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).
Rare earth elements (REEs), which are essential for many modern technologies such as batteries and wind turbines, have characteristic absorption features in visible to shortwave infra-red (VNIR-SWIR) reflectance spectra. Neodymium (Nd) has amongst the most prominent absorption features of the REEs and thus represents a key pathfinder element for the element group as a whole. Given that the world’s largest REE deposits are associated with carbonatites, we collected spectral, petrographic and geochemical data from a predominantly carbonatitic suite of rocks to assess the feasibility of imaging REE deposits using remote sensing. Rock samples were drawn from a number of sources including the Harker Collection in Cambridge, Cambourne School of Mines and the Natural History Museum in London. REE ores from the Bayan Obo (China) and Mountain Pass (USA) mines, as well as REE-rich alkaline rocks from the Motzfeldt and Ilímaussaq intrusions in Greenland were included in the sample suite.
By simulating the response of a number of remote sensing instruments, we demonstrated that hyperspectral instruments with capabilities equivalent to the operational Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and planned Environmental Mapping and Analysis Program (EnMAP) instruments have the spectral resolution necessary to detect Nd absorption features, especially in high grade samples with economically relevant REE accumulations.
Unfortunatly, most REE-rich outcrops are too small to be detected by satellite-based platforms. However, Nd absorption features should be identifiable in high-quality, airborne, hyperspectral datasets collected at meter-scale spatial resolutions. Future deployment of hyperspectral instruments on Unmanned Aerial Vehicles (UAVs) could enable REE grade to be mapped at the cm-scale across whole deposits.
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.
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.
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.
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.
Dissolved volatile elements play important roles in driving volcanic eruptions and controlling the physical properties of magmas. Degassing of magmatic volatiles also links deep geochemcial reservoirs with the Earth’s surface, closing global element cycles (e.g., Marty & Tolstikhin, 1998). However, determing the original CO2 content of mantle melts is difficult because most melts reach volatile saturation long before eruption. Measuring melt inclusions isolated hosted in primitive crystals that remained isolated from their carrier melt provides one way of investigating the CO2 content of basaltic magmas (e.g., Moore, 2008).
In this paper, we presented major, trace and volatile element analyses from >100 primitive olivine-hosted melt inclusions from a sub-glacial eruption in the Eastern Volcanic Zone of Iceland – the Skuggafjöll eruption. While our melt inclusion compositions preserved a record of primitive melt heterogeneity similar to that observed in other Icelandic systems including Laki (Neave et al., 2013), the most striking feature of our dataset was an enigmatic negative correlation between CO2 and incompatible trace element enrichment:
We suggested that a negative correlation between CO2 and incompatibe trace element enrichment may result from the concurrent mixing, crystallisation and exsolution of CO2 from melts that have experienced varying degrees of previous CO2 loss: mixing may have been triggered by the injection of a depleted and possibily CO2-supersaturated melt (CO2/Nb > 350) into a relatively shallow magma reservoir containing an enriched melt that has already lost much of its CO2.
Another inportant finding concerned the CO2 content of shrinkage bubbles in melt inclusions. Many recent studies have demonstrated that CO2 can be sequestered into bubbles during the cooling of melt inclusions (e.g., Hartley et al., 2014; Mironov et al., 2015; Wallace et al., 2015; Moore et al., 2015). However, despite investigating a large number number of shrinkage bubbles by Raman spectroscopy and microthermometry, we found no CO2-bearing bubbles. We therefore suggested that our subglacially quenched samples cooled sufficiently quickly to for CO2 sequestration to have been kinetically inhibited, an observation that has implications for interpreting the CO2 content of inclusions from other settings that experince rapidly quenched, such as those from mid-ocean ridges.
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).
Pantellerites are Fe- and volatile-rich, peralkaline rhyolites that erupt primarily in continental rift settings. As no eruptions of pantelleritic magma have been observed, interpreting the diversity of volcanic phenomena at pantelleritic volcanoes is challenging. Explosive eruptions range in scale from large ignimbrite-forming events like the ~45 ka Green Tuff eruption on Pantelleria to small cone-forming events. Effusive eruptions form structures as diverse as low-aspect-ratio lava domes and high-aspect-ratio lava shields. Although fewer in number than their calcalkaline counterparts, peralkaline rhyolite volcanoes nevertheless present a range of hazards.
The evolution of peralkaline magmas has been the subject of much recent debate, with some authors advocating for pantellerite genesis by melting alkali gabbros (e.g., Avanzinelli et al., 2004), and others favouring extensive fractional crystallisation (e.g., White et al., 2009). The volatile content of pantellerite melts had also been the subject of considerable uncertainty until a recent studied have confirmed the water-rich nature of pantelleritic melts.
In this paper, we presented major element, trace element and volatile compositions from glasses, crystals and melt inclusions from a number of post-Green Tuff eruptions from Pantelleria. The main outcomes were:
A quantification of the degree of aluminous lherzolite melting required to generate the alkali basalts present around northwest coast of Pantelleria (~2%).
A confirmation that pantellerites can be generated by extensive fractional crystallisation (~95%) of alkali basalts.
High precision analyses of glass and melt inclusion volatile contents (H2O, CO2, Li, F, Cl, S) that confirm the H2O- and halogen-rich of pantellerite melts
An evaluation that explosive peralkaline eruptions may emit much more sulphur than metauluminous eruptions of an equivalent size, up to ~100 Mt for a Green Tuff-sized eruption, becasue of the high sulphur solubilty in Fe- and alkali-rich melts.