Garnet-Carbonated fluid Interaction at high pressures and temperatures

Measurements of the mantle redox state through the application of oxy-thermobarometry on peridotite and eclogite show evidence of their interaction with metasomatic fluids likely originating from subducted slabs [1]. In particular, garnets from these rocks often reveal heterogeneous (rim-to-core) ferric iron contents associated with variations in the trace element distribution [2] that have been explained in light of redox reactions with circulating carbonated fluids. To date, however, an experimental study is missing that allows modeling the Fe2+ to Fe3+ oxidation resulting from mineral-fluid interaction. At this aim, we performed experiments to investigate redox reaction between natural garnets and carbonated fluid [3] . Two sets of natural garnets, peridotitic FSM1 from Kaapvaal craton (South Africa; [4]) and eclogitic G3-1 from V. Grib kimberlite pipe (Arkhangelsk Diamondiferous Province, NW Russia; [5]) with known Fe³⁺/Fetot determined by Mössbauer spectroscopy were used as starting crystals. The experiments were performed using a diamond anvil cell (DAC) equipped with two ultralow-fluorescence type IIa diamond anvils (500 μm culet diameter). An 80 μm-thick pre-indented iridium gasket was drilled to produce two holes that were filled with single crystal of peridotitic and eclogitic garnet, respectively. A drop of 1 mol/kg Na₂CO₃ acqueous solution acted as both the pressure-transmitting medium and reactive fluid. Pressures were determined monitoring the shifts in the v3(SiO4) band of the 20 μm zircon crystals inserted as pressure marker. In situ micro-Raman spectroscopy of the garnet–Na₂CO₃-H2O system was performed from room temperature to 800 °C and up to 8 GPa using a Renishaw Reflex spectrometer equipped with a 532 nm Nd:YAG laser at the HPSTAR facility in Beijing (China). Heating was provided by a Ni-Cr resistance coil at a rate of 10 °C min⁻¹ and monitored with a K-type thermocouple. Preliminary data shows the formation of new crystalline phases from the C-fluid whose stability will be discussed in terms of P, T, Fe3+/ΣFe and fo2. [1] Stagno, V., and Fei, Y. Elements, (2020). [2] McCammon, C., et al. Contrib. Mineral. Petrol., (1998). [3] Tao and Fei, Commun. Earth Environ., (2021). [4] Woodland, A., and M. Koch. EPSL (2003). [5] Marras et al., J. Petrol., (2024).