Implications Of Light And Trace Elements Signatures in Melt Inclusions Of St Vincent And Grenada Island On The Lesser Antilles Arc Behavior

St. Vincent and Grenada islands are located in the south part of the Lesser Antilles arc, generated by the subduction of the Atlantic plate beneath the Caribbean plate. In the both islands, the erupted high-MgO basalts (MgO > 10.0 wt%) are thought to be representative of the primary magmas and to be generated by the melting of a MORB mantle source enriched by fluids derived from the subducted slab [1]. We present here trace element compositions determined by ion probe in melt inclusions (M.I.) trapped in olivines (Fo84-91) from magnesian scoriae, for which light and volatile elements, as stable isotopes were previously measured [2-3]. Their major elements compositions point out a broad variability, but show the primitive character of these M.I. (SiO2 < 50.0 wt%). Grenada M.I. are enriched in K2O (0.4-2.5 wt%) and MgO (up to 12.5 wt%) compared to St. Vincent M.I. (up to 0.85 wt% and 10.0 wt%, respectively). Their trace element patterns encompass those of whole rocks. Compared to St. Vincent, Grenada M.I. recorded more variable trace element compositions. All M.I. patterns are characteristic of subduction zones, with Ba and Sr enrichments associated with pronounced negative Nb anomalies, implying slab-fluids influence, as also demonstrated by high Cl/F ratios (up tp 10.9) [4]. As a whole, REE patterns are enriched and poorly fractionated compared to MORB. The trace element patterns, combined with light element and isotopic compositions, are interpreted in term of variations of degree of mantle partial melting and slab influence, with variable contributions of aqueous fluids released from altered oceanic crust and from sediments, and of sediment melt. Both St. Vincent and Grenada primary magmas record the influence of aqueous fluids, whereas the addition of sediment melt is only identified in Grenada M.I., as Zr positive anomalies (Zr contents up to 1200 ppm), a rare feature in basaltic melts. Although light trace elements and stable isotopes illustrated mostly aqueous fluids from AOC and sediments dehydrations [2-3], trace elements highlight the influence of silicate melts. This comparison underlines the contribution of several different sources in the arc melt formation. [1] Macdonald et al., 2000 [2] Bouvier et al., 2008 [3] Bouvier et al., submitted [4] Straub and Layne, 2003