The extent and geochemical impact of crustal contamination during magmatic evolution in intra-oceanic subduction zone settings is often assumed to be of minimal significance but remains poorly constrained. Acquiring such information is crucial if meaningful timescales of magma generation and crustal residence beneath volcanoes are to be determined. High-MgO basalts and differentiates erupted over the last 100 years at Lopevi volcano display a strong negative correlation between 87Sr/86Sr isotope ratio and indices of differentiation (e.g. SiO2) even though these lavas have a restricted Sr-Nd isotopic range compared to other Vanuatu arc lavas. This trend provides compelling evidence for the interaction of rising mafic magmas with 'primitive' sub-arc crust and so this volcano affords an excellent opportunity to investigate and ascertain timescales of crustal interaction using U-series data. Quantitative geochemical modelling of whole-rock trace element ratios, 87Sr/86Sr isotope compositions and U-series data shows that assimilation of a relatively small-degree partial melt of > 380 Ka mafic oceanic crust (similar to Pacific- or Indian-MORB in 87Sr/86Sr isotopic composition) during fractional crystallisation of magma exerts major control on the (230Th/232Th) and (226Ra/230Th) activity ratios of the lavas. The incorporation of higher (230Th/232Th) and lower (226Ra/230Th) assimilated material drives compositions closer to secular equilibrium than simple closed-system differentiation, reducing calculated apparent timescales of closed-system differentiation from Th isotope composition (104-105) by orders of magnitude. Modelling suggests that assimilation occurs extremely rapidly at Lopevi with timescales for magma generation, differentiation and eruption on the order of 102 years.