Crystallisation conditions of low-pressure cumulate nodules from the Lesser Antilles island arc

Cumulate nodules with the low-pressure mineralogy of olivine-orthopyroxene-Ca-rich clinopyroxene-anorthite-amphibole-magnetite provide direct evidence for high-level fractionation of magmas in the Lesser Antilles volcanic island arc. The nodules only occur in volcanic centres comprising the sequence basalt-basaltic andesite-andesite-dacite, suggesting that this sequence is the product of fractional crystallisation of basaltic magma. Thermodynamic analysis of mineral equilibria in the nodules provides important information on the conditions under which fractionation occurred. Assuming a temperature of crystallisation in the range 1100–1200°C, the pressure of crystallisation of the nodules decreases from north to south along the arc, from approximately 9–10 kbars in Statia to 3 kbars in Grenada. This variation in the pressure of crystallisation should be directly related to variation in the depth to crustal magma chambers along the arc. Oxygen activity is uniformly high (>NNO) for all the available analysed nodules and shows no variation along the arc. P_H2O is generally less than 1000 bars except for Grenada where it is much higher (1000–3000 bars).