A field and petrological study of an intermediate composition fissure eruption on Ascension Island

Abstract

Ascension Island is a Holocene, intraplate volcanic island in the South Atlantic Ocean and exhibits a wide compositional range of magma from basalt to rhyolite, but intermediate products make up <5% of the surface deposits. I 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. I present detailed field work, petrographic, geochemical, and textural data to reconstruct the evolution of the DIP trachyandesite eruption, understand the genesis of intermediate magmas and shallow-surface processes occurring at low-flux ocean island volcanoes, and provide insights to future eruption scenarios on Ascension Island. The fissure is composed of 18 cones, 3 lava flow fields and tephra fall deposits up to 2 m thick. Two contrasting eruption styles are evidenced in cone deposits along the fissure. Despite the morphological and lithological differences, whole rock major 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 DIP fissure, therefore demonstrates that additional processes must be involved in the generation of intermediate melts. Analysis of feldspar microlite textures and anorthite contents indicate variations in the time spent in the upper conduit for the tephra and lava/spatter samples and the conduit dynamics, which can heavily influence the style of eruption. Small volume eruptions may be missing from the geological record, leading to a potential underestimation of their frequency. However, even small-volume eruptions are significant on small islands with limited options for self-evacuation of local inhabitants, therefore planning for eruptions with shifts in eruption styles should be incorporated into eruption scenarios on Ascension Island.