Slab-derived Fluids in Magma Sources of StVincent (Lesser Antilles Arc): Volatile and Light Element Imprints

It is generally accepted that the parental magmas of the Lesser Antilles arc were generated by partial melting of a mid-ocean ridge basalt (MORB)-type mantle source modified by slab-derived components.To determine the nature of these components, the H2O, S, Cl, F, Li and B contents and 7Li, 11B, 18O and 34S composi-tions were systematically determined in olivine-hosted melt inclusions from St.Vincent (southern part of the arc). Both the geochemical and isotopic data define a broad compositional spectrum. On the whole, the melt inclusions have basaltic to CaO-rich (4150 wt %), SiO2-poor (5457 wt %) compositions. Most of the entrapped melts result from 10^20 % batch partial melting of a MORB-type mantle source modified initially by dehydration fluids with low solute contents and a seawater-like chemical signature. As a result, the melt inclusions are enriched in B, Cl and H2O compared with MORB and have 11B up toþ15ø, 34S of 2ø and 18O down toþ 3ø. In contrast, some others record initial magmatic heterogeneities that require input of fluids derived from (1) the dehy-dration of altered oceanic crust in agreement with the selective B enrichment (up to 53 ppm) in the melt and negative lithium isotopic compositions, and (2) the dehydration of sediments resulting in distinctive 11B and 34S (down to ^20ø and ^8ø, respectively) and high Li contents in the melts. The CaO-rich melt inclusions cannot be distinguished from the others on the basis of their isotopic signatures.They possibly reflect magma interactions with CaO-rich, amphibole-bearing lithologies. Combination of our results with lit-erature experimental data leads to the conclusion that St. Vincent basaltic melt inclusionsçwhose water content varies from 22 to 36 wt %çrepresent magmas derived from a rather limited portion of the mantle wedge, by partial melting at between 13 and 145 kbar and a restricted temperature range (1220^11908C)