Abstract
Peralkaline rhyolites are volatile-rich magmas that typically erupt in continental rift settings. The high alkali and halogen content of these magmas results in viscosities two to three orders of magnitude lower than in calc-alkaline rhyolites. Unless extensive microlite crystallisation occurs, the calculated strain rates required for fragmentation are unrealistically high, yet peralkaline pumices from explosive eruptions of varying scales are commonly microlite-free. Here we present a combined 2D scanning electron microscopy and 3D X-ray microtomography study of peralkaline rhyolite vesicle textures designed to investigate fragmentation processes. Microlite-free peralkaline pumice textures from Pantelleria, Italy, strongly resemble those from calc-alkaline rhyolites on both macro and micro scales. These textures imply that the pumices fragmented in a brittle fashion and that their peralkaline chemistry had little direct effect on textural evolution during bubble nucleation and growth. We suggest that the observed pumice textures evolved in response to high decompression rates and that peralkaline rhyolite magmas can fragment when strain localisation and high bubble overpressures develop during rapid ascent.
Highlights
Peralkaline rhyolites, less common than their calc-alkaline counterparts, are found in many settings including continental rifts, ocean islands and back-arc basins
Peralkaline rhyolite viscosities are so low that the fragmentation threshold for brittle failure (108 to 109 Pa·s; Papale, 1999) should never be reached during magma ascent and degassing unless significant microlite crystallisation takes place (Di Genova et al, 2013), though numerical modelling has suggested that initial temperature may exert a strong control on the depth of brittle fragmentation and whether it can occur at all (Campagnola et al, 2016)
Our largest vesicle population (L N 5 × 10−1 mm) returns to a power law exponent typical of continuous nucleation and free growth (d = 2.06), which we suggest could be related to dynamic processes such as tearing and deformation during fragmentation, but has not been noted in previous studies
Summary
Peralkaline rhyolites, less common than their calc-alkaline counterparts, are found in many settings including continental rifts, ocean islands and back-arc basins. During the Holocene, central volcanoes along the East African Rift, from Afar to Tanzania, have produced explosive ignimbrite-forming eruptions of peralkaline magma (Macdonald et al, 1987) Today, these volcanic centres threaten many hundreds of thousands of people, yet the dynamics of peralkaline eruptions are poorly understood and have never been observed directly. These volcanic centres threaten many hundreds of thousands of people, yet the dynamics of peralkaline eruptions are poorly understood and have never been observed directly Despite their high silica contents, peralkaline melts have a relatively low viscosity (equivalent to calc-alkaline andesite for similar water contents) as a result of their alkali-rich nature (molar (Na2O + K2O) / Al2O3 N 1, e.g., Dingwell et al, 1998; Di Genova et al, 2013). Peralkaline rhyolite viscosities are so low that the fragmentation threshold for brittle failure (108 to 109 Pa·s; Papale, 1999) should never be reached during magma ascent and degassing unless significant microlite crystallisation takes place (Di Genova et al, 2013), though numerical modelling has suggested that initial temperature may exert a strong control on the depth of brittle fragmentation and whether it can occur at all (Campagnola et al, 2016)
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