Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano

Botcharnikov, Roman; Wilke, Max; Garrevoet, Jan; Portnyagin, Maxim; Klimm, Kevin; Buhre, Stephan; Krasheninnikov, Stepan; Almeev, Renat; Moune, Severine; Falkenberg, Gerald

Here we present a confocal Fe K-edge inline-formulaμ-XANES method (where XANES stands for X-ray absorption near-edge spectroscopy) for the analysis of Fe oxidation state in heterogeneous and one-side-polished samples. The new technique allows for an analysis of small volumes with high spatial 3D resolution of inline-formula<100inline-formulaµminline-formula3. The probed volume is restricted to that just beneath the surface of the exposed object. This protocol avoids contamination of the signal by the host material and minimizes self-absorption effects. This technique has been tested on a set of experimental glasses with a wide range of Feinline-formula3+inline-formula M8inlinescrollmathml / 8pt14ptsvg-formulamathimgfb147fccdcf98a9911cf3d26a8f6dc33 ejm-36-195-2024-ie00001.svg8pt14ptejm-36-195-2024-ie00001.png inline-formulaΣFe ratios. The method was applied to the analysis of natural melt inclusions trapped in forsteritic to fayalitic olivine crystals of the Hekla volcano, Iceland. Our measurements reveal changes in Feinline-formula3+inline-formula M11inlinescrollmathml / 8pt14ptsvg-formulamathimg165b352473919034209a9d51d0eaf41d ejm-36-195-2024-ie00002.svg8pt14ptejm-36-195-2024-ie00002.png inline-formulaΣFe from 0.17 in basaltic up to 0.45 in dacitic melts, whereas the magnetite–ilmenite equilibrium shows redox conditions with Feinline-formula3+inline-formula M14inlinescrollmathml / 8pt14ptsvg-formulamathimg539a58614ea8688159b8effbc6d3da8d ejm-36-195-2024-ie00003.svg8pt14ptejm-36-195-2024-ie00003.png inline-formulaΣFe inline-formula≤0.20 (close to FMQ, fayalite–magnetite–quartz redox equilibrium) along the entire range of Hekla melt compositions. This discrepancy indicates that the oxidized nature of glasses in the melt inclusions could be related to the post-entrapment process of diffusive hydrogen loss from inclusions and associated oxidation of Fe in the melt. The Feinline-formula3+inline-formula M18inlinescrollmathml / 8pt14ptsvg-formulamathimg62d2c8208bbdf49afb8db19c9f7b6b50 ejm-36-195-2024-ie00004.svg8pt14ptejm-36-195-2024-ie00004.png inline-formulaΣFe ratio in silicic melts is particularly susceptible to this process due to their low FeO content, and it should be critically evaluated before petrological interpretation.

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Botcharnikov, Roman / Wilke, Max / Garrevoet, Jan / et al: Confocal μ-XANES as a tool to analyze Fe oxidation state in heterogeneous samples: the case of melt inclusions in olivine from the Hekla volcano. 2024. Copernicus Publications.

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