Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland

Igneous rock textures encode important information about magma reservoir dynamics. Specifically, the size, shape and abundance of crystals can record multiple phases of crystallisation and magma mixing. However, characterising rock textures using traditional manual methods is extremely time consuming. However, the potential for quantifying textures with automated mineralogical methods, which have seen widespread use in the ore petrology community for some time, has yet to be evaluated.

We investigated samples from across the long-lasting Laki fissure eruption, Iceland, in order determine whether crystal mush occurred at the start of the eruption, or throughout its eight-month duration – an important consideration for understanding magma reservoir dynamics and geometry. We did this by using traditional approaches to determine phase proportions and plagioclase size distribtuions, as well as novel QEMSCAN-based approaches. Although we found significant differences between the manaul and automated datasets, largely because of the inability to easily segment glomerocrysts in the latter, being able to easily combine textural and compositional data was a powerful advantage of the automated approach.

Combined composition-size distributions of plagioclase in samples from the Laki eruption. A0.5 is the square root of crystal area. Figure from Neave et al. (2017).

By fitting high-quality, manually derived plagioclase size distributions, we estimated that mush disaggregation occurred around ten days before the eruption of each sample. These observations, which align well with findings from other stidies (Hartley et al. 2015; 2016), suggest that mush disaggregation was progressive and occurred throughout the eruption: the total volume of eruptable magma active at any given time was much less than the final erupoted volume of 15.1 km3.

Publication

Neave, D. A., Buisman, I. & Maclennan, J. 2017. Continuous mush disaggregation during the long-lasting Laki fissure eruption, Iceland. American Mineralogist 102, 2007–2021. <Open Access>

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>

Goldschmidt: Magma plumbing systems and plagioclase-hosted melt inclusions

In summer 2016, I presented two abstracts at Goldschmidt in Yokohama, Japan. In my invited contribution, I summarised how a range of petrological and geochemical observations can be combined to reconstruct magma plumbing system characteristics (slides). In my second contribution, I discussed the reliability of estimating magma volatile contents by measuring primitve plagioclase-hosted melt inclusions (slides).

Magma pluming systems in th EVZ.
Magma plumbing systems in the Eastern Volcanic Zone of Iceland.

 

Crystal storage and transfer in basaltic systems: the Skuggafjöll eruption, Iceland

Basaltic lavas rich in large, high-anorthite plagioclase crystals are commonly erupted along slow spreading ridges and at ocean islands. Such plagioclase is often too primitive to be in equilibrium with the melts in which it is carried (Cullen et al., 1989). While some authors have preferred flotation as a mechanism for accumualting large amounts of primitve plagioclase in basatlic magmas (e.g., Flower, 1980), Lange et al. (2013) proposed that entraiment of earlier-formed cumulates represents a more feasible model. Understanding such mush disaggregation in basaltic magma reservoirs is crucial for a number of reasons: (1) timescales between disaggregation and eruption are often thought to be short (e.g., Costa et al., 2010); (2) mush crystals record information about conditions of magma storage at depth; and (3) disaggregated crystals provide a link between volcanic and plutonic realms.

We thus carried out a detailed petrological and geochemical study on the highly plagioclase-phyric Skuggafjöll eruption within the Eastern Volcanic Zone of Iceland in order to investigate crystal storage and transport processes. By using a range of petrographic and geochemical tools, including novel QEMSCAN technology, we evaluated the origin of crystals on a case-by-case basis and thus distinguished crystals grown from the carrier melt from crystals entrained from mushes.

QEMSCAN image of a glassy basalt sample from Skuggafjöll. Large pale blue crystals plagioclase crystals, khaki olivine crystals and dark green clinopyroxene crystals can be observed against a glassy and vesiculated orange groundmass. The field of view is ~20 mm across.
QEMSCAN image of a glassy basalt sample from Skuggafjöll. Large pale blue crystals plagioclase crystals, khaki olivine crystals and dark green clinopyroxene crystals can be observed against a glassy and vesiculated orange groundmass. The field of view is ~20 mm across.

Variability in whole-rock, macrocryst and melt inclusion compositions suggested that the Skuggafjöll magma experienced two stages of crystallisation. Primitive crystals from an earlier stage of crystallisation were stored in crystal mushes prior to disaggregating into to an evolved and geochemcially distinct magma, which then underwent further crystallisation before eruption. The timescale between crystal entrainment and eruption, during which crystal accumulation occurred, was short – of the order of days – and is being investigated further by PhD student I am co-supervising. Striking petrological similarities between Skuggafjöll and other highly phyric eruptions in Iceland (e.g., Halldorsson et al., 2008), as well as along mid-ocean ridges, indicate that crystal accumulation by mush disaggregation is an important mechanism for generating highly phyric magmas.

Publication

Neave, D.A., Maclennan, J., Hartley, M.E., Edmonds, M. & Thordarson, T. 2014. Crystal storage and transfer in basaltic systems: the Skuggafjöll eruption, Iceland. Journal of Petrology 55, 2311–2346. <Open Access>