How to fragment peralkaline rhyolites: Observations on pumice using combined multi-scale 2D and 3D imaging

Thanks to their alkali-rich compositions, pantellerites have much lower viscosities than other rhyolites. As a consequence, these peralkaline magmas erupt in myriad ways to create volcanic landforms that range from lava domes to ignimbrites and fountain-fed lava shields. However, the mechanisms by which such low viscosity melts are able to fragment in explosive eruptions are poorly understood despite the hazards presented by pantellerite volcanoes, above all in the East African Rift.

Building on her MSci project, Ery Hughes investigated this problem of magma fragmentation under the supervision of Marie Edmonds, Kate Dobson and myself by combining 2D (electron microscopy) and 3D (X-ray microtomography) measurements of pumice samples during my MSci.

Slices through an X-ray tomography volume of a pumice from Pantelleria (Hughes et al. 2017).

We found that pantelleritic pumices from Pantelleria are texturally indistinguishable from calc-alkaline pumices from a range of rhyolitic systems, implying that our peralkaline pumices fragmented in a brittle fashion and that their unusual chemistry had little effect on their syn-eruptive textural evolution. We therefore propose that the observed pumice textures developed in response to high decompression rates and that peralkaline rhyolite magmas can fragment when strain localisation and high bubble overpressures develop during rapid ascent.

Publication

Hughes, E.C., Neave, D.A., Dobson, K.J., Withers, P.J. & Edmonds, A. 2017. How to fragment peralkaline rhyolites: Observations on pumice using combined multi-scale 2D and 3D imaging. Journal of Volcanology and Geothermal Research 336, 179–191. <Open Access>

Melting, differentiation and degassing at the Pantelleria volcano, Italy

Pantellerites are Fe- and volatile-rich, peralkaline rhyolites that erupt primarily in continental rift settings. As no eruptions of pantelleritic magma have been observed, interpreting the diversity of volcanic phenomena at pantelleritic volcanoes is challenging. Explosive eruptions range in scale from large ignimbrite-forming events like the ~45 ka Green Tuff eruption on Pantelleria to small cone-forming events. Effusive eruptions form structures as diverse as low-aspect-ratio lava domes and high-aspect-ratio lava shields. Although fewer in number than their calcalkaline counterparts, peralkaline rhyolite volcanoes nevertheless present a range of hazards.

The evolution of peralkaline magmas has been the subject of much recent debate, with some authors advocating for pantellerite genesis by melting alkali gabbros (e.g., Avanzinelli et al., 2004), and others favouring extensive fractional crystallisation (e.g., White et al., 2009). The volatile content of pantellerite melts had also been the subject of considerable uncertainty until a recent studied have confirmed the water-rich nature of pantelleritic melts.

In this paper, we presented major element, trace element and volatile compositions from glasses, crystals and melt inclusions from a number of post-Green Tuff eruptions from Pantelleria. The main outcomes were:

  1. A quantification of the degree of aluminous lherzolite melting required to generate the alkali basalts present around northwest coast of Pantelleria (~2%).
  2. A confirmation that pantellerites can be generated by extensive fractional crystallisation (~95%) of alkali basalts.
  3. High precision analyses of glass and melt inclusion volatile contents (H2O, CO2, Li, F, Cl, S) that confirm the H2O- and halogen-rich of pantellerite melts
  4. An evaluation that explosive peralkaline eruptions may emit much more sulphur than metauluminous eruptions of an equivalent size, up to ~100 Mt for a Green Tuff-sized eruption, becasue of the high sulphur solubilty in Fe- and alkali-rich melts.

Dammusi on Pantellieria
Dammusi on Pantellieria

Publication

Neave, D.A., Fabbro, G., Herd, R.A., Petrone, C.M. & Edmonds, M. 2012. Melting, Differentiation and Degassing at the Pantelleria Volcano, Italy. Journal of Petrology 53, 637–663.