Improved understanding of mantle melting processes and melt transport requires knowledge of how fast magma is generated and transferred from source region to surface. The rate of magma transfer can in favorable cases be estimated from radioactive disequilibria between nuclides of the 2 3 8 U series. Young lavas from southern Chile, in which 2 3 8 U- 2 3 0 Th disequilibria have been measured [Sigmarsson et al., Nature 346 (1990) 163-165; Sigmarsson et al., Nature 394 (1998) 566-569], were analyzed for 2 2 6 Ra abundances. The disequilibrium between 2 2 6 Ra and 2 3 0 Th in these lavas is found to correlate with 2 3 8 U- 2 3 0 Th disequilibria and 1 0 Be/Be [Morris et al., Nature 344 (1990) 31-36]. These correlations strongly suggest that the excess of 2 2 6 Ra over 2 3 0 Th is due to the addition of a slab-derived fluid to the magma source, since Ra and U are fluid-mobile elements and the cosmogenic 1 0 Be is most likely derived from the subducting Nazca plate beneath the Andes. The largest slab signature is observed in the lavas of Villarrica volcano, which is the most active volcano in South America. A model for subduction fluxing is discussed, in which the U series disequilibria in arc lavas will reflect the integrated dehydration process during metamorphism of the subducting plate and the metasomatized mantle, but be principally controlled by the latest hydrous mineral breakdown in the mantle wedge. Repeated precipitation and dehydration mineral reactions of the hydrated mantle could be the homogenization process of the slab input needed to explain the 1 0 Be/Be-B/Be correlation for different arcs [Morris et al., Nature 344 (1990) 31-36]. The fact that excesses of 2 2 6 Ra and 2 3 8 U over 2 3 0 Th are correlated indicates that linear arrays on the ( 2 3 0 Th/ 2 3 2 Th)-( 2 3 8 U/ 2 3 2 Th) diagram are not isochrons reflecting time elapsed since a fluid addition but rather mixing lines between a fluid phase and melts. The 2 2 6 Ra- 2 3 0 Th disequilibrium in arc lavas suggests significantly shorter timescales for magma transfer, or less than 8000 years. This disequilibrium is consistent with minimum magma transfer rate through the mantle wedge on the order of 10 m/year. Finally, the correlations of ( 2 2 6 Ra / 2 3 0 Th) with ( 2 3 8 U/ 2 3 2 Th) and 1 0 Be/Be in Andean magmas imply that magma chamber residence time is of the same order of magnitude beneath the stratovolcanoes studied.