Can Multiphase Inclusions in Garnet From Eclogite Facies Rocks Be a Product of Melt or Supercritical Fluid Entrapment During Subduction?

ABSTRACT Dehydration of rocks and minerals during high‐pressure metamorphism is controlled by the thermal structure of the subduction zone and can be traced by the stability fields of hydrous phases. Consistent with the results of experimental works, the content of silicate components in fluids is assumed to increase from the fields of subcritical to supercritical fluid at higher pressure and to hydrous melt at higher temperature. As there is no direct information about the composition of these fluids along the subduction zone, the multiphase (polyphase) inclusions in anhydrous minerals in eclogite facies rocks are frequently interpreted as products of such fluids or melt, which became entrapped and subsequently crystallised. Although some of these multiphase inclusions are formed by quartz and feldspars and have a composition of granitic melt, most of them contain phases that do not support their formation from granitic or other types of melt. The assumption of their formation from supercritical fluids or melt has stimulated laboratory research to reproduce the original fluids or melts from which these multiphase inclusions could have originated. Therefore, petrological data useful to verify or falsify this assumption is essential for further direction in multidisciplinary research on fluid and melt generation in the subduction zone. This paper presents textural and compositional relations among multiphase inclusions in garnet from various lithologies subjected to high‐ to ultrahigh‐pressure metamorphism. The aim is to substantiate the in situ formation of multiphase inclusions through recrystallisation of pre‐existing (hydrous) solid phases enclosed within host minerals. In addition to the presence of original hydrous solid phases, evidence is provided for their transformation into high‐pressure phases during prograde metamorphism as well as their subsequent modification and decomposition during retrogression or heating associated with decompression. Furthermore, it is discussed whether the crystallisation of garnet with inclusions is in agreement with bulk‐rock fractionation and thermodynamic modelling and whether the compositions of multiphase inclusions correspond to the pressure–temperature fields of hydrous melts or supercritical fluids from experimental data. The results of the study on the textural and compositional relationships of the multiphase inclusions, along with the pressure stability fields of the host minerals, support the interpretation that these inclusions formed by recrystallisation of originally enclosed aqueous and anhydrous phases, rather than by entrapment from a melt.