Fate of fluid inclusions after high- to ultrahigh-temperature metamorphism and implications for multiphase fluid inclusions as a petrological tool

High-density monophase CO2 inclusions remain the focus of many studies on high-grade metamorphic rocks, although their origin as near-peak fluids is disputable. Conversely, multiphase fluid inclusions require more analytical effort and are often deemed unsuitable for determining the original fluid composition. This study demonstrates that this is not the case. We investigate rocks from the Napier Complex (Enderby Land, East Antarctica), which is the type locality of ultrahigh temperature (UHT) granulites and where only pure, highdensity CO2 inclusions were previously described. We document the widespread presence of multiphase fluid inclusions, composed of fluids and solids, resulting from fluid-host interaction during cooling. Composition and volume of the crystallized phases and fluid inside the inclusions were estimated by Raman spectroscopy, both by point analyses and 2D and 3D mapping. Fe-Mg carbonates and pyrophyllite are the most common solid phases, besides SiO2 (quartz and cristobalite). The fluid phase occupies ca. 66 vol% of the inclusions, is composed of CO2 with traces of N2 and has a density up to 1.09 g/cm3. These data are used to determine the initial H2O content (up to 10–20 mol%) and density (up to 1.2 g/cm3) of the fluid inclusions. Our results are consistent with entrapment at the peak P-T conditions previously estimated in the Napier Complex (~1050 ◦C, ~10 kbar). Our approach highlights the potential of multiphase fluid inclusions as petrological tools because their investigation enables to retrieve both density and bulk composition of the fluid phase present along the prograde path to peak conditions. Furthermore, since multiphase fluid inclusions provide insights into the nature of carbonate-forming reactions in a variety of minerals and lithologies, these inclusions may also reveal important information for carbon capture and storage projects.