Paul B. Tomascak (914-365-8655; Lamont-Doherty Earth Observatory, Rt.9W, Palisades, NY 10964; tomascak@ldeo.columbia.edu, and DTM, Carnegie Institution, Washington, DC 20742

Jeffrey G Ryan (813-974-1598; ryan@chuma.cas.usf.edu), and Marc J Defant (813-974-1593; defant@chuma.cas.usf.edu) Department of Geology, Univ. of South Florida, Tampa, FL 33620

We have examined the Li isotope compositions of a suite of "adakite" lavas from Panama in order to assess further the viability of the slab melting concept. Our measurements were made with the VG Plasma 54-30 multiple collector sector ICP-MS at the Carnegie Institution of Washington. Long term reproducibility is estimated at ±1.1‰ (2s pop.).

The lavas range from andesites to dacites and rhyolites. They comprise an "old" group (>5 Ma), with calc-alkaline major element trends and ordinary arc lava trace element distributions, and a "young" group (<1.5 Ma), which bear the characteristics of adakites: high La/Yb, Sr/Y, and Sr contents over a range in MgO (Defant and Drummond, 1990; Nature). Old group samples have d7Li that range from +3.9 to +11.2‰ (d7Li: 7Li/6Li relative to the L-SVEC standard). The lower end of this range overlaps the accepted range for MORB. The Young group possess restricted, low d7Li (+1.7 to +4.2‰). The high d7Li (+8.2 to +11.2‰) of older samples from La Yeguada (northern Panama) is approached by lavas from Nicaragua and Costa Rica (+7.1 to +7.9‰; Chan et al. 1999; Chem.Geol.) that show trace element evidence of slab fluid input. Slab fluids are expected to be 7Li-rich. That the La Yeguada samples do not have the high B/Nb and B/Be typical of lavas with strong slab signatures may result from subduction of warmer crust, depleted in B and other highly fluid-mobile elements prior to magmatic arc development. Lithium is less mobile than B under low to moderate P-T conditions, so 7Li enrichment in the old group samples may still be interpreted to reflect slab fluid input.

The (adakitic) young group show very restricted B/Be (range: 5-8), whereas old group samples show significant variability (6-24). The clustering of adakite data from three different volcanic centers is consistent with a relatively homogeneous source, dominated by broadly MORB-like d7Li. Higher d7Li of the majority of the old group lavas are consistent with a model in which 7Li-rich fluids are released into the mantle from the upper part of the slab.