Melt Inclusion Study of Water and Chlorine Abundances in HIMU- and EM-type Mantle

该项目将检测来自南岛岩链的熔岩中以橄榄石为宿主的熔体包裹体中的挥发性丰度(水、二氧化碳、氯和S)。之前对这些岛屿熔岩的研究表明，有三种不同的地幔成分参与其中，其中一种成分据信来自古老的再循环洋壳。挥发性元素丰度和形态数据将与常量和微量元素数据以及现场铅同位素数据相结合，以评估有关这些熔岩的成因及其地幔来源性质的几个基本问题。将讨论的具体问题包括：(1)澳大利亚群岛熔体包裹体中氯浓度和氯/K2O比值的变化是来自浅层熔体/地壳还是熔体/水圈相互作用，或者这些变化是否跟踪了玄武岩成因所涉及的不同地幔储集层中挥发性元素的丰度？(2)水和氯的丰度在多大程度上与表明存在再循环洋壳的其他同位素和微量元素特征有关？(3)俯冲洋壳中有多少水或氯在俯冲导致脱水后返回到地幔深处？地幔中水和其他挥发性物质的丰度和分布对地幔对流、熔体生成和参与地球内部化学和物理演化的其他基本过程有重要影响。地球内部与水圈和大气的相互作用也可能在地质时间尺度上改变地球表面环境。目前，水和其他挥发物在地幔中的数量和分布受到很少的约束。目前也不清楚板块形成和后来的俯冲过程是否会导致地球表面水和其他挥发物的净增加或净损失。这项研究将更好地了解不同地幔储集层中挥发性物质的丰度和来源，以及板块构造和板块俯冲在控制地幔随时间的脱气(以及可能的再气化)方面所起的作用。

This project will examine volatile abundances (water, CO2, Cl, and S) in olivine-hosted melt inclusions from lavas from the Austral Island chain. Previous study of lavas from these islands indicates the involvement of three distinct mantle components, including a component believed to derive from ancient recycled oceanic crust. Volatile element abundance and speciation data will be combined with major and trace element data as well as in situ Pb-isotope data to evaluate several fundamental questions regarding the genesis of these lavas and the nature of their mantle sources. Specific questions that will be addressed include: (i) do variations in Cl concentrations and Cl/K2O ratios in Austral Islands melt inclusions derive from shallow melt/crust or melt/hydrosphere interaction, or do these variations track volatile element abundances in the different mantle reservoirs involved in basalt genesis? (ii) to what extent do water and chlorine abundances correlate with other isotopic and trace element signatures indicative of the presence of recycled oceanic crust? (iii) how much water or chlorine is retained in subducted oceanic crust and returned to the deep mantle after subduction-induced dehydration?The abundance and distribution of water and other volatile species in the mantle has a significant effect on mantle convection, melt generation, and other fundamental processes involved in the chemical and physical evolution of the Earth's interior. Interaction of the Earth's interior with the hydrosphere and atmosphere may also alter the Earth's surface environment on geologic timescales. At present, the amount and distribution of water and other volatiles in the mantle are poorly constrained. It is also unclear whether the processes of plate creation and later subduction result in a net gain or a net loss of water and other volatiles at the Earth's surface. This study will provide a better understanding of the abundance and origin of volatile species in different mantle reservoirs and the role that plate tectonics and plate subduction have played in controlling degassing (and possibly re-gassing) of the mantle over time.