The systematics of fluid-mobile trace elements in arc lavas from Panama, relative to their Li isotopic compositions, provide unique evidence for the fertilization and subsequent differential extraction of mobile species from the subarc mantle. Calc-alkaline lavas that crystallized between 20 and 5 Ma (Old Group) that possess δ7Li as high as +11.2 have low B/Be. Otherwise identical (and similarly old) calc-alkaline lavas with high B/Be (to 23), have mid-ocean ridge basalt (MORB) like δ7Li (+4.7 to +5.6). Adakite lavas (<3 Ma; Young Group) possess δ7Li from +1.4 to +4.2 and have consistently lower B/Be than Old Group lavas, consistent with derivation from melting of a devolatilized MORB slab. If Li and B had comparable fluid mobility in the subarc mantle, then slab fluids would carry both high B concentrations and elevated δ7Li signatures into arc sources, and samples with the highest δ7Li would also have the highest B/Be. Our data suggest that although both Li and B are initially derived from the slab, older δ7Li signatures may be preserved in the mantle beneath arcs. As a result, regions of the lithospheric mantle will develop Li isotope signatures that are heavier than typical MORB mantle.