Although it is widely accepted that shallow silicic magma reservoirs exist, and can feed eruptions, their dynamics and longevity are a topic of debate. Here, we use field mapping, geochemistry, 3D pluton reconstruction and a thermal model to investigate the assembly and eruptive history of the shallow Reyoarartindur Pluton, southeast Iceland. Primarily, the exposed pluton is constructed of a single rock unit, the Main Granite (69.9-77.7 wt.% SiO2). Two further units are locally exposed as enclaves at the base of the exposure, the Granite Enclaves (67.4-70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8-67.3 wt.% SiO2). Geochemically, the units are related and were likely derived from the same source reservoir. In 3D, the pluton has a shape characterized by flat roof segments that are vertically offset and a volume of >2.5 km(3). The pluton roof is intruded by dikes from the pluton, and in two locations displays depressions associated with large dikes. Within these particular dikes the rock is partially to wholly tuffisitic, and rock compositions range from quartz monzonite to granite. We interpret these zones as eruption-feeding conduits from the pluton. A lack of cooling contacts throughout the pluton indicates rapid magma emplacement and a thermal model calculates the top 75 m would have rheologically locked up within 1,000 years. Hence, we argue that the Reyoarartindur Pluton was an ephemeral part of the wider plumbing system that feeds a volcano, and that timeframes from emplacement to eruption were rapid.

Shallow magmatic reservoirs have been identified at many volcanoes worldwide. However, questions still remain regarding their size, dynamics and longevity. The Reyðarártindur Pluton exposed in Southeast Iceland provides a superb example to investigate the above questions. Here, we use field mapping, sampling, geochemistry, 3D pluton shape modelling and a numerical thermal model to reconstruct the assembly and eruptive history of the shallow magma body.

In 3D, the c. 2.5 km3 pluton has a castle-like shape characterised by flat roof segments that are vertically offset along steep faults. The exposed pluton is constructed largely of a single rock unit, the Main Granite (69.9 to 77.6 wt.% SiO2). Two additional units occur only as enclaves: the Granite Enclaves (67.4 to 70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8 to 67.3 wt.% SiO2). However, geochemistry clearly indicates that the units are related and hence were likely derived from the same source reservoir. 

In two locations, the pluton roof displays depressions associated with large dykes. Within these two dykes the rock is partially to wholly tuffisitic, and geochemical compositions range from quartz monzonite to granite. We interpret these dykes as eruption-feeding conduits from the pluton. Additionally, we speculate that the mingling of magmatic units with compositional ranges from quartz monzonite to granite within the conduits indicates that injection of new magma into the reservoir triggered eruption. 

Rapid pluton construction is indicated by ductile contacts between units in the pluton and a thermal model calculates the top 75 m would have rheologically locked up within 1000 years. Hence, we argue that the pluton was a short-lived part of the wider magmatic system that fed the associated volcano, and that timeframes from emplacement to eruption were limited to 1000 years.

Rhodes, E. Barker, A. K. Burchardt, S. et al. (2021). Rapid assembly and eruption of a shallow silicic magma reservoir, Reyðarártindur Pluton, Southeast Iceland. G-Cubed. DOI: 10.1029/2021GC009999

Reyðarártindur is one of several felsic plutons exposed in Southeast Iceland, interpreted to be the shallow plumbing systems of late Neogene volcanic centres (Cargill et al., 1928; Furman et al., 1992; Padilla, 2015). These plutons are considered to preserve analogous plumbing systems to the central volcanoes active in Iceland today (Furman et al., 1992). Reyðarártindur is the oldest pluton in Southeast Iceland at 7.30 ± 0.06 Ma (Padilla, 2015), and has been conveniently incised by the Reyðará River, making it ideal for an in-depth study of the external and internal geometry of a shallow rift-zone magma plumbing system.

In order to analyse mechanisms of magma emplacement, we have conducted detailed structural mapping of the pluton and its basaltic host rock using drone-based photogrammetry. To complement this, we have also extensively sampled and analysed the geochemistry and petrology of the pluton interior. An outline of the pluton is shown in Figure 1, highlighting that the pluton is NNW-SSE trending, which is in contrast to the NE-SW regional dyke trend. A total thickness of 500 m and a calculated volume of 1.5 km3 is exposed. While the pluton walls are steeply-dipping, the pluton roof is mostly flat. Deviations from the flat roof occur in the form of areas that are cut by steep dip-slip faults with displacements of up to 100 m. Roof faulting creates both structural highs (horsts) and lows (grabens, as well as a monoclinal structure) in the roof. Many of the faults are intruded by felsic dykes, some of them seem to have been the feeders of surface eruptions.

An estimated 95% of the pluton volume is rhyolitic in composition, with 73-76 wt.% SiO2. Geochemically, the magma in the majority of the pluton is similar, but hand samples and thin sections show a large variety of textures. In the lower part of the exposure there is a zone of mingling and mixing between a matrix magma and several different types of silicic enclaves (Figure 1). The matrix magma is more mafic with an SiO2 content of 68-73 wt.% and the enclaves vary in nature with no systematic shape, size or aspect ratio. There are at least two types of enclaves, and the predominant type is a coarse grained trachydacite with 64-69 wt.% SiO2. These less evolved compositions are limited to a 1 km stretch of the riverbed in the centre of the pluton. Closer to the wall contacts (i.e. to the north and south of the mingling zone), the composition of the magma returns to that of the main magma body, as observed at higher elevations.

Our poster aims to summarise our results and present interpretations of the magmatic processes preserved in the Reyðarártindur pluton. Our preliminary results indicate that the pluton was emplaced by a combination of floor subsidence and roof doming, and that the pluton structure was modified during further magma intrusion into, and eruption from, the pluton.

 Fig. 1 – Map of the Reyðarártindur Pluton, South-East Iceland.

References

Cargill, H., Hawkes, L., and Ledeboen, J. (1928). The major intrustions of South-Eastern Iceland. Quarterly Journal of the Geological Society of London 84, 505–539.

Furman, T., Meyer, P. S., and Frey, F. (1992). Evolution of Icelandic central volcanoes: evidence from the Austurhorn intrusion, southeastern Iceland. Bulletin of Volcanology. 55, 45–62.

Padilla, A. (2015). Elemental and isotopic geochemistry of crystal-melt systems: Elucidating the construction and evolution of silicic magmas in the shallow crust, using examples from southeast Iceland and southwest USA [PhD Dissertation: Vanderbilt University].

Glass art practice is indivisible from the material behaviour of glass at a range of working conditions, providing a direct link with the science of glass and melts. The use of non-standard, non-commercial, or natural glass compositions in art usually brings with it challenges associated with unexpected or undesirable processes, such as bubble formation and growth, liquid-liquid immiscibility, heterogeneities, and devitrification. For these reasons, natural geological compositions, including obsidian, have typically been avoided in glass art, with a few pioneering exceptions. Here, we bring together the results of mutual experimentation, knowledge-exchange workshops, and successful obsidian and magma use-cases in glass art in order to constrain the usability of obsidian and the techniques most suitable for rendering the material amenable to glass art practice. We conclude by exploring opportunities for collaboration between volcanologists and glass artists, which we propose would develop both fields in novel directions.

Changes in rhyolite melt viscosity during magma decompression and degassing exert a first order control on ascent through the crust and volcanic eruption style. These changes have as yet unknown hazard implications for geothermal drilling in pursuit of particularly hot fluids close to magma storage regions. Here, we exploit the situation at Krafla volcano in which rhyolite has both erupted at Earth's surface and been sampled at shallow storage depths via drilling of the 2009 IDDP-1 and 2008 KJ-39 boreholes. We use differential scanning calorimetry to constrain that the IDDP-1 magma quenched to glass at similar to 700 K, at a rate of between 7 and 80 K.min(-1). We measure the equilibrium viscosity of the IDDP-1 rhyolite at temperatures close to the glass transition interval and show that the rhyolite viscosity is consistent with generalized viscosity models assuming a dissolved H2O concentration of 2.12 wt%. We couple these results with micro-penetration and concentric cylinder rheometry over a range of potential magma storage temperatures to constrain the response of surficial Krafla rhyolites to stress. The surficial rhyolites at Krafla match the same viscosity model, assuming a lower dissolved H2O concentration of 0.12 wt%. Our results show that at a storage temperature of 1123-1193 K, the viscosity of the stored magma is similar to 3x10(5) Pa.s. At the same temperature, the viscosity following degassing during ascent to the surface rises to similar to 2x10(9) Pa.s. Finally, we use high-stress compression tests on the Hrafntinnuhryggur surface obsidian to determine the onset of unrelaxed behavior and viscoelastic melt rupture or fragmentation pertinent to understanding the melt response to rapid pressure changes that may be associated with further (near-) magma exploration at Krafla. Taken together, we characterize the relaxation and viscosity of these magmas from source-to-surface.

Silicic magma in the shallow crust has the potential to violently erupt, depending on its ability to release overpressures caused by magmatic volatiles (outgassing). Deformation-induced outgassing is prevalent along volcanic conduit margins, where ascending magma is sheared at high rates. However, this mechanism limits outgassing to the contact with the host rock, leaving the bulk of the magma untouched and full of volatiles. This thesis presents a different mechanism of silicic outgassing that affects the interior volume of a magma body as well as the margins. Here, we present a case study of deformation features within the Miocene Sandfell laccolith, Eastern Iceland: a 0.57 km³ dome-shaped rhyolitic magma body with ~5 vol% phenocrysts and a microcrystalline groundmass. Similar textures have been reported in lava domes and intrusions with various compositions and crystallinities.

The range of deformation features are 1. porous flow bands, 2. elongated pores within flow bands, 3. 1–5 cm long tensile fractures aligned in bands, 4. 5–20 cm fractures within bands (often multiple fracture sets), and 5. breccia (densely spaced bands that are no longer distinguishable). The bands in each category range in length from ‘lenses’ (~15 cm) to laterally expansive (several meters), and usually taper at the tips. The bands are interlayered with coherent, undeformed rhyolite, and their morphology varies between planar, undulating, and anastomosing. The chapters within this thesis characterize the spatial distribution of each stage of ‘fracture banding’ and interpret their role in magma emplacement (Paper I); analyze the textures of each deformation stage on a micro-scale to interpret the rheology of the magma during formation (Paper II); investigate the mineral assemblage of fracture fillings and apply results to metal separation from parent magma in early ore systems (Paper III); and attempt to experimentally recreate fracture bands in a laboratory setting (Paper IV).

The results of these chapters suggest the deformation features formed from a rheological contrast between flow bands with different crystallinity. Emplacement-related stress localized along the weaker, more melt-rich flow bands, driving the ductile magma to deform through viscous and brittle processes. The fractures arrested against the stiffer rhyolite in the more crystalline flow bands, while drawing in surrounding melt and fluids. This, plus the interconnectedness of the fracture bands, implies an efficient outgassing system.

Here we show that fracture banding is an outgassing mechanism taking place in silicic magma undergoing deformation.

Efficient outgassing of shallow magma bodies reduces the risk of explosive eruption. Silica-rich magmas are too viscous for exsolved gas bubbles to escape the system through buoyant forces alone, and so volatile overpressure is often released through deformation-related processes. Here we present a case study on magma-emplacement-related deformation in a shallow (500 m depth) rhyolite intrusion (the Sandfell laccolith, Eastern Iceland) to investigate the establishment and evolution of degassing and outgassing networks in silicic sub-volcanic intrusions. We observe viscous and brittle deformation features: from vesiculated flow bands that organized into 'pore channels' in the ductile regime, to uniform bands of tensile fractures (‘fracture bands’) that grade into breccia and gouge in the brittle regime. Through field mapping, structural analysis, and anisotropy of magnetic susceptibility (AMS), we show that the deformation spectrum, observed all over the laccolith, represents stages of degassing (viscous processes) and outgassing (brittle processes) that resulted in the formation of interconnected permeable networks through the growth and linkage of fracture bands. Areas with concentrations of higher degrees of brittle deformation are proximal to abruptly changing AMS fabrics and point to laccolith-scale strain partitioning in the magma linked to different stages of laccolith growth. The establishment of intrusion-scale permeable networks through the cumulation of discrete magma fractures would have profoundly assisted the outgassing of the entire laccolith. Therefore, fracture banding captures viscous and brittle processes working in tandem as an efficient outgassing mechanism, and should be considered in sub-volcanic intrusions elsewhere.