Spinel+chlorite assemblage forms only at ultrahigh pressures in garnet-hosted inclusions

The action of slab-derived fluids on mantle rocks frequently brings about the formation of microcavities in minerals, which represent microenvironments where the interaction between fluid and host mineral is preserved during the whole geological path of the rock. The entrapped fluid and the host matrix constitute a system evolving towards equilibrium under peculiar chemical-physical constraints. By expliciting these constraints through mass conservation laws and stoichiometry relations accounting for the composition of the host mineral and of the initial fluid, and for the presence of a finite fluid-solid interface, we show that cavities in minerals filled with slab-derived fluids can re-equilibrate following a crystallisation mechanism which is not predictable through simple equilibrium arguments holding for open systems (Campione et al., 2017). The occurrence of phases stable at ultrahigh pressure (UHP), such as coesite or microdiamond, has long been considered evidence of precipitation from such fluids at P > 3 GPa. Here, we demonstrate that even a mineral association characterised by phases usually considered stable at relatively low pressure in mantle systems (e.g., spinel + chlorite), would potentially crystallise at UHP by chemical fluid/host interaction. We will consider as a case study a well-known example of multiphase solid inclusions occurring in the cores of garnets forming orthopyroxenites from the Maowu Ultramafic Complex (Eastern China), interpreted as hybrid rocks resulting from the interaction of previous harzburgites and slab-derived silica-rich liquids (P = 4 GPa, T = 800 °C) at the slab-mantle interface. Malaspina et al. (2015) demonstrated the epitaxial relationship between spinel and garnet, which suggested nucleation of spinel under near-to-equilibrium conditions. On the contrary, hydrous phases (amphiboles, chlorite and ±talc ±phlogopite) nucleate in a non- registered manner and likely under far-from-equilibrium conditions. The epitaxial growth of spinel with respect to garnet and the chlorite rim + water assemblage filling the space between the host garnet and the other inclusion minerals (Malaspina et al., 2017), suggest that spinel and chlorite formed at UHP together with the garnet cores. Campione, M., Tumiati, S., Malaspina, N. (2017): Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure. Geochem. Perspect. Lett., 4, 19-23. Malaspina, N., Alvaro, M., Campione, M., Wilhelm, H. & Nestola, F. (2015): Dynamics of mineral crystallization from precipitated slab- derived fluid phase: first in situ synchrotron X-ray measurements. Contrib. Mineral. Petrol., 169, 26. Malaspina, N., Langenhorst, F., Tumiati, S., Campione, M., Frezzotti, M.L., Poli, S. (2017): The redox budget of crust- derived fluid phases at the slab-mantle interface. Geochim. Cosmochim. Acta, 209, 70-84.