Muscovite dehydration melting: Reaction mechanisms, microstructures, and implications for anatexis

Abstract

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour‐absent) crust, muscovite reacts with quartz to produce K‐feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K‐feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K‐feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K‐feldspar from K‐feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K‐feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K‐feldspar replaces plagioclase by K+–Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K‐feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K‐feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic‐tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K‐feldspar trap fine‐grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K‐feldspar grains overgrow and trap fabric‐aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K‐feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K‐feldspar in the Himalayan metamorphic core requires vapour‐absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.

Novel extraction route of lithium from α-spodumene by dry chlorination

“…There is a foreknowledge that under pressure, the decomposition of muscovite 43,44 and biotite, 45,46 (the two mica species identified in the spodumene concentrate) form K-feldspar as shown from eqn (12)–(15) . The formation is said to be an ionic exchange process 44 which is similar to observation in our previous study. 32 KAl 3 Si 3 O 10 (OH) 2 + SiO 2 → [KAlSi 3 O 8 ] melt + Al 2 SiO 5 + H 2 O (g) .…”

Metamorphic evolution of the Great Slave Lake shear zone

“…6.1 | Pressure-temperature estimates 6.1.1 | GSL-18-C47-Granulite facies paragneiss the vapour-absent muscovite dehydration reaction does any significant melting occur, which is accompanied by the growth of an aluminosilicate phase (Dyck et al, 2020). We interpret the aluminosilicate microstructures to have formed along a prograde path that started in the kyanite stability field with 3-4 modal% growth during muscovite melting.…”

Microstructural evidence for convection in high-silica granite

“…The crystallization sequence of the six HSG samples used in this study was calculated to predict, and provide context for, the relative timing of microstructure formation. Calculations were made with Theriak-Domino software (de Capitani and Petrakakis, 2010), a Gibbs free-energy minimization program, using the Holland and Powell (1998) thermodynamic database and the activity composition relations employed by Dyck et al (2020).…”