Diamond Annual Review 2020/21

86 87 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 small-angle and wide-angle X-ray scattering (SAXS-WAXS) measurements were made at beamlines I15 and I22 to observe the dynamics of nanolite nucleation and growth in Mt. Etna basalt. Figure 1a illustrates the XRD patterns collected at different temperatures during slow, 1° C min −1 (0.017° C s −1 ) cooling of the magma from 1,300° C. The appearance of clear diffraction peaks in the XRD pattern (e.g. 4.4, 5.1 and 5.4 Å −1 ) shows the onset of microlites crystallization between ~1,224° and ~1,208° C. Fig. 1b shows XRD patterns taken at the end of a series of relatively fast (~10° to 20° C s −1 ) single cooling steps from 1,475° C down to four different target temperatures. Here, while sharp diffraction peaks were not observed, a marked increase in the intensity at 0.3° (0.2 Å −1 ) was. That is consistent with the coexistence of amelt matrix and a separated phase on the order of nanometers in size.Therefore, results suggest that nanoscale phase separation was formed with arapidperturbationofthetemperatureofthemagma,which isexpectedtooccur before an explosive eruption 5 . The SAXS modelling allowed the determination of the size and time scale of nanolite formation and growth (Fig. 2) in a sample rapidly cooled in a single step from above the liquidus temperature and then held at 950° C. During the first 50 s of experiment (Fig. 2) the melt evolved into a suspension carrying a first population of spherical iron-bearing nanolites up to 50 nm. Afterwards, a second population of spherical polydisperse iron-bearing nanolites emerges in the melt that measures ~8 nm by the end of the cycle. Moreover, the analysis of SAXS patterns suggested the formation of particle aggregates in the melt.This is consistent with the agglomeration seen in natural samples, including Mt. Etna products erupted in 122 BCE. Viscosity measurements of magma analog allowed the observation and quantification (Fig. 3) of the effect of nanolite on the viscosity of Mt. Etna basalt. The results suggest that during the aggregation process of nanolites some liquid that makes up the magma can be trapped between the nanolites and stick to their surface. This immobile liquid behaves like a solid and can eventually lead the magma to approach the explosive condition of ~10 6 Pa s at 8, 11 and 32 volume%of nanolites at shear rates of 1.0, 3.5 and 100 s −1 , respectively. Notably, approximately 65 volume % of microlites (in orange) is required to reach such a condition. This study demonstrates that nanolite formation, growth and agglomeration occurs within seconds and triggers a rapid and substantial increase in magma Figure 2: Nanolite growth with time. The evolution of the nanolite radius with time for the two populations of nanolites is derived by modelling the SAXS patterns collected during nanolite crystallisation. The colour changes from hot (red) to cold (blue) with time. A nanoscale perspective of explosive volcanic eruptions Related publication: Di Genova D., Brooker R. A., Mader H. M., Drewitt J.W. E., Longo A., Deubener J., Neuville D. R., Fanara S., Shebanova O., Anzellini S., Arzilli F., Bamber E. C., Hennet L., La Spina G. &Miyajima N. In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions. Sci. Adv. 6 , eabb0413 (2020). DOI: 10.1126/sciadv.abb0413 Publication keywords: Magma; Volcanoes; Volcanic eruptions; Iron oxide nanoparticle; X-ray diffraction T he viscosity of magma is a critical factor in the explosiveness of volcanic eruptions. However, our current knowledge of the mechanisms that regulate the style of volcanic eruptions fails to explain an anomaly; some volcanoes with low viscosity magmas have unexpectedly explosive eruptions. We know that micro-sized crystals (microlites) can significantly increase the viscosity of magmas and promote bubble formation. However, a relatively large volume of these crystals (more than 30%) is needed to impact the viscosity. Recent observations of volcanic products have shown that magmas may contain nanometric crystals (i.e. nanolites), 10,000 times smaller than the width of a human hair, whose formation and influence on both viscosity and bubble formation is unknown. An international team of researchers used Diamond Light Source’s Extreme Conditions beamline (I15) and Small Angle Scattering & Diffraction beamline (I22) to subject a basaltic magma erupted explosively by Mount Etna in 122 BCE to X-ray diffraction measurements at high temperature (from about 900 up to 1,500 Celsius) by varying the cooling rate. This allowed the in situ observation of nanolite formation and growth. Their results showed that nanolites can form in the magma within milliseconds during rapid cooling, grow to ~50 nm in two minutes and even aggregate. These new aggregates effectively disrupt the remaining free liquid flow, increasing the magma’s viscosity and resulting in an explosive eruption. This information will inform new numerical models of volcanic eruptions, used to establish risk scenarios in volcanic areas. Volcanoes are messengers from the depth and connect the interior of the earth with its surface and atmosphere. Volcanic activity contributed to set the stage for life and its evolution through the injection of gas into the primordial atmosphere and the generation of new and fertile land. Nonetheless, increasing population growth and urbanization mean that volcanoes can also pose a threat to our society and economy. This is the case of explosive eruptions, such as those of Vesuvius in 79 BCE and Tambora in 1815, where the violent sustained volcanic explosions can eject ash and gas over several kilometres on timescales of minutes to hours or days.Yet, the current understanding of volcanic processes has identified an apparent anomaly; the occurrence of explosive eruptions fed by low viscous magmas, such as basalt that is the object of this study. Viscosity describes a fluid’s internal resistance to flow and it is known 1 to play a key role in controlling the eruptive style of volcanoes. If the magma is relatively low viscosity (very fluid), then most of the exsolving buoyant gas bubbles, which are generated due to the decompression of the magma during its ascent towards the Earth’s surface, have a good chance of escaping before that magma nears the surface, averting an explosive outcome. By contrast, when the magma viscosity is relatively high (>10 6 Pa s, 100,000 times higher the viscosity of honey) the gas bubbles remain trapped into the magma and the gas pressure builds up inexorably during the ascent and consequent decompression of the magma. Furthermore, the continuous exsolution of gas upon decompression promotes both the formation of crystals and the increase of magma buoyancy by reducing its density. The occurrence of these conditions, associated with the enormousshearstressestowhichthemagma issubjected inthevolcanicconduit, is dominantly responsible for explosive eruptions. The viscosity of bulk magma varies by orders of magnitude depending on the chemical composition of the melting phase, temperature, water and crystal content of the magma. At low crystal content (less than 20-30 vol.%), the SiO 2 content and temperature heavily control the magma viscosity. SiO 2 - poor magmas such as basalts, which are erupted at a temperature of about 1,100° C, do not exceed viscosity of 10 2 Pa s and thus these magmas commonly feed effusive eruption. Yet, several pieces of evidence both in the geological record and from recent observations that basaltic volcanoes can interrupt their normal and calm effusive activity with sudden explosive eruptions such as the Plinian eruption of 122 BCE 2 at Mt. Etna volcano in Italy. One common explanation for this explosivity is late stage, rapid growth of small crystals (microlites, typically from1 to >100 μm) above 20-30 vol. % in the fast-rising magma that causes a sudden increase in magma viscosity. However, the role of microlites is sometimes controversial since their volume percent appears to be too low. Interestingly and at a different size scale, nanocrystals (nanolites, typically <100 nm) are increasingly being discovered in volcanic products 3 . This study reveals the conditions leading to their formation and their effect onmagma viscosity. Nanolite formation is suspected to occur within seconds during the fast magma ascent within the volcanic conduit and impact magma rheology 3,4 . However, standard laboratory facilities alone cannot allow answering many questions behind nanolite formation and its effect on volcanic eruptions. Therefore, high-temperature andmilliseconds-resolution X-ray diffraction (XRD), Crystallography Group Beamline I15 (and beamline I22 from the Soft CondensedMatter Group) viscosity of several orders of magnitude. The increase in viscosity is sufficient to exceed the maximum stresses that can be supported by the magma during the eruptionandhencetriggeringexplosiveeruptions.Futurestudieswillfocusonthe understanding of the subtle factors that define the nanolite-controlled window between high (explosive) and low (effusive) viscosities that may be fundamental in predicting the“unpredictable”switches in eruptive style of volcanoes. References: 1. Gonnermann H. M. et al. The fluidmechanics inside a volcano. Annu. Rev. FluidMech. 39 , 321–356 (2007). DOI: 10.1146/annurev. fluid.39.050905.110207 2. Coltelli M. et al. Discovery of a Plinian basaltic eruption of Roman age at Etna volcano, Italy. Geology 26 , 1095–1098 (1998). DOI: 10.1130 /0091-7613(1998)026 <1095:DOAPBE >2.3.CO ;2 3. Mujin M. et al. A nanolite record of eruption style transition. Geology 42 , 611–614 (2014). DOI: 10.1130/G35553.1 4. Di Genova D. et al. A compositional tipping point governing the mobilization and eruption style of rhyolitic magma. Nature 552 , 235–238 (2017). DOI: 10.1038/nature24488 5. Arzilli F. et al. Magma fragmentation in highly explosive basaltic eruptions induced by rapid crystallization. Nat. Geosci. 12 , 1023–1028 (2019). DOI: 10.1038/s41561-019-0468-6 Funding acknowledgement: This project was supported by the NSFGEO-NERC grant‘Quantifying disequilibriumprocesses in basaltic volcanism’(NE/N018567/1) to H.M.M., R.A.B., F.A., G.L.S. and D.D.G. J.W.E.D. was funded by a NERC standard grant NE/P002951/1. Access to DLS was via experiment sessions EE17615-1 (XRD, February 2018) and SM20447-1 (SAXS-WAXS, February 2019).We acknowledge facility funding fromNE/N018567/1, which supports the ARS STEM at Oxford Material Sciences department.The German Research Foundation is acknowledged (DFG grant INST 91/251-1 FUGG) for the STEM-EDS analysis at the Bayerisches Geoinstitut. Corresponding author: Dr Danilo Di Genova, Universität Bayreuth, Germany, danilo.di-genova@uni-bayreuth.de Figure 1: In situ synchrotron XRD patterns of molten Mt. Etna basalt during slow and fast cooling. (a) the basalt was slow cooled from 1,300° C down to 1,083° C. Results show the microlite crystallisation (e.g., plagioclase) for 60-s acquisitions at the temperatures indicated. The sharp peak observed at 2.9 Å −1 at 1224° C is the Bragg diffraction peak of crystalline Pt heating wire; (b) spectra collected from a much faster cooling of the basalt from 1,475° C to room temperature (in black) and then to each target temperature in the legend. The asterisk (*) indicates the small-angle ‘nanolite peak,’ which increases systematically with deeper cooling. Figure 3: The effect of ~15nm diameter nanolite on viscosity at different shear rates. The calculated expected viscosity of nanolite-bearing melt with an initial viscosity of 200 Pa s, equivalent to Mt. Etna basalt at pre-eruptive conditions (temperature and water content). We also show the equivalent curve for microlites at a shear rate of 1 s −1 . It is clear that <10 volume % of nanoparticles is equivalent to >60 volume % of microparticles and can raise the viscosity to ~10 6 Pa s required for magma fragmentation.