Slab-Derived Fluids in the Magma Sources of St. Vincent (Lesser Antilles Arc): Volatile and Light Element Imprints

Abstract

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 H 2 O, S, Cl, F, Li and B contents and 7 Li, 11 B, 18 O and 34 S 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 (4 150 wt %), SiO 2 -poor (5457 wt %) compositions. Most of the entrapped melts result from $10^20% batch partial melting of a MORBtype 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 H 2 O compared with MORB and have 11 B up to 15, 34 S of $2 and 18 O down to 3. 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 11 B and 34 S (down to ^20 and ^8, 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 inclusionswhose water content varies from 22 to 36 wt %represent 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).