Melt Production beneath subduction zones: Using numerical models to evaluate melt production under batch and fractional melt conditions

Melt production at subduction zones depends on numerous variables, including mineral composition, water content, age of the plate, dip angle of the plate subducting, rate of convergence, age of the slab, and forearc dimensions. To evaluate the importance of individual variables and their interaction with each other, we constructed 2D numerical models of subduction, tracking temperature, mantle flow, and melt production. This project examines differences in batch and fractional melting sensitivity to the changes of the different variables. Variables include modal clinopyroxene (cpx) and its exhaustion, mantle hydration, dip angle, convergence rate, and forearc depth. Models tracked total melt as parameters were altered. For this project, the dip angle of the slab varied from 45 to 60°, rate of the slab between 20 and 90 km/Myr, age of the plate between 20 and 90 Myr, forearc depth between 40-50 km, and hydration between 0.01 and 0.1 wt%. The slab age and initial modal cpx levels are held constant throughout all the trials at 60 Myr and 15%, respectively. With batch melting, melting peaks for models set with hydration content > 0.1%, a dip angle at 60°, the highest convergence rates, and the youngest ages. Melting decreases with greater ages and lower convergence rates. In both fractional and batch melting, increasing the hydration leads to an increase in melt production overall. For fractional melting with hydration less than 0.05wt%, the difference in amount of melt compared to batch melting is negligible. At greater initial hydration the difference becomes greater with less produced under fractional melting. Changes in forearc extent also affect total melt with longer forearcs resulting in less melt than shorter ones. Additionally, we explored the effects of permeability on the melt production. Most notably, a secondary region of melt begins to form for when permeability is about 0.02 or greater. The secondary region encompasses melting above the harzburgite solidus. While two melting regions were nearly always observed under batch melt conditions, typically only one region of melting was observed under fractional melt conditions. In both cases, hydration and the dip of subducting slab have the most effect on melt production, while the convergence rate and the depth of the forearc have a smaller effect on melt production.