Exploring eruption dynamics in 3D; textural and geochemical analysis of archetypal Ocean Island eruptive deposits, Ascension Island, South Atlantic

Volatile escape, and the impact of volatiles on melt rheology is a key factor determining eruptive explosivity. The microscopic scale heterogeneity within a magma body that controls volatile escape therefore ultimately influences volcanic eruptive style - and hence has potentially global impacts. Volcanism on Ascension Island is expressed through a broad range of compositions, water content and eruptive behaviour, from flows and domes to Plinian pumice falls. This makes Ascension the perfect natural laboratory in which to investigate volatile escape and eruption dynamics.

Pumice clasts represent the melt at the moment of fragmentation, providing information about volatile escape. We apply 3D and 2D textural and chemical analytical techniques in an integrated study to determine the dynamics and controls on eruptive behaviour of evolved alkali-rich melts from Ascension Island for the first time. Representative pyroclasts from three end members of magmatism and eruptive behaviour have been sub-sampled stratigraphically. These deposits come from explosive eruptions associated with: magma mingling, steady compositional change and an effusive to explosive transition. We combine; X-ray computed tomography (XCT), scanning electron microscope images and electron microprobe analyses to determine how ascent dynamics and eruptive style change during the course of these eruptive phases.

We show bubble size distributions and bubble-crystal-melt geometries obtained from XCT and interrogate how nucleation and coalescence are effecting bubble growth, the resulting connectivity within and between regions of differing composition, the crystallinity and vesicularity. By comparing this 3D and 2D textural analysis with the inferred rheological conditions, and our detailed reconstruction of eruptive behaviour from our stratigraphic analysis. We are able to determine the dynamics and controls on eruptive behaviour of evolved alkali-rich melts on Ascension Island.

The results of this Zeiss/Geological Society of London funded project will be presented in the context of intra-plate Ocean Island eruptive behaviour of the Atlantic with implications for our broader understanding of eruption dynamics at alkaline volcanoes worldwide.