Premelting polymerization of crustal and mantle fluids, as indicated by the solubility of albite + paragonite + quartz in H2O at 1 GPa and 350–620 °C

The composition and structure of dissolved silicates in model crustal and upper-mantle fluids was assessed by measuring the solubility of the assemblage albite + paragonite + quartz in H2O from 350 to 620 °C, at 1 GPa. Natural, low albite and quartz were equilibrated with H2O; paragonite grew in all experiments due to incongruent dissolution of albite + quartz. Melting occurred at 635 ± 5 °C. Solute concentrations at subsolidus conditions were determined by analysis of quenched fluids or mineral weight-loss and mass balance. Bulk solubility of the mineral assemblage increased from ∼ 1 to ∼ 8 oxide wt.% with rising temperature. Si, Al, and Na all increase in concert. The solutions were slightly peralkaline, and possess Si/(Na + Al) ≫ 1.5 (molar) at all conditions studied. Extrapolated thermodynamic data were used to predict solubility at the conditions investigated experimentally. Calculated solubility agreed with that measured from 350 to ∼ 500 °C; however, above 500 °C, the calculations underpredict solubility to an increasing degree as the hydrothermal melting point is approached. The excess measured solubility points to increasing abundance of aqueous Si, Al–Si, and Na–Al–Si polymers. Polymerized solutes predominate in all near-solidus solutions, rising to > 80% of total dissolved solids at the melting point. The observations support a conceptual model in which, as the temperature of the system rises isobarically at 1 GPa, the silicate components dissolved in the aqueous phase begin to polymerize significantly within ∼ 100 °C of the melting point. The polymerized solutes may facilitate condensation of more polymerized hydrous silicate liquid at the hydrothermal melting point. From the perspective of isobaric cooling, the fluid crossing the solidus retains Na–Al–Si–O clusters or fragments that are less polymerized than those which comprised the melt, but more polymerized than has previously been inferred for the aqueous phase. The high concentrations of Na, Al, and Si in the form of polymeric clusters will yield substantial mass transfer by near-solidus metamorphic and magmatic fluids in the crust and mantle, and likely promote dissolution, transport and precipitation of nominally insoluble, refractory rock components.