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.