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.
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).
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 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.
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.
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>
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.
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.
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>
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).
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>