Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc.

Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufrière, St Vincent, and the Ilet à Ramiers basalt Martinique. High-pressure experiments performed on a Soufrière basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufrière melt under relatively dry conditions (C. 2 wt% H2O in melt). using estimates of the H2O content of mantle sources fluxed by an hydrous slab-derived component, H2O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles Arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H2O concentrations (6-8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050-1100°C. Their crystallization at 950-1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufrière Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts