Iron oxidation state in garnet from a subduction setting : a micro-XANES and electron microprobe (“flank method”) comparative study

The variation of the Fe-redox state across a strongly zoned garnet crystal occurring in a FeTi-oxide metagabbro from a subduction setting has been investigated comparing micro-XANES and electron microprobe ("flank method") results. This sample represents an ideal monitor of the redox processes associated with devolatilization reactions occurring in the subducting slab and likely involved in the oxidation of the mantle wedge, as well as a challenge for the analytical methods. Since in the studied garnet the Fe 3+/ΣFe variations are small and occur on a micrometric scale, we exploited the X-ray microprobe available at the ESRF ID22 beamline to reach a spot size of 1.7 μm × 5.3 μm. Our study represents a first attempt to measure the Fe 3+/ΣFe variations across a single crystal of garnet, ca. 400 μm in diameter, using the micro-XANES technique. The absolute Fe 3+/ΣFe values obtained with micro-XANES and "flank method" are perfectly comparable, ranging from a maximum of about 0.1 in the garnet core to a minimum of 0.0 in the garnet rim.