Basaltic fissure eruptions emit an array of volatile and environmentally reactive gases and particulate matter (PM) into the lower troposphere (e.g., SO2, HCl, and HF in the gas phase; Se, As, Pb as complexes in the PM phase). Lava flows from fissure eruptions can be spatially extensive, but the composition and fluxes of their emissions are poorly characterized compared to those from main vent(s). Using UAS-mounted (drone) samplers and ground-based remote FTIR spectroscopy we investigate the down-flow compositional evolution of emissions from active lava flows during the Fagradalsfjall 2021-2023 eruptions. The calculated fluxes of volatile trace metals from lava flows are considerable relative to both main vent degassing and anthropogenic fluxes in Iceland. We demonstrate a fractionation in major gas emissions, with decreasing S/halogen ratio down-flow. This S-Cl fractionation is reflected in the trace element degassing profile, where the abundance of predominantly sulfur-complexing elements (e.g., Se, Te, As, Pb) decreases more rapidly in down-flow emissions relative to elements complexing as chlorides (e.g., Cu, Rb, Cs), oxides (e.g., La, Ce) and hydroxides (e.g., Fe, Mg, Al). Using thermochemical modeling, we explain this relationship through temperature and composition dependent element speciation as the lava flow ages and cools. As a result, some chloride-complexing elements (such as Cu) become relatively more abundant in emissions further down-flow, compared to emissions from the main vent or more proximal lava flows. This variability in down-flow element fluxes suggests that the output of metals to the environment may change depending on lava flow age and thermal evolution.