Magmagenesis at Soufriere Volcano, St Vincent, Lesser Antilles arc

Soufriere volcano of St Vincent (<0.6 Ma) is composed of basalts and basaltic andesites, the most mafic of which (mg-number 75) may be representative of the parental magmas of the calc-alkaline suites of the Lesser Antilles are. Parental, possibly primary magmas at Soufriere had MgO similar to 12.5 wt % and were probably nepheline-normative. They last equilibrated with mantle at similar to 17 kbar pressure, at temperatures of around 1130 degrees C and f(O-2) exceeding FMQ (fayalite-magnetite-quartz) +1. They fractionated, along several liquid lines of descent, through to basaltic andesites and rarer andesites over a range of crustal pressures (5-10 kbar) and temperatures (1000-1100 degrees C), separating initially olivine + Cr-spinel + clinopyroxene + plagioclase + titanomagnetite and then clinopyroxene + plagioclase + titanomagnetite + orthopyroxene assemblages. The total amount of crystallization mas some 76 Let %. Amphibole was apparently not a fractionating phase. Sr and Nd isotopic and trace element systematics show no evidence for significant crustal assimilation. There is conflicting evidence as to the pre-eruptive water contents of Soufriere magmas; compositions of clinopyroxene phenocrysts and melt inclusions suggest H2O >3 mt %, whereas various projections onto phase diagrams are more consistent with relatively anhydrous magmas. Primary magmas at Soufriere were generated by around 15% melting of mid-ocean mid-ocean ridge basalt type mantle sources which had been modified by addition of fluids released from the slab containing contribution from subducted sediments and mafic crust.