Arc magma acquires continental crust-like trace element signatures through selective recycling of incompatible elements from the subducted slab. The long-standing model of element recycling through aqueous fluid from altered oceanic crust (AOC) and sediment melt has been challenged by the resurgence of mélange diapir (a mix of AOC, sediment, and serpentinite) and saline aqueous fluid models. Here, we present experimental data for near-solidus sediment melts and a framework for calculating trace element concentrations in subduction fluids from metamorphosed sediment and oceanic crust. We observe that variation of element ratios in global primitive arc basalts is comparable with that of sediment and/or oceanic crustal melt, rather than (saline) aqueous fluid or mélange melt. In particular, the systematic correlation of element ratios in arc basalt corresponds to element fractionation in slab melt with temperature and therefore follows a power function. Our findings suggest that slab melt is primarily responsible for element recycling to the arc.