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

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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.