Basalt Melting in Dry and Wet Systems: Thermodynamic Modeling, Parameterization, and Comparison with Experimental Data

Metabasite melting is a large-scale geological process that contributes to the formation of felsic volcanics and, to a greater extent, tonalite-trondhjemite-granodiorite (TTG) complexes, which make up a significant part of ancient continental crust. Based on the results of phase equilibrium modelling using the Perple_X software package, melting parameterisation was performed for three compositions: anhydrous mid-ocean ridge basalt (MORB), MORB-HO (2.78 wt % HO) and hydrated basalt (AOC, altered oceanic crust, 2.78 wt % HO) for temperatures of 500–1600°C and pressures of 0.0001–3 GPa. The obtained expressions are in good agreement with the few experimental data and show that for hydrous compositions (MORB-HO and AOC) there is a sharp increase in melt volume (up to 20 vol %) in the first 20–30°C after passing the water solidus temperature, the subsequent increase in temperature leads to a more restrained increase in the degree of melting. Modelling has shown that near-solidus melts in hydrous systems have rhyolitic and trachyadacitic compositions. A further increase in the degree of melting leads to a decrease in SiO and alkaline elements and an increase in CaO, MgO and FeO. The change in volume and composition of the melt is considered in the context of peritectic reactions, as well as changes in HO content. Application of the melting parameterisation to metabasalts from subducting slabs in the Cascadia and Central Aleutian subduction hot zones has revealed different degrees of melting of these rocks along the corresponding geotherms; the products of such melting are adakite magmas. The proposed parameterisation of rock melting can be used to analyse the mechanisms of felsic rock formation in different geodynamic settings and can be integrated into existing petrological and petrological-thermomechanical models.