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.

The Krafla area in north Iceland hosts a high-temperature geothermal system within a volcanic caldera. Temperature measurements from boreholes drilled for power generation reveal enigmatic contrasts throughout the drilled area. While wells in the western part of the production field indicate a 0.5-1 km thick near-isothermal (~210 °C) liquid-dominated reservoir underlain by a deeper boiling reservoir, wells in the east indicate boiling conditions extending from the surface to the maximum depth of drilled wells (~2 km). Understanding these systematic temperature contrasts in terms of the subsurface permeability structure and overall dynamics of fluid flow has remained challenging. Here, we present a new numerical model of the natural, pre-exploitation state of the Krafla system, incorporating a new geologic/conceptual model and a version of TOUGH2 extending to supercritical conditions. The model shows how the characteristic temperature distribution results from structural partitioning of the system by a rift-parallel eruptive fissure and an aquitard at the transition between deeper basement intrusions and high-permeability extrusive volcanic rocks. As model calibration is performed using a Bayesian framework, the posterior results reveal significant uncertainty in the inferred permeability values for the different rock types, often exceeding two orders of magnitude. While the model shows how zones of single-phase vapor develop above the deep intrusive heat source, more data from deep wells is needed to better constrain the extent and temperature of the deep vapor zones. However, the model suggests the presence of a significant untapped resource at Krafla.