Significance: The increasingly popular notion that steady-state magma chambers are highly crystallized, and thus only capable of erupting during brief (100 ka) timescales. Together with data from other volcanic complexes, we show that arc magmas may generally be stored warm (are able to erupt for >100 ka). Thus geophysical detection of melt beneath volcanoes represents the normal state of magma storage and holds little potential as an indicator of volcanic hazard.

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 δ⁷Li, δ¹¹B, δ¹⁸O and δ³⁴S 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 ∼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 δ¹¹B up to + 15‰, δ³⁴S of ∼2‰ and δ¹⁸O 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 δ¹¹B and δ³⁴S (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 inclusions—whose water content varies from 2·2 to 3·6 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°C).