Petrology and geochemistry of the 2014–2015 Holuhraun eruption, central Iceland

The 2014–2015 Holuhraun eruption in Iceland was one of the most closely monitored and sampled basaltic fissure eruptions to have ever taken place. In this paper lead by Sæmundur A. Halldórsson and many other scholars from Iceland and beyond we present a comprehensive collection of glass, mineral and whole-rock data. The geochemistry of the eruption products firmly locate it within the Bárðarbunga volcanic system. By carrying out careful geothermobarometry, we infer that the magma was stored at 8 ± 5 km prior to eruption, in excellent agreement with independent petrological, geophysical and geodetic observations (e.g., Hartley et al., 2018; Gudmundsson et al., 2016). Although the erupted magma is extremely homogeneous in composition, complexity in its crystal cargo reveals that the it was ultimately assembled from heterogeneous mantle melts that underwent crystallisation and mixing in the lower- to mid-crust.

Backscattered electron (BSE) image of a complexly zoned clinopyroxene from the 2014–2015 Holuhraun lava.

Publication

Halldórsson, S.A., Bali, E., Hartley, M.E., Neave, D.A., Peate, D.W., Gudfinnson, G., Bindeman, I., Whitehouse, M., et al. Petrology and geochemistry of the 2014–2015 Holuhraun eruption, central Iceland: Compositional and mineralogical characteristics, temporal variability and magma storage. Contributions to Mineralogy and Petrology,173:64.

Melt inclusion constraints on petrogenesis of the 2014–2015 Holuhraun eruption, Iceland

The 2014–2015 Holuhraun eruption in Iceland was the largest volume eruption on the island since the 1783–1784 Laki erution (e.g., Neave et al., 2013; 2017), and was one of the most closely monitored eruptions ever to have taken place (Gudmundsson et al., 2016). In this paper, lead by Margaret Hartley and Enikö Bali (who also recently published an associated paper), we present melt inclusion data from a suite of samples collected throughout the eruption.

Variability in melt inclusion compositions indicates that the erupted magma evolved from diverse primary melts by concurrent mixing and crystallisation. Using a refined method of olivine–plagioclase–augite–melt (OPAM) barometry, we place this evolution at mid-crustal depths, in agreement with geophysical indicators of magma storage. Re-equilibration of melt inclusion H2O contents indicates that crystals spent at least 1–12 days in their carrier liquid before eruption, consistent with lateral transport in a mid-crustal dyke from the Bárðarbunga central volcano to the eruption site.

A conceptual model for the entrapment of melt inclusions erupted during the 2014–2015 Holuhraun eruption. Figure from Hartley et al. (2018).

Publication

Hartley, M.E., Bali, E., Maclennan, J., Neave, D.A. & Halldórsson, S.A. 2018. Melt inclusion constraints on petrogenesis of the 2014–2015 Holuhraun eruption, Iceland. Contributions to Mineralogy and Petrology, 173: 10. <Open Access>

Mantle-derived trace element variability in olivines and their melt inclusions

Olivine is almost ubiquitous in primitive basalts, making it an excellent tool for investigating early phases of magmatic evolution and mantle melting conditions. For example, studies of olivine-hosted melt inclusions have provided crucial insights into primitive melt variability, deep magma mixing (e.g., Neave et al., 2013) and the behaviour of volatiles during magma transport from the mantle to the surface (e.g., Neave et al., 2012; 2014). It has also been proposed that the compatible trace-element (CTE) content of olivines themselves provides information about lithological heterogeneity in the mantle (e.g., Sobolev et al., 2007), though the causes of such CTE variability remain highly debated (e.g., Matzen et al., 2017).

In this paper with Oliver Shorttle and Martin Oeser, I present both CTE and incompatible trace-element (ITE) data from primitive Icelandic olivines that we use check the validity of melt inclusion records and investigate causes of geochemical variability in olivine macrocrysts themselves. We demonstrate that olivine macrocrysts are capable of preserving similar patterns of compositional variability to melt inclusions on intra- and inter-eruption lengthscales, and may allow degrees of magma enrichment to be reconstructed in samples where matrix glasses are degraded or absent.

An X-ray map of P in an olivine from the Stapafell eruption. Almost no P zoning can be observed in the olivine; boundary layer crystallisation seems unimportant. This image is approximately 1 mm across.

Although olivines from our enriched case study eruption, Stapafell, are slightly richer in Ni than those from our depleted case study eruption, Háleyjabunga, the CTE content of both eruptions are wholly consistent with melt supply from a peridotitic source. However, independent constraints from the combined major and trace element systematics of Icelandic basalts indicate that enriched melts come from a modally enriched source (Shorttle & Maclennan, 2011); enriched Icelandic basalts are too rich in iron to be derived by melting of depleted mantle. We therefore conclude that enriched domains in the Icelandic mantle are composed of modally enriched peridotite not pyroxenite, and that olivine CTE contents provide an incomplete picture of lithological heterogeneity in the mantle.

Publication

Neave, D.A., Shorttle, O., Oeser, M., Weyer, S. & Katsura, K. 2018. Mantle-derived trace element variability in olivines and their melt inclusions. Earth and Planetary Science Letters 483, 90–104.

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>

GeoBremen2017: Biases in the geochemical record of oceanic magmatism

At the end of summer 2017, I gave a talk at the annual meeting of the Deutsche Mineralogische Gesellschaft (DMG) at GeoBremen2017. My talk focussed on the results of my 3-kbar experiments on primtive Icelandic basalts, and how they show that depleted mantle melts are much less likely to survive being processed during their ascent through the crust than enriched melts. In other words, enriched melts are more likely to erupt at the surface and depleted melts are more likely to freeze at depth, fundamentally biasing the record of oceanic magmatism we see at the surface. You can download my slides here.

Experimental liquid lines of descent (LLD) for melts from the depleted Háleyjabunga (Hál) and enriched Stapafell (Sta) eruptions. Grey dots show Icelandic compositions from the Western Volcanic Zone and Reykjanes Peninsula (Shorttle & Maclennan, 2011). The arrow shows where compositions were resynthesised to mimic fractional crystallisation.

IAVCEI: The effect of mantle-derived variability on the mineralogy of primitive basalts: Experimental constraints from Icelandic systems

In summer 2017, I presented a poster at the excellent IAVCEI Scientific Assembly in Portland. My contribution summarised the findings of my experimental work in Hannover so far. In particular, I focussed the effects of mantle-dervied heterogeneity on the phase equilibria of primitive Icelandic basalts in the 1–7 kbar range. You can download a copy of my poster here.

False-colour backscattered electron image of the run products of an experiment on the Háleyjabunga eruption, Iceland

A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones.

Pressure is one of the key intensive variables that controls magmatic phase equilibria, which thus raises the possibily of using mineral and melt compositions to estimate magma storage pressures from erupted products. Such estimations are crucial for addressing geological problems ranging from understanding crustal accretion through to interpreting signals of unrest at active volcanoes.

In this paper with Keith Putirka, I assess the performance of some commonly used barometers that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. We find that many current barometers overestimate the pressure of phase equilibria experiments carried out on H2O-poor basalts at 1–7 kbar by up to 3 bar. Many published magma storage pressure estimates may thus need to be re-evaluated, and revised towards lower pressures.

In order to resolve the of barometer inaccuracy at low pressures, we thus present a newly calibrated Jd-in-clinopyroxene. Our new barometer is suitable for use on hydrous and anhydrous samples that are ultramafic to intermediate in composition. However, we do not recommend using the barometer at temperatures below 1100 °C and at oxygen fugacities above QFM+1 because of reduced accuracy under these conditions. The barometer reproduces its calibration data with a standard error of estimate (SEE) of 1.4 kbar, and tests performed using experiments on bastilc compositons confirm that it is significantly more accurate than previous models.

Calibration and test datasets used to develop and assess a new Jd-in-clinopyroxene barometer for use on ultramafic to intermediate compositions.

We apply our new barometer to a range previously studied eruptions from Iceland’s neovolcanic rift zones. Most eruptions preserve records of mid-crustal crystallisation at 2.6–3.6 kbar; only the highly primtive Borgarhraun eruption recrds crystallisation in the lower crust at 5.7 kbar. While some magma processing takes place immediately beneath Iceland’s central volcanoes, magma evolution under the island’s neovolcanic rift zones is thus strongly mediated by mid-crustal processes.

Spreadsheet for estimating P-T conditions from clinopyroxene-liquid equilibria

Clinopyroxene P-T Dec16

Publication

Neave, D.A. & Putirka, K.D. 2017. A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones. American Mineralogist 102.

Note that there is an erratum concerning a typographical error in the barometric equation published in this paper

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.

 

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>