Magma supply from the lower crust is often proposed as a trigger mechanism for volcanic eruptions. The timescales over which magma can be transported from the deepest parts of volcanic systems are, however, poorly constrained. This uncertainty poses problems for the construction of physical models and for assessment of volcanic hazards. Here, we combined geothermobarometry with Bayesian inversion diffusion chronometry on primitive olivine crystals from the Borgarhraun eruption, northern Iceland. We find that magma took about 10 days to ascend from near-Moho storage at 24 km depth before its eruption, and therefore present timescales for transcrustal magma transport on the global spreading ridge system. Our results reveal a rapid connection between the lower and upper crust with melt transport rates of 0.02 to 0.1 m s−1, which are consistent with the propagation rates observed in seismic swarms in the Icelandic lower crust. Monitoring of such events using surface CO2 fluxes may provide one of the earliest indicators that an eruption is imminent. At the high transport rates and low CO2 contents estimated for the Borgarhraun eruption, any effect of rising magma on surface CO2 fluxes is limited to a period of less than two days before eruption.