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Plutonic xenoliths from St Vincent, Lesser Antilles, act as ‘windows into the deep’

Coarse-grained igneous rocks, sourced directly from a sub-volcanic magma chamber, provide glimpses into the prevailing conditions beneath an active volcano.

Tollan and co-workers used several different techniques to analyse the major and isotopic composition of mineral phases in cumulates sourced from the active volcanic island of St Vincent in the Lesser Antilles. Cumulates are igneous rocks comprise the first fractionating minerals that form from a crystallising melt. The combinations of minerals and their composition are modulated by the conditions imposed upon the magma when it is cooling, and thus each rock represents a unique ‘snapshots’ of magmatic evolution.

The study revealed that the rounded cumulate nodules are distinctively rich in anorthitic plagioclase and pargasitic hornblende, accompanied by fresh olivine and pyroxenes. The composition of the minerals indicates the cumulates formed at ~970 – 1150°C at 5-6 km below the Earth’s surface.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

The paper concludes that all the cumulates collected from St Vincent formed from relatively evolved melts rich in calcium, aluminium and water. These types of magmas are produced from early crystallisation of mafic phases (olivine, clinopyroxene and Cr-rich spinel) before their low density facilitates ascent through the crust, where they stall and deposit the observed cumulus minerals in shallow magma chambers. Evidence from oxygen isotopes suggests the cumulates have a residence time of ~50,000 years before being entrained and fragmented by newly injected magmas, and transported to the surface during explosive volcanic eruptions.

Full reference: Tollan, PME, Bindeman, I & Blundy, JD (2012) ‘Oxygen and hydrogen isotope compositions of plutonic xenoliths from St. Vincent, Lesser Antilles Island Arc.’ Contributions to Minerology and Petrology, no. 163, pp. 189-208.