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Lithium isotope evidence for light element decoupling in the Panama subarc mantle

¹ Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C. 20015, USA
² Department of Geology, University of South Florida, Tampa, Florida 33620, USA

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 ⁷Li 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 ⁷Li (+4.7 to +5.6). Adakite lavas (<3 Ma; Young Group) possess ⁷Li 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 ⁷Li signatures into arc sources, and samples with the highest ⁷Li would also have the highest B/Be. Our data suggest that although both Li and B are initially derived from the slab, older ⁷Li 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.

Key Words: lithium isotopes • boron • adakites • MORB • subduction

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