Fluid Inputs to Magma Sources of St. Vincent and Grenada (Lesser Antilles): New Insights from Trace Elements in Olivine-hosted Melt Inclusions

Abstract

St.Vincent and Grenada, known for the presence of high Mg-basalts, are two of the southernmost islands of the Lesser Antilles arc, located at the boundary between the Atlantic and Caribbean Plates. We report here secondary ionization mass spectrometry measurements of trace elements (V, Sr, Y, Zr, Nb, Ba, La to Eu) in basaltic melt inclusions trapped in olivine (Fo90^86) within high-MgO basalts. Combined with previous data on light elements and stable isotopes obtained on the same sample set, these results allow us to decipher the nature of the slab-derived fluids that modified the mantle wedge beneath these two islands. The melt inclusions exhibit trace element patterns that encompass those of the whole-rocks, but with a larger range of variability at Grenada. On the whole, such patterns are trace element enriched but not very fractionated compared with mid-ocean ridge basalt and display typical Ba and Sr enrichment. The variations of mobile/immobile element ratios (e.g. K/La, Sr/ Y) are poorly reconciled with crustal contamination processes but rather track the involvement of early aqueous fluids, which are also responsible for significant increases in Cl/F, B and Cl recorded by the melt inclusions. The variations of immobile element ratios such as La/Sm and Nb/Y cannot be accounted for by variable extents of melting alone, but also imply the influence of a hydrous sediment-derived melt for St. Vincent, whereas the by far larger enrichments of La and Nb in the Grenada samples require contributions of solute-rich fluids extracted at high temperature and pressure from both subducted oceanic crust and sediments. Moreover, a few Grenada melt inclusions display unusual Zr enrichment (up to 1274 ppm), a rare feature in basaltic melts that possibly involves the localized dissolution of zircon. The comparison of trace element distribution patterns in the melt inclusions (MI) and whole-rocks indicates the primary composition of the MI and the interaction of the melt with mantle paragenesis after their entrapment in olivine.