Melting kinetics of granitic powder aggregates at 1175°C, 1 atm.
Résumé
Melting experiments of pressed granitic powders (10% < 3.23 m, 50% < 31.63 m, 90% < 169.85 m) have been performed at 1175°C, 1 atm for different run durations (5, 10, 40 mn and 3, 24, 68 h). During partial melting, quartz and feldspars progressively decrease in abundance, the amount of melt produced increases sharply after 40 mn and leads to a strong decrease of the porosity of the powder (from 42% before heat-treatment to 10% after 68 h). In the 5 mn charge, thin glass films (1-2 m) appear cementing small particles grains and narrow glass channels are formed and trapped within both albite and K-feldspar. Newly-formed K-feldspars appear in the 10 and 40 mn charges. They are both texturally and compositionally distinctive from the original feldspars. All feldspars have totally disappeared after 3 h and quartz (up to 330 m) persists in the longest experiment (68 h). For durations longer than 40 mn, glass compositions are chemically zoned. They vary mainly between two end-members, one rich in normative feldspar components and poor in normative quartz and the other rich in normative quartz corresponding to areas close to the quartz interface. The melting reaction is governed by disequilibrium melting. Two melting regimes were identified: (1) a low melt fraction regime corresponding to heterogeneous nucleation of melt at the surface and within interiors of both albite and K-feldspar and (2) a high melt fraction regime kinetically controlled by diffusive mass transport within the melt layer. The lack of leucite implies that the melting relations of K-feldspar observed in this study are congruent. About 50 days are needed for the melting reaction to approach chemical equilibrium at 1175°C.