮助下急剧降低。在 300 到 2500 K 的温度下计算了两个不同晶格位点之间的两种锂同位素的温度依赖性分馏。这种行为在近地表地幔衍生的岩石中产生了较轻的锂储存，并提供了对锂的分带和组成的深入了解橄榄石及其高压相中的同位素。