We present a critical reappraisal of U-series data from arc volcanoes to constrain slab dehydration and melting processes using a global subduction zone data set. There is no clear evidence for significant mobilization of Th or Pa in dehydration fluids while the source region of arc rocks is relatively oxidized and mobility of U is strongly enhanced. It is argued that along-arc U/Th and U/Pa isotope data reflect time-integrated addition of U from the slab to the mantle wedge. The presence of large Ra-Th disequilibrium correlated with Ba/Th ratios provides evidence for some very recent fluid addition and fast magma ascent. This is consistent with radiogenic Os isotope signatures in arc lavas that can only be preserved if there is no melt-peridotite equilibration during melt transport. The 231Pa excesses found in most arc lavas provide clear evidence for a melting signature and require that the timescale of melt production cannot be as short as the timescale of melt migration (<100 annum). We propose a dynamic model in which melting is initiated by fluid fluxing and where melt is rapidly extracted out of the mantle via high-velocity channels while 231Pa excess is produced at melting rates consistent with literature estimates of melt productivity. An alternative model with continuous flux melting would require a long melting timescale (1--5 Ma), a large melting region and the mobility of Th and Pa in fluids. A model with a time lag between melting and initial dehydration is more consistent with a thermal structure where the wet solidus is not reached before an intermediate depth.