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Magmagenesis at Soufriere volcano St Vincent, Lesser Antilles Arc

E. Heath·R. Macdonald·H. Belkin·C. Hawkesworth·Haraldur Sigurdsson
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Abstract

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 arc. Parental, possibly primary, magmas at Soufriere had MgO ∼12.5 wt % and were probably nepheline-normative. They last equilibrated with mantle at ∼17 kbar pressure, at temperatures of around 1130°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°C), separating initially olivine + Cr-spinel + clinopyroxene + plagioclase ± titanomagnetite and then clinopyroxene + plagioclase + titanomagnetite + orthopyroxene assemblages. The total amount of crystallization was some 76 wt %. 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 H 2 O >3 wt %, 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 ridge basalt type mantle sources which had been modified by addition of fluids released from the slab containing contributions from subducted sediments and mafic crust.

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E. Heath, R. Macdonald, H. Belkin, C. Hawkesworth, Haraldur Sigurdsson. 1998. Magmagenesis at Soufriere volcano St Vincent, Lesser Antilles Arc. https://doi.org/10.1093/petroj%2F39.10.1721

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