Precise measurements of U and Th concentrations and Sr, Th and O isotopes in a suite of samples from the 1783-84 Lakagigar eruption, reveal an extreme homogeneity in the 15 km3 lave flow. However, geochemical constraints suggest that the quartz-tholeiite magma results from the assimilation of approximately 20% of the lower crust by a mantle derived olivine-tholeiite magma. The constant magma composition thus implies vigorous convection and efficient mixing before eruption, probably in a reservoir at the crust mantle boundary. The same deep reservoir probably also fed a shallow magma chamber below Grimsv¿tn, inducing simultaneous activity in both Lakagigar and Grimsv¿tn central volcano. ¿American Geophysical Union 1991