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

Abstract

Melting of metabasic rocks is a large-scale geologic process contributing to the formation of silicic volcanics and, especially, tonalite–trondhjemite–granodiorite (TTG) complexes, which make up a considerable portion of the ancient continental crust. Based on the phase equilibria modeling using the Perple_X program package, parameterization of melting was conducted for three compositions: anhydrous mid-ocean ridge basalt (MORB), MORB-H₂O (2.78 wt % H₂O), and hydrated basalt (altered oceanic crust, AOC, 2.78 wt % H₂O) at 500–1600°C and 0.0001–3 GPa. The obtained relations show good consistency with limited experimental data and indicate that the volume of melt produced in hydrous systems (MORB-H₂O and AOC) increases rapidly (up to 20 vol %) within 20–30°C above the hydrous solidus, which is followed by a more moderate increase in the degree of melting with increasing temperature. The modeling demonstrated that the near-solidus melts of the hydrous systems are rhyolitic and trachydacitic in composition. An increase in the degree of melting results in a decrease in SiO₂ and alkalis and an increase in CaO, MgO, and FeO contents. Changes in melt volume and composition are considered in connection with peritectic reactions and variations in H₂O content. The application of the parameterization of melting to metabasalts from the downgoing slabs in the Cascadia and Central Aleutian hot subduction zones revealed that these rocks underwent different degrees of melting along respective geotherms, and adakitic magmas are produced by such melting. The proposed parameterization of rock melting is useful for the analysis of the mechanisms of silicic rock formation in different geodynamic environments and can be implemented in the existing petrological and petrological–thermomechanical models.