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

在过去的几十年里，地幔氧化还原状态的精确测定是许多研究的主题，但仍然存在激烈的争论。地幔氧化还原态的主要信息来自海洋地壳玻璃的分析，这些玻璃被认为冻结了地幔中生成的玄武岩的Fe3+/Fe2+比值。玄武岩氧化还原态的争论主要与测定玻璃中Fe3+/Fe2+比值的不同分析方法和可能出现的分析问题（如光束损伤）有关。由于玄武岩的大玻璃碎片很少，特别是如果要分析矿物中的玻璃包裹体，因此必须采用高空间分辨率和允许我们常规分析大量样品的分析方法。在本研究中，我们计划开发微探针侧翼法，作为一种准确、快速、低成本地测量由IODP采集的自然淬火玄武岩玻璃（枕状玄武岩外壳以及矿物中的玻璃包裹体）中铁铁比的工具，这些玻璃是由不同地球动力学环境的关键地点（如海洋中脊、海洋岛屿、海洋高原和岛弧）制成的。使用各种已知Fe3+/Fe2+比例的玻璃标准，微探针侧面方法将用于最近在汉诺威获得的新微探针进行校准。新仪器在分析可能性方面的进展将用于提高侧翼法测定玻璃中Fe3+/∑Fe比的精度。天然玻璃的分析将用于澄清关于地幔氧化态的争议，并检查不同大地构造环境之间的差异。来自大西洋的洋中脊玄武岩玻璃覆盖了近0至160 Ma的年龄，将用于检查氧化态随时间可能发生的变化。来自伊扎-波宁马里亚纳弧的样品将为俯冲带开始期间玄武岩氧化还原条件的变化提供重要信息。海洋高原玻璃中Fe3+/Fe2+比值的测定将有助于讨论这些玄武岩的羽流成因和非羽流成因。最后，研究结果对探讨玄武岩与地幔渣之间的熔融反应和多价元素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.