Subsolidus fluid generation via coupled muscovite dehydroxylation and biotite dehydration: experimental constraints for metasomatism in granites

Subsolidus muscovite dehydroxylation represents a significant internal fluid source in felsic rocks. Non-equilibrium low-pressure differential-heating and high-pressure piston–cylinder experiments show that muscovite dehydroxylation generates fluids capable of reactivating mineral-replacement reactions without external fluid addition. Structurally bound OH released during dehydroxylation forms transient fluids that enhance the instability of adjacent biotite, transforming it into fine-grained orthopyroxene–spinel aggregates with total K and Ti loss. This contrasts with biotite orthogneiss where only limited transformations occur. In two-mica orthogneiss, internally derived fluid triggers titanite growth and pervasive alkali metasomatism in feldspars: plagioclase relicts are replaced by transitional Or–Ab feldspar, porous albite, and newly formed K-feldspar, producing textures indistinguishable from those of natural hydrothermal or lower-crustal metasomatic systems. These transformations proceed via coupled dissolution–precipitation, recorded by extensive porosity, patchy domains, and K-feldspar veins parallel to foliation. The resulting microstructures indicate efficient fluid-mediated mass transfer even at extremely low fluid volumes. This behaviour is consistent with transport-dominated kinetic regimes in which grain-boundary solubility and connected fluid networks enable rapid re-equilibration. The contrasting behaviour of the two lithologies highlights the influence of muscovite on fluid availability, alkali mobility, and subsolidus reaction pathways. Our experiments indicate that mica-derived fluids may act as effective metasomatic agents during thermal shock events in subsolidus natural systems where muscovite-out reactions are overstepped, allowing its dehydroxylation. Therefore, subsolidus metastable muscovite devolatilisation provides a mechanistic link between dehydration reactions, feldspar re-equilibration, and the preconditioning of felsic rocks for anatexis.