Li isotope geochemistry ofHawaiian plume

研究表明，玄武岩橄榄石中玻璃包裹体的化学成分代表了熔体的主要化学成分。因此，预计熔体包裹体的数据将为理解地幔的化学非均质性和演化提供有用的信息。为了评价橄榄石玻璃包裹体中的锂同位素组成反映了原生熔体组成，并评价锂同位素作为地球化学示踪剂的作用，以了解地幔非均质性，本研究利用SIMS测量了夏威夷和冰岛玄武岩橄榄石玻璃包裹体及其寄主橄榄石的锂同位素组成。为了使夏威夷玄武岩橄榄石中的玻璃包裹体均质化，在冷却条件下形成的矿物包裹体包含在玻璃包裹体中，因此必须进行加热预处理。在尺寸大于100 pm的玻璃夹杂物中，锂同位素组成经再均质处理后呈非均质性。本文提出了三种解释非均质性的可能性：(1)在重均质过程中，玻璃包裹体中的锂被蒸发，导致了较大的同位素分馏。(2)熔融体作为包裹体在橄榄石基体中被捕获后，锂从玻璃包裹体向周围的橄榄石基体扩散。(3)熔体在被捕获为熔体之前，具有亚微米尺度的非均质性。为了评估这些可能性，对冰岛玄武岩橄榄石中的玻璃包裹体进行了锂同位素分析。冰岛样品不需要再均质处理，因为熔融夹杂物不包括在玻璃夹杂物中。结果表明，玻璃夹杂物的锂同位素组成具有非均质性。如果气泡包含在玻璃包裹体中，则锂同位素组成发生了系统的变化。也就是说，δ^7Li随着它接近气泡而增加。这表明^6Li比^7Li更快地转移到气泡中。因此，观察到的非均质性可能是岩浆房起泡和/或再均质处理造成的，导致原生锂同位素组成不能保存在玻璃包裹体中。此外，用SIMS测定了寄主橄榄石的锂同位素组成。在变形橄榄石中获得了异常高的δ 7Li （+120 permil）。在高温条件下，平衡条件下的同位素分馏不能产生如此高的δ^7Li值。因此，这种同位素组成的异常可能不是原始组成，而是反映了岩石圈地幔条件下橄榄石变形过程中的动力学作用。少

It has been suggested that the chemical composition of glass inclusion included in olivine from basaltic rocks represent the primary chemical compositions of the melt. It has been expected that, therefore, the data of the melt inclusion bring useful information for the understanding of the chemical heterogeneity and evolution of mantle. In order to assess that the lithium isotopic composition preserved in glass inclusion in olivine reflect the primary melt composition and to evaluate lithium isotope as a geochemical tracer to understand the mantle heterogeneity, in this study, lithium isotopic compositions of glass inclusions in olivine from Hawaiian and Icelandic basaltic rocks together with host olivines were measured by SIMS.In order to homogenize glass inclusion in olivine from Hawaiian basalts, heating processes are necessary as an pretreatment because mineral inclusions formed in cooling condition are included in the glass inclusion. After rehomogenization processes, heterogeneit … More y observed in lithium isotopic composition in glass inclusion whose size is larger than 100 pm. We suggested that the following three possibilities to explain the heterogeneity: (1) During rehomogenization processes, lithium included in glass inclusion is evaporated, resulting that large isotopic fractionation occur. (2) After trap of melt as inclusion in olivine host, lithium was diffusive out from glass inclusion to the surrounding host olivine. (3) There was originally submicron scale heterogeneity in melts before it was trapped as melts. In order to evaluate these possibilities, lithium isotope analyses were undertaken for glass inclusions in olivine from Icelandic basalts. Rehomogenization treatment is not necessary for the Icelandic samples because melt inclusions are not included in the glass inclusions. The results indicate that the heterogeneity was observed in lithium isotopic composition of the glass inclusion. If the bubble is included in the glass inclusion, lithium isotopic compositions were changed systematically. That is, δ^7Li was increased as it approaches the bubble. This indicates that ^6Li was transferred to the bubble faster than ^7Li. Therefore, observed heterogeneity can be caused by foaming in magma chamber and/or rehomogenization treatment, resulting that primary lithium isotopic composition cannot be preserved in glass inclusions.In addition, lithium isotopic compositions of the host olivine were measured by SIMS. Extraordinary high δ^7Li (+120 permil) were obtained in deformed olivine. At high temperature condition, such extraordinary high δ^7Li values cannot be produced by isotopic fractionation under equilibrium condition. Therefore, this anomaly observed in the isotopic composition may not show original composition but reflect kinetic effect during deformation processes in olivine at lithospheric mantle conditions. Less