Contrasting eruption styles from the intermediate composition, Devil’s Ink Pot fissure eruption, Ascension Island

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. The 1.3 km long Devil’s Ink Pot fissure (DIP), in the south-east of the island represents one of the youngest and best-preserved intermediate-composition eruptions on the island. We present detailed field work, petrographic and geochemical data to reconstruct the evolution of the DIP trachyandesite eruption and provide insights to future eruption scenarios on Ascension. The fissure is composed of 18 craters, 3 lava flow fields and tephra fall deposits up to 2 m thick. Two contrasting eruption styles are evidenced in crater deposits along the fissure. Some craters are characterised by moderately- to densely-welded spatter and fed lava flows. Other craters are characterised by weakly agglutinated spatter, loose lapilli and bomb clasts and an abundance of lithic and ballistics clasts (>1 m in diameter). Tephra fall deposits are composed of scoria, pumice, dense clasts, and lithic clast components. Despite the morphological and lithological differences along the fissure, whole rock major and trace element analyses show that the erupted magma is chemically uniform. Initial 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. This suggests that the magma feeding the lava/spatter-forming eruption style was characterised by a greater overall residence time in the shallowest portions of the upper conduit zone, consistent with recent conceptual models of fissure eruption localisation, and so-called hybrid explosive-effusive eruption styles. 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.