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>

AGU: Disentangling disequilibrium and tuning thermobarometers in Eastern Volcanic Zone of Iceland

At the end of 2015, I presented the following abstract at the AGU Fall Meeting in San Francisco. My contribution summarised the main findings of my work in Iceland so far and outlined my next research directions: calibrating new thermobarometric models optimised for mid-crustal pressures and performing new phase equilibria experiments on basalts in the 1–7 kbar pressure range. You can download a copy of my poster here.

The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)

The environmentally impacting AD 1783–84 Laki eruption was the largest Icelandic eruption to have been directly obseved by humans (Thordarson et al., 1996). However, it is by no means unique in Iceland’s volcanic history: Thordarson & Höskuldsson (2008) note that over 50 eruptions >1 km3 in volume have taken place in Iceland since the end of the last glaciation. The 10 ka Grímsvötn tephra series, or Saksunarvatn Ash, which is distributed across the North Atlantic from Greenland to Germany, is thought to have been generated in a series of large, phreatomagmatic eruptions within the Grímsvötn volcanic zone at the end of the last glacial period (Grönvold et al., 1995; Thordarson, 2014). In this first petrological study of the tephra, we (a team from the universities of Cambridge, Manchester and Iceland) exploited the abundance of primitive crystals and melt inclusions in samples from Lake Hvítárvatn in central Iceland in order to investigate magma evolution and storage processes.

http://static-content.springer.com/image/art%3A10.1007%2Fs00410-015-1170-3/MediaObjects/410_2015_1170_Fig2_HTML.gif
Crystal textures in the 10 ka Grímsvötn tephra series from lake Hvítárvatn in central Iceland. a) zoning in plagioclase, b) melt inclusions in plagioclase, c) sector zoning in clinopyroxene and d) inclusions in olivine. Figure from Neave et al. (2015).

Following the approaches laid out by our recent work on Laki and Skuggafjöll, we defined evolved and primtive macrocryst assemblages in tephra samples, the latter of which was out of equilibrium with the matrix glass and probably derived from disaggregated crystal mushes (e.g., Halldorsson et al., 2008). High-anorthite plagioclase-hosted melt inclusions provided the first direct evidence for the supply of high-Mg#, incompatible trace element-depleted mantle melts to the base of the lithosphere in Iceland’s Eastern Volcanic Zone. Through the critical application of clinopyroxene-melt and melt barometers (Putirka, 2008; Yang et al., 1996) , we suggested that the primtive macrocryst assemblage formed within the mid-crust (4±1.5 kbar) and that the evolved assemblage formed in the shallow crust (<2 kbar) shortly before eruption. We showed, however, that clinopyroxene-melt equilibria are not well calibrated at conditions relevant for the tephra’s pre-eruptive storage. We therefore made the case for further exploration of basalt phase equilibria in the critical 1–7 kbar interval, which is a primary aim of my Humboldt Research Fellowship in Hannover.

Publication

Neave, D.A., Maclennan, J., Thordarson, T. & Hartley, M.E. 2015. The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash). Contributions to Mineralogy and Petrology 171, 21. <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 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>

Melt mixing causes negative correlation of trace element enrichment and CO2 content prior to an Icelandic eruption

Dissolved volatile elements play important roles in driving volcanic eruptions and controlling the physical properties of magmas. Degassing of magmatic volatiles also links deep geochemcial reservoirs with the Earth’s surface, closing global element cycles (e.g., Marty & Tolstikhin, 1998). However, determing the original CO2 content of mantle melts is difficult because most melts reach volatile saturation long before eruption. Measuring melt inclusions isolated hosted in primitive crystals that remained isolated from their carrier melt provides one way of investigating the CO2 content of basaltic magmas (e.g., Moore, 2008).

In this paper, we presented major, trace and volatile element analyses from >100 primitive olivine-hosted melt inclusions from a sub-glacial eruption in the Eastern Volcanic Zone of Iceland – the Skuggafjöll eruption. While our melt inclusion compositions preserved a record of primitive melt heterogeneity similar to that observed in other Icelandic systems including Laki (Neave et al., 2013), the most striking feature of our dataset was an enigmatic negative correlation between CO2 and incompatible trace element enrichment:

 

Negative correlation between melt inclusion CO2 and Ce/Y contents. The solid black line shows a mixing line between a depleted end-member shown with a black diamond and an enriched end-member shown with a white diamond that explains much of the correlated varibility in tha sample suite. Many inclusions have experineced further exsolution as illustrated by the discrepancy between predicted and measured CO2 contents. Modifued from Neave et al. (2014).
Negative correlation between melt inclusion CO2 and Ce/Y contents. The solid black line shows a mixing line between a depleted end-member shown with a black diamond and an enriched end-member shown with a white diamond that explains much of the correlated varibility in tha sample suite. Many inclusions have experineced further exsolution as illustrated by the discrepancy between predicted and measured CO2 contents. Modified from Neave et al. (2014).

We suggested that a negative correlation between CO2 and incompatibe trace element enrichment may result from the concurrent mixing, crystallisation and exsolution of CO2 from melts that have experienced varying degrees of previous CO2 loss: mixing may have been triggered by the injection of a depleted and possibily CO2-supersaturated melt (CO2/Nb > 350) into a relatively shallow magma reservoir containing an enriched melt that has already lost much of its CO2.

Another inportant finding  concerned the CO2 content of shrinkage bubbles in melt inclusions. Many recent studies have demonstrated that CO2 can be sequestered into bubbles during the cooling of melt inclusions (e.g., Hartley et al., 2014; Mironov et al., 2015; Wallace et al., 2015; Moore et al., 2015). However, despite investigating a large number number of shrinkage bubbles by Raman spectroscopy and microthermometry, we found no CO2-bearing bubbles. We therefore suggested that our subglacially quenched samples cooled sufficiently quickly to for CO2 sequestration to have been kinetically inhibited, an observation that has implications for interpreting the CO2 content of inclusions from other settings that experince rapidly quenched, such as those from mid-ocean ridges.

Comparison of the CO2 content of melt inclusions with and without inclusion-hosted bubbles: bubbles have no systematic effect on inclusion CO2 content.
Comparison of the CO2 content of melt inclusions with and without inclusion-hosted bubbles: bubbles have no systematic effect on inclusion CO2 content.

Publication

Neave, D.A., Maclennan, J., Edmonds, M. & Thordarson, T. 2014. Melt mixing causes negative correlation of trace element enrichment and CO2 content prior to an Icelandic eruption. Earth and Planetary Science Letters 400, 272–283. <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>