| Budget Amount *help |
¥3,950,000 (Direct Cost: ¥3,500,000、Indirect Cost: ¥450,000)
Fiscal Year 2007: ¥1,950,000 (Direct Cost: ¥1,500,000、Indirect Cost: ¥450,000)
Fiscal Year 2006: ¥2,000,000 (Direct Cost: ¥2,000,000)
|
|---|
| Research Abstract |
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
|
