The redox budget of crust-derived fluid phases at the slab-mantle interface

作者：Malaspina N; Langenhorst F; Tumiati S; Campione M; Frezzotti M L; Poli S

来源：Geochimica et Cosmochimica Acta, 2017, 209: 70-84.

DOI：10.1016/j.gca.2017.04.004

• Flank method
• Electron energy loss spectroscopy
• Multiphase inclusions
• Oxygen chemical potential
• Oxygen fugacity
• Subduction zone

The redox processes taking place in the portion of the mantle on top of the subducting slab are poorly investigated and the redox potential 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 represented by garnet orthopyroxenites from the Maowu Ultramafic Complex (China) deriving from harzburgite precursors metasomatised at +/- 4 GPa, 750-800 degrees 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 Raman spectroscopy, together with X-ray microtomography performed on single inclusions indicate that liquid water is still preserved at least in some inclusions (+/- spinel).
To investigate the redox budget of these fluid phases, we measured for the first time the Fe3+ concentration of the micronsized precipitates of the multiphase inclusions using EELS on a TEM. Results indicate that spinel contains up to 12% of Fe3+ with respect to the total iron, 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 fO(2) calculations indicate that the garnet orthopyroxenites record Delta FMQ = -1.8 : 1.5, therefore 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 (nO(2)) is the same (10 mol m(-3)). The oxygen mass balance of the crust-derived fluids (multiphase inclusions) also indicates that the fluid precipitates are more oxidised than the host rock, reaching up to 400 mol m(-3) of nO(2). This suggests that even after their interaction with the metasomatic orthopyroxenites, the residual fluid phases could be potentially carrier of oxidised components when it escapes the slab-mantle interface. Because of this gradient in nO(2), a metasomatic front develops from the oxidised slab to the overlying lithospheric mantle wedge passing through a transitional layer of hybrid rocks at the slab-mantle interface.

• 出版日期2017-7-15