Generation, storage, and eruption of an intermediate composition magma on Ascension Island.

Ascension Island is a Holocene, intraplate volcanic island in the South Atlantic Ocean. The island exhibits a wide compositional range of basalt-trachybasalt-basaltic trachyandesite-trachyandesite-trachyte-rhyolite(1) magma, but intermediate products make up <5% of the surface deposits. Here, we focus on the 1.3 km long, trachyandesite Devil’s Ink Pot fissure (DIP), located in the south east corner of the island, and one of the youngest and best-preserved intermediate composition eruption on the island. We use a combination of petrological and geochemical data collected from juvenile components and present whole rock major and trace element data, crystal textures, and compositions, melt inclusion data and associated modelling of intensive variables to understand the genesis and triggers of intermediate melts, and relate this to the processes of magmatic evolution at low-flux ocean island volcanoes. Whole rock and trace element analyses show that the erupted magma is chemically uniform. However, petrological, and geochemical analysis of plagioclase and olivine crystals identified textural and chemical variations. MELT’s modelling from the least evolved Ascension Island deposits is not able to reproduce the same intermediate composition of the Devil’s Ink Pot fissure. Previous island wide studies have shown that intermediate melts formed by fractional crystallisation, but MELTs modelling and eruption specific studies suggest that additional processes are involved in the generation of intermediate melts. (1) Weaver, B., Kar, A., Davidson, J. and Colucci, M., 1996. Geochemical characteristics of volcanic rocks from Ascension Island, south Atlantic Ocean. Geothermics, 25(4-5), pp.449-470.