Compositional variability is present for almost every element analyzed in 70 whole rock samples and 40 olivine-hosted melt inclusions from the Borgarhraun lava flow in NE Iceland. The variation of incompatible element concentrations can be produced by incomplete mixing of fractional melts, while the compatible element variations in the whole rock samples can be explained by addition/removal of crystals found in the flow. The melt inclusion incompatible element compositions are more variable than the whole rock compositions, and the magnitude of variability of the whole rock samples can be matched if each hand specimen is made of magma that was formed by mixing of 20--30 batches of melt with the composition of the inclusions. Clinopyroxene barometry and the major element composition of the whole rock samples suggest that mixing took place at pressures of ~0.9 GPa in sub-Moho magma chambers. The spatial distribution of the concentrations of incompatible elements is not random in Borgarhraun and samples with a separation of <4 km have concentrations that are more similar than expected from a random distribution of geochemistry. This separation distance corresponds to a lava volume of 0.014--0.14 km3. This volume may be controlled by magma mixing and episodic eruption of melts from a sub-Moho chamber. The concentration variations of compatible elements with position in the flow are little different to those expected for random variation. It is likely that the crystal removal and addition processes that control major element compositions generate variability on short length scales.