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>