Lithium isotope evidence for light element decoupling in the Panama subarcmantle

The systematics of fluid-mobile trace elements in are lavas from Panama, re lative to their Li isotopic compositions, provide unique evidence for the f ertilization and subsequent differential extraction of mobile species from the subarc mantle. Calc-alkaline lavas that crystallized between 20 and 5 M a (Old Group) that possess delta(7)Li as high as +11.2 have low B/Be. Other wise identical (and similarly old) calc-alkaline lavas with high B/Be (to 2 3), have mid-ocean ridge basalt (MORB) like delta(7)Li (+4.7 to +5.6). Adak ite lavas (<3 Ma; Young Group) possess delta(7)Li from +1.4 to +4.2 and hav e 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 co ncentrations and elevated delta(7)Li signatures into are sources, and sampl es with the highest delta(7)Li would also have the highest B/Be. Our data s uggest that although bath Li and B are initially derived from the slab, old er delta(7)Li signatures may be preserved in the mantle beneath arts. As a result, regions of the lithospheric mantle will develop Li isotope signatur es that are heavier than typical MORB mantle.