The redox budget of crust-derived fluid phases: a snapshot of the slab-mantle interface

The redox processes taking place in the portion of the mantle on top of the subducting slab are poorly investigated and the redox budget of crust-derived fluid phases is still poorly constrained. A case study of supra-subduction mantle affected by metasomatism from crust-derived fluid phases is the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatised at ∼4 GPa, 750-800 °C by a silica- and incompatible trace element- rich fluid phase. This metasomatism produced poikilitic orthopyroxene and inclusion-rich garnet porphyroblasts. Solid multiphase primary micro-inclusions in garnet display negative crystal shapes and infilling minerals (spinel, ±orthopyroxene, amphiboles, chlorite, ±talc, ±mica) occur with constant modal proportions, indicating that they derive from trapped solute- rich aqueous fluids. FT-IR hyper spectral imaging analyses and micro-Raman spectroscopy, together with X-Ray microtomography performed on single inclusions indicate that liquid water is still preserved at least in some inclusions. To investigate the redox budget of these fluid phases, the Fe3+ concentration of the micron-sized precipitates of the multiphase inclusions has been measured for the first time using EELS on a TEM. Results indicate that spinel contains up to 12% of Fe3+/ΣFe, amphibole about 30%, while the ratio in inclusion phases such as chlorite and phlogopite may reach 70%. The Fe3+ fraction of the host garnet is equal to that measured in spinel as also confirmed by Flank Method EPMA measurements. Forward modelling fO2 calculations indicate that the garnet orthopyroxenites record ΔFMQ=−1.8 ÷ −1.5, resulting apparently more reduced with respect to metasomatised supra-subduction garnet-peridotites. On the other hand, oxygen mass balance, performed both on the Maowu hybrid orthopyroxenite and on metasomatised supra-subduction garnet peridotites, indicate that the excess of oxygen (nO2) is the same (10 mol m-3). An oxygen mass balance of the crust- derived fluids also indicates that the fluid precipitates are more oxidised than the host rock, reaching up to 400 mol m-3 of nO2. This suggests that even after their interaction with the metasomatic orthopyroxenites, the residual fluid phases could be potentially carrier of oxidised components when escaping the slab-mantle interface.