Determination of ferric-ferrous ratio in natural glasses by microprobe: application of the flank method for determination of the redox state of basalts from mid-oceanic ridges, oceanic islands, oceanic plateaus and island arcs

在过去的几十年中，地幔氧化还原状态的确定是许多研究的主题，仍然存在激烈的争论。地幔氧化还原状态的主要信息来自于对收集于洋壳中的玻璃的分析，这些玻璃被认为冻结了地幔中生成的玄武岩的Fe 3 +/Fe 2+比值。关于玄武岩氧化还原状态的争论主要涉及确定玻璃中Fe 3 +/Fe 2+比值的不同分析方法以及可能的分析问题（例如，梁损伤）。由于玄武岩的大玻璃碎片很少，特别是如果必须分析矿物中的玻璃包裹体，因此必须应用具有高空间分辨率的分析方法，并允许我们常规分析大量样品。在本提案中，我们计划开发微探针侧翼法作为一种工具，以准确，快速和低成本地测量自然淬火玄武岩玻璃（枕状玄武岩外皮以及矿物中的玻璃包裹体）的铁-铁比，由IODP收集在典型的不同地球动力学环境的关键地点，如大洋中脊，海洋岛屿，海洋高原和岛弧。使用各种玻璃标准与已知的Fe 3 +/Fe 2+的比例，微探针侧翼法将校准一个新的微探针最近在汉诺威收购。新仪器分析性能的提高将用于提高侧面法测定玻璃中Fe ~（3+）/∑Fe比值的精度。天然玻璃的分析将用于澄清关于地幔氧化状态的争议，并检查不同大地构造环境之间的差异。大西洋洋中脊玄武岩玻璃的年龄范围从0到160 Ma，将用于检查氧化态随时间的可能变化。来自伊豆-小笠原马里亚纳弧的样品将提供有关俯冲带形成过程中玄武岩氧化还原条件变化的重要信息。大洋高原玄武岩玻璃中Fe ~（3+）/Fe ~（2+）比值的测定，将有助于探讨这些玄武岩的地幔柱成因与非地幔柱成因。研究结果对探讨地幔熔融反应和多价元素在玄武岩和地幔残留物之间的分配行为具有重要意义。

The determination of the exact redox state of the mantle was the subject of numerous studies in the last decades and still remains strongly debated. The main information on the redox state of the mantle is derived from the analysis of glasses collected in the oceanic crust, which are considered to freeze the Fe3+/Fe2+ ratio of basalts generated in the mantle. The controversy on the redox state of basalts is mainly related to different analytical approaches to determine the Fe3+/Fe2+ ratio in glasses and to possible analytical problems (e.g., beam damage). Since large glass fragments of basalts are rarely available, especially if glass inclusions in minerals have to be analyzed, analytical methods with high spatial resolution and allowing us to analyze a large number of samples routinely have to be applied. In this proposal we plan to develop the microprobe flank method as a tool to measure accurately, quickly and at low costs the ferric-ferrous ratio in naturally quenched basaltic glasses (pillow basalt rinds as well as glass inclusions in minerals) collected by IODP in key localities typical of different geodynamic environments, such as mid oceanic ridge, oceanic island, oceanic plateaus and island arcs. Using a variety of glass standards with known Fe3+/Fe2+ ratio, the microprobe flank method will be calibrated for a new microprobe recently acquired at Hannover. The advances in the analytical possibilities of the new instrument will be used to improve the precision of flank method for the determination of Fe3+/∑Fe ratios in glasses. The analysis of natural glasses will be used to clarify the controversy on the oxidation state of the mantle and to check for differences between the different geotectonic environments. Mid oceanic ridge basaltic glasses from the Atlantic covering an age ranging from nearly 0 to 160 Ma will be used to check for possible variation of the oxidation state with time. Samples from Izu-Bonin Mariana arc will provide crucial information on changing redox conditions in basalts during the initiation of a subduction zone. The determination of the Fe3+/Fe2+ ratio of glasses from oceanic plateaus will be helpful to discuss the plume vs. non-plume origin of these basalts. Finally, the results are of importance to discuss melting reactions in the mantle and the partitioning behaviors of multivalent elements between basalts and mantle residue.