How long does it take to assemble the magmas that feed large basaltic fissure eruptions?

Petrological constraints on timescales of pre-eruptive crystal storage and transport provide a vital framework for interpretation of seismic, geodetic and gas monitoring data in regions where large fissure eruptions occur. The AD 1783 Laki eruption, southeast Iceland, is one of the best-documented small-scale analogues to a flood basalt eruption. Here we use Fe-Mg interdiffusion in olivine and Mg diffusion in plagioclase to assess the timescales required to assemble the Laki magma prior to eruption. Olivine macrocrysts in Laki magmatic tephra display two to three distinct compositional zones, indicative of crystallization and storage in different magmatic environments. Most olivines have core compositions of Fo<76, and rim compositions of Fo69-Fo74 that are close to equilibrium with the Laki melt. The most probable Fe-Mg diffusion timescale for these olivines is 7.8 days, representing a characteristic crystal residence time in the carrier melt. The timescales coincide with historical accounts of strong earthquakes in southeast Iceland, which we interpret as being associated with dissociation of crystal mushes and initiation of dyke propagation to the surface. A small population of olivines with Fo>81 cores preserve diffusion timescales of 125 days, reflecting crystallization and storage in mid-crustal magma chambers. Plagioclase macrocrysts from Laki display more complex textures, with high-anorthite (An≥85) cores, oscillatory-zoned mantles, and low-anorthite (An 60) rims. Reverse fractional crystallization models demonstrate that the high-An plagioclase cores could not have crystallized from the Laki parental melt, but grew from depleted, high-Ca/Na primary mantle melts. High-An plagioclase cores were thus entrained into the Laki magma at an early stage of its evolution. Element diffusion across plagioclase core-mantle boundaries provides an upper estimate for timescales of magma assembly in the lower to mid-crust. Modelling Mg diffusion across these zones indicates that magma assembly occurred on timescales on the order of months prior to eruption. Lower to mid-crustal seismicity associated with magma mixing and mush entrainment, and elevated CO2 fluxes from degassing magmas in the mid-crust, could provide longer-term indicators for the potential onset of future large eruptions.