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>

Diffusive over-hydration of olivine-hosted melt inclusions

Olivine-hosted melt inclusions are ofen used to estimate the pre-eruptive H2O content of magmas (Métrich & Wallace, 2008). However, it has been noted for a number of years that H2O appears to ‘leak’ out of melt inclusions during ascent and eruption (Massare et al., 2002; Chen et al., 2013). Rare cases of H2O gain have also been noted (Kolezsar et al., 2009). Recent experiments and modelling has clarified the mechanisms of H2O loss– by diffusive re-equilibration through the host crystal – and has opened up the possibily of extracting timescales from the extent of H2O exchange (Gaetani et al., 2012; Bucholz et al., 2013).

In this study, led by Margaret Hartley at the University of Manchester, we showed that different populations of melt inclusions from the Laki and Skuggafjöll eruptions in the Eastern Volcanic Zone of Iceland experienced diffusive loss or diffusive gain of H2O. Some rapidly quenched melt inclusions from the Laki tephra and subglacially-quenched pillow glasses from Skuggafjöll had coherent H2O/Ce values of ~180 that we interpreted as the primary mantle value (e.g., Michael, 1995). However, many inclusions from the Laki lava flow had very low H2O/Ce values consistent with H2O loss during transport in an extensive lava tube network at the surface. Conversely, most inclusions from Skuggafjöll, as well as most low-Ce, primitive inclusions from Laki, had elevated H2O/Ce values of up to ~1000 that are indicative of H2O gain during storage in the crust.

A figure from Hartley et al. (2015) summarising the processes by which diffusive loss and gain of H2O has been observed in the Laki and Skuggafjöll eruptions.

Using the diffusive re-equilibration model of Bucholz et al. (2013), we placed minimum constraints on the residence times of dehydrated inclusions in the Laki lava flow and over-hydrated inclusions in evolved melts immediately prior to the eruptions. The timescales were on the order of days to tens of days in both cases. Finally, we demonstrated that diffusive gain, as well as diffusive loss, can be observed in a number of global datasets where primitive, H2O-poor inclusions are mixed into more enriched and/or evolved melts before eruption. Thus, rather than viewing the open system nature of olivine-hosted melt inclusions as weakness, it can be exploited to gain further insights into pre-eruptive magma processes.

Publication

Hartley, M.E., Neave, D.A., Maclennan, J., Edmonds, M. & Thordarson, T. 2015. Diffusive over-hydration of olivine-hosted melt inclusions. Earth and Planetary Science Letters 425, 168–178. <Open Access>

Crystal-melt relationships and the record of deep mixing and crystallisation in the AD 1783 Laki eruption, Iceland

Basaltic magmas are often assembled from a diversity of mantle melts that mix and crystallise en route to the Earth’s surface (Sobolev & Shimizu, 1993; Maclennan, 2008). Thus, before any attempt can be made at determining the depths of any pre-eruptive processes, it is essential to understand how melts and and crystals relate to each other.

In this paper, we investigated how the magma that fed the large and environmentally impacting AD 1783–84 Laki eruption was assembled. Olivine-hosted melt inclusion compositions revealed that concurrent mixing and crystallisation of variable mantle melts occurred deep within Laki plumbing system. Indeed, the presence of high-anorthite plagioclase compositions more primitive than any other crystal or melt inclusion composition measured confirmed that the difference components of the Laki lava cannot all be related to the carrier liquid by single liquid line of descent. Furthermore, crystal zonation patterns indicated that multiple crystal mush formation and disaggregation events took place prior to eventual eruption. Combining clinopyroxene-melt barometry with information from crystal textures indicates that most crystallisation took place within the mid-crust, the depth of much recent seismogenic magmatism in the Eastern Volcanic Zone of Iceland (Tarasewicz et al. 2012).

 

Syhthesis of deep magmatic processes in the Laki plumbing system. Modified from Neave et al. (2013)
Deep magmatic processes in the Laki plumbing system. Modified from Neave et al. (2013).

Publication

Neave, D.A., Passmore, E., Maclennan, J., Fitton, J.G. & Thordarson, T. 2013. Crystal-Melt Relationships and the Record of Deep Mixing and Crystallization in the AD 1783 Laki Eruption, Iceland. Journal of  Petrology 54, 1661–1690. <Open Access>