西太平洋弧后盆地火山岩玻璃氧逸度的XANES测定及其应用

Subduction zone is the most important place for material recycling and also an important metallogenic belt. Many ore deposits have close relationship with high oxygen fugacity. Previous studies show that arc magma in subduction zone usually has higher oxygen fugacity than Mid-ocean ridge basalt, and it is also found that fluid released from the subducted oceanic plate has high oxygen fugacity. It is, however, still controversial as regards to causal mechanism of the high oxygen fugacity in arc magmas, due to the complexity of subduction process. There have been three major models,subducted material addition, melting of subduction slab and magmatic evolution. In this project, we plan to analyse basic volcanic glass with diffenrent distance from subduction zone from the Lau back-arc basin, volcanic glass (ranging from basic to acidic) of different evolution degree from the Manus back-arc basin, and volcanic glass representing evolution of arc magmas from the primary arc magmas formed during the initiation of the subduction zone to well developed backarc extension in Amami Sankaku basin. The Fe3+/∑Fe and S6+/∑S ratios of volcanic glass will be determined using new synchrotron radiation micro X-ray absorption near edge structure spectra (XANES) technique, to get reliable oxygen fugacity information. Combining with other geochemical and petrological studies, e.g., major and trace elements, the project aims at firmer constraints on the formation of high oxygen fugacity in subduction zones.

俯冲带是全球最重要的物质再循环场所也是重要的成矿带。研究表明俯冲带岩浆往往具有高氧逸度，但是由于板块俯冲的复杂性，有关俯冲带岩浆高氧逸度的成因问题目前仍存在广泛争议。本项目拟在预研究基础上，选取Lau弧后盆离俯冲带不同距离的基性火山岩玻璃来研究板块俯冲析出的流体对氧逸度的影响；选取Manus弧后盆演化程度不同（涵盖从基性到酸性）的火山岩玻璃，研究岩浆演化对氧逸度的影响。通过参加IODP 351航次，系统采集Amami Sankaku弧后盆代表该岛弧形成、演化的火山岩玻璃，重点研究俯冲带氧逸度随俯冲历史的变化。在此基础上，结合其地球化学组分特征，分析俯冲带氧逸度变化规律和影响因素，以揭示俯冲带高氧逸度的形成机制。氧逸度的测定将利用国内先进的同步辐射X射线吸收近边结构谱微区分析技术，准确测定火山岩玻璃Fe3+/∑Fe和 S6+/∑S比值，计算氧逸度，以期揭示俯冲带高氧逸度及其变化的控制因素。

俯冲带是全球最重要的物质再循环场所也是重要的成矿带。许多斑岩矿床与岩浆高氧逸度有关。俯冲带岩浆氧逸度普遍高于大洋中脊玄武岩，但是俯冲带岩浆高氧逸度的成因一直存在争议。本课题选取了Lau 弧后盆离俯冲带不同距离的基性火山岩玻璃来研究板块俯冲析出的流体对氧逸度的影响；选取Manus 弧后盆演化程度不同（涵盖从基性到酸性）的火山岩玻璃，研究岩浆演化对氧逸度的影响。通过参加IODP 351 航次，系统采集Amami Sankaku 弧后盆代表该岛弧起始阶段、俯冲过程的火山岩熔体包裹体，重点研究俯冲带氧逸度随俯冲历史的变化。由于部分样品中S的含量偏低，影响了S价态的测定，本课题主要选取变价元素Fe的氧化态来反应火山岩玻璃的氧化态信息，并探讨其变化规律。为准确测定火山岩玻璃中氧化态信息，本课题建立了利用上海光源同步辐射微聚焦X射线近边吸收谱技术测定火山岩玻璃Fe的X射线近边吸收谱方法，以及解谱计算铁比值的方法。通过对来自三个弧后盆地的样品进行Fe比值测定计算和主微量元素的分析，综合对比铁比值与主微量元素的变化规律。结果表明，以Lau弧后盆地为例，距离俯冲带由远及近受到俯冲流体的作用增强，Ba/La和Ba/Nb等俯冲流体指示剂逐渐增大，初始岩浆的氧化态逐渐升高。以Manus弧后盆地从玄武质到流纹质火山岩玻璃为例，Manus火山岩受俯冲流体作用高于Lau盆地的火山岩，初始岩浆氧化态也较高，但是早期岩浆演化没有造成岩浆氧化态的变化，尽在岩浆演化最末期由氧化态的升高。受俯冲流体影响较大的岛弧岩浆具有较高程度的岩浆演化。奄美三角盆地内记录的板块俯冲起始以来40Ma至30Ma期间的代表岛弧岩浆的熔体包裹体铁比值显示早在俯冲初期40Ma时岛弧岩浆已经较高，随着俯冲的进行氧化态增量不明显，说明俯冲带的氧逸度随俯冲历史并无较大变化。