Slab-derived fluids in the magma sources of St. Vincent (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 delta Li-7, delta B-11, delta O-18 and delta S-34 compositions 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 (>15.0 wt %), SiO2-poor (<45.7 wt %) compositions. Most of the entrapped melts result from similar to 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 delta B-11 up to + 15 parts per thousand, delta S-34 of similar to 2 parts per thousand and delta O-18 down to + 3 parts per thousand. In contrast, some others record initial magmatic heterogeneities that require input of fluids derived from (1) the dehydration 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 delta B-11 and delta S-34 (down to -20 parts per thousand and -8 parts per thousand, 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 literature experimental data leads to the conclusion that St. Vincent basaltic melt inclusions-whose water content varies from 2.2 to 36 wt %-represent magmas derived from a rather limited portion of the mantle wedge, by partial melting at between 13 and 14.5 kbar and a restricted temperature range (1220-1190 degrees C).