Li isotope geochemistry ofHawaiian plume

有人认为，玄武质岩石中橄榄石所含玻璃包裹体的化学成分代表了熔体的主要化学成分。因此，熔融包裹体的数据有望为地幔化学不均一性和地幔演化的研究提供有用的信息。为了评价橄榄石中玻璃包裹体中锂同位素组成是否反映了初始熔体组成，并评价锂同位素作为地球化学示踪剂对地幔不均一性的认识，用西姆斯法测定了夏威夷和冰岛玄武岩中橄榄石玻璃包裹体的锂同位素组成，对于玄武岩，加热过程作为预处理是必要的，因为在冷却条件下形成的矿物夹杂物包括在玻璃夹杂物中。在再均质化过程之后， 关于我们 在大于100 μ m的玻璃包裹体中，锂同位素组成中观察到γ。我们认为，这种不均一性可能有以下三种解释：（1）在再均一化过程中，玻璃包裹体中的锂被蒸发，导致大的同位素分馏。(2)熔体作为包裹体进入橄榄石基质后，锂从玻璃包裹体中扩散到周围的橄榄石基质中。(3)在熔体被捕获之前，熔体中原本存在亚微米尺度的非均匀性。为了评估这些可能性，锂同位素分析进行了从冰岛玄武岩橄榄石的玻璃包裹体。冰岛样品不需要再均匀化处理，因为熔融包裹体不包括在玻璃包裹体中。结果表明，玻璃包裹体锂同位素组成具有不均一性。当玻璃包裹体中含有气泡时，锂同位素组成发生了系统的变化。也就是说，δ ^7 Li随着接近气泡而增加。这表明^6Li比^7Li更快地转移到气泡中。因此，所观察到的不均匀性可能是由于岩浆房中的泡沫作用和（或）再均一化处理造成的，从而导致玻璃包裹体中的原生锂同位素组成不能被保留。在变形橄榄石中得到了非常高的δ ^7 Li（+120 permil）。在高温条件下，如此高的δ ^7 Li值不可能是平衡条件下同位素分馏产生的。因此，这种同位素组成的异常可能不是原始组成，而是岩石圈地幔条件下橄榄石变形过程中的动力学效应。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