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doi: 10.7185/geochemlet.1730 Multiphase inclusions represent microenvironments where the interaction between fluid and host mineral is preserved during the rock geological path. Under its peculiar chemical-physical constraints, the entrapped solute-rich fluid might follow a crystallisation mechanism which is not predictable through simple equilibrium arguments. In this letter, by the modelling of solid-solution equilibrium and the application of principles of mass conservation, we demonstrate that cavities in mantle garnet filled with slab-derived fluids can re-equilibrate to a pyrope + spinel + chlorite assemblage at the same high P-T of their formation. The basis of this occurrence is a dissolution-reprecipitation mechanism, triggered by a dilute, non-equilibrated slab fluid. Received 13 April 2017 | Accepted 25 July 2017 | Published 23 August 2017 1. Department of Earth and Environmental Sciences, Università degli Studi di Milano Bicocca, Piazza della Scienza 4, I-20126 Milano, Italy * Corresponding author (email: marcello.campione@unimib.it) 2. Department of Earth Sciences, Università degli Studi di Milano, Via Mangiagalli 34, I-20133 Milano, Italy Introduction Primary inclusions (fluid or solid multiphase) in mantle minerals represent the remnant of the fluid phase produced by dehydration reactions in the subducted slab and subsequently equilibrated with the overlying supra-subduction mantle peridotites. Natural and experimental studies demonstrated that saline aqueous inclusions with variable solute load prevail in high pressure (HP) rocks (Scambelluri and Philippot, 2001; Touret, 2001), whereas multiphase solid inclusions in some ultrahigh pressure (UHP) rocks have been attributed to silicate-rich fluids or hydrous melts at supercritical conditions, namely supercritical liquids (Bureau and Keppler, 1999; Stöckhert et al., 2001; Carswell and van Roermund, 2005; Ferrando et al., 2005; Korsakov and Hermann, 2006; Malaspina et al., 2006). These inclusions are frequently hosted by minerals stable at mantle depths, such as garnet, and show the same textural features as fluid inclusions (Frezzotti and Ferrando, 2015; Fig. 1). The mineral infillings of the solid multiphase inclusions are generally assumed to have crystallised by a simultaneous precipitation from the solute load of dense supercritical liquids equilibrating with the host rock. Moreover, the occurrence of phases stable at UHP, such as coesite or microdiamond, has long been considered evidence of precipitation from such liquids at pressures above 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., 2006; Chen et al., 2017). These multiphase inclusions have negative crystal shapes and constant volume proportions of the mineral infillings consisting of spinel ± orthopyroxene, and hydrous phases gedrite/pargasite, chlorite, phlogopite, ±talc, ±apatite (Malaspina et al., 2006, 2015; Fig. 1). Some inclusions still preserve liquid water at the interface between mineral infillings and the cavity wall (Malaspina et al., 2017). Recently, Malaspina et al. (2015) demonstrated the epitaxial relationship between spinel and garnet (Fig. 1a), 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. However, spinel should be stable at UHP conditions neither in hydrous peridotites (Niida and Green, 1999), nor in a chemical system characterised by pyrope + H2O, at any P-T range (Fig. 2). Surprisingly, in nature we can count a number of examples where spinel occurs in peridotites in garnet-hosted primary diamond-bearing (hence UHP) multiphase inclusions from Bardane, Ugelvik and Svartberget in the Western Gneiss Region (van Roermund and Drury, 1998; Van Roermund et al., 2002; Carswell and van Roermund, 2005; Vrijmoed et al., 2006, 2008; van Roermund, 2009; Malaspina et al., 2010; Scambelluri et al., 2010). We will show that the growth of spinel, driven by a garnet dissolution-reprecipitation mechanism triggered by precise constraints on the composition of the supercritical liquid phase released by the slab, is possible also at UHP conditions. Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2017) 4, 19-23 | doi: 10.7185/geochemlet.1730 20 Figure 1 (a) Photomicrograph of a multiphase solid inclusion in metasomatic garnet from Maowu Ultramafic Complex (Dabie Shan, China). Inset represents the relative orientation of the spinel {100} surface lattice (light blue) with respect to the garnet {100} surface lattice (violet) for the coincidence at θ = −45° (from Malaspina et al., 2015). (b) and (c) Negative-crystal shaped multiphase solid inclusion (plane polarised transmitted light and Secondary Electron image) with evident microstructural relations between spinel, chlorite and amphibole (gedrite). Figure 2 Isochemical P-T section in the MASH system showing the predicted mineral assemblages calculated with Perple_X software package (Connolly, 1990), for a bulk composition corresponding to pyrope [MgO (3 mol) – SiO2 (3 mol) – Al2O3 (1 mol)] + excess H2O. Mineral abbreviations: chl = chlorite, mctd = Mg-clorithoid, ta = talc, mcar = Mg-carpholite, ky = kyanite, q = quartz, opx = orthopyroxene, sud = sudoite, cor = corundum, sill = sillimanite, crd = cordierite, sapp = sapphirine.

Primary spinel + chlorite inclusions in mantle garnet formed at ultrahigh-pressure