The chemistry and mineralogy of slabs subducted into lower mantle control slab rheology and impact the deep volatile cycle. It is known that the metamorphism of little-altered oceanic crust results in eclogite rocks with subequal proportions of garnet and clinopyroxene. With increasing pressure, these minerals react to stabilize pyrope-rich tetragonal majoritic garnet. However, some eclogites contain higher proportions of omphacitic clinopyroxene, caused by Na- and Si-rich metasomatism on the ocean floor or during subduction. The mineralogy of such eclogites is expected to evolve differently. Here, we discuss the results of the crystallization products of omphacitic glass at ~ 18 and ~ 25 GPa and 1000 °C to simulate P-T regimes of cold subduction. The full characterization of the recovered samples indicates evidence of crystallization of Na-, Si-rich cubic instead of tetragonal majorite. This cubic majorite can incorporate large amounts of ferric iron, promoting redox reactions with surrounding volatile-bearing fluids and, ultimately, diamond formation. In addition, the occurrence of cubic majorite in the slab would affect the local density, favoring the continued buoyancy of the slab as previously proposed by seismic observations. Attention must be paid to omphacitic inclusions in sublithospheric diamonds as these might have experienced back-transformation from the HP isochemical cubic phase.
Open-conduit basaltic volcanoes can be characterised by sudden large explosive events (paroxysms) that interrupt normal effusive and mild explosive activity. In June-August 2019, one major explosion and two paroxysms occurred at Stromboli volcano (Italy) within only 64 days. Here, via a multifaceted approach using clinopyroxene, we show arrival of mafic recharges up to a few days before the onset of these events and their effects on the eruption pattern at Stromboli, as a prime example of a persistently active, open-conduit basaltic volcano. Our data indicate a rejuvenated Stromboli plumbing system where the extant crystal mush is efficiently permeated by recharge magmas with minimum remobilisation promoting a direct linkage between the deeper and the shallow reservoirs that sustains the currently observed larger variability of eruptive behaviour. Our approach provides vital insights into magma dynamics and their effects on monitoring signals demonstrating the power of petrological studies in interpreting patterns of surficial activity.
Identifying accurate topographic variations associated with volcanic eruptions plays a key role in obtaining information on eruptive parameters, volcano structure, input data for volcano processes modelling, and civil protection and recovery actions. The 2021 eruption of Cumbre Vieja volcano is the largest eruptive event in the recorded history for La Palma Island. Over the course of almost 3 months, the volcano produced profound morphological changes in the landscape affecting both the natural and the anthropic environment over an area of tens of km2. We present the results of a UAS (Unoccupied Aircraft System) survey consisting of >12,000 photographs coupled with Structure-from-Motion photogrammetry that allowed us to produce a very-high-resolution (0.2 m/pixel) Digital Surface Model (DSM). We characterised the surface topography of the newly formed volcanic landforms and produced an elevation difference map by differencing our survey and a pre-event surface, identifying morphological changes in detail. The present DSM, the first one with such a high resolution to our knowledge, represents a relevant contribution to both the scientific community and the local authorities.
In 2019, Stromboli volcano experienced one of the most violent eruptive crises in the last hundred years. Two paroxysmal explosions interrupted the 'normal' mild explosive activity during the tourist season. Here we integrate visual and field observations, textural and chemical data of eruptive products, and numerical simulations to analyze the eruptive patterns leading to the paroxysmal explosions. Heralded by 24 days of intensified normal activity and 45 min of lava outpouring, on 3 July a paroxysm ejected ~6 × 107 kg of bombs, lapilli and ash up to 6 km high, damaging the monitoring network and falling towards SW on the inhabited areas. Intensified activity continued until the less energetic, 28 August paroxysm, which dispersed tephra mainly towards NE. We argue that all paroxysms at Stromboli share a common pre-eruptive weeks-to months-long unrest phase, marking the perturbation of the magmatic system. Our analysis points to an urgent implementation of volcanic monitoring at Stromboli to detect such long-term precursors.
Radon (222Rn) and thoron (220Rn) are two isotopes belonging to the noble gas radon (sensu lato) that is frequently employed for the geochemical surveillance of active volcanoes. Temperature gradients operating at subvolcanic conditions may induce chemical and structural modifications in rock-forming minerals and their related 222Rn-220Rn emissions. Additionally, CO2 fluxes may also contribute enormously to the transport of radionuclides through the microcracks and pores of subvolcanic rocks. In view of these articulated phenomena, we have experimentally quantified the changes of 220Rn signal caused by dehydration of a zeolitized tuff exposed to variable CO2 fluxes. Results indicate that, at low CO2 fluxes, water molecules and hydroxyl groups adsorbed on the glassy surface of macro- and micropores are physically removed by an intermolecular proton transfer mechanism, leading to an increase of the 220Rn signal. By contrast, at high CO2 fluxes, 220Rn emissions dramatically decrease because of the strong dilution capacity of CO2 that overprints the advective effect of carrier fluids. We conclude that the sign and magnitude of radon (sensu lato) changes observed in volcanic settings depend on the flux rate of carrier fluids and the rival effects between advective transport and radionuclide dilution.
Rock substrates beneath active volcanoes are frequently subjected to temperature changes caused by the input of new magma from the depth and/or the intrusion of magma bodies of variable thickness within the subvolcanic rocks. The primary effect of the influx of hot magma is the heating of surrounding host rocks with the consequent modification of their physical and chemical properties. To assess mobilization in subvolcanic thermal regimes, we have performed radon (220Rn) thermal experiments on a phonolitic lava exposed to temperatures in the range of 100-900°C. Results from these experiments indicate that transient Rn signals are not unequivocally related to substrate deformation caused by tectonic stresses, but rather to the temperature-dependent diffusion of radionuclides through the structural discontinuities of rocks which serve as preferential pathways for gas release. Intense heating/cooling cycles are accompanied by rapid expansion and contraction of minerals. Rapid thermal cycling produced both inter- and intra-crystal microfracturing, as well as the formation of macroscopic faults. The increased number of diffusion paths dramatically intensified Rn migration, leading to much higher emissions than temperature-dependent transient changes. This geochemical behaviour is analogous to positive anomalies recorded on active volcanoes where dyke injections produce thermal stress and deformation in the host rocks. An increased Rn signal far away from the location of a magmatic intrusion is also consistent with microfracturing of subsurface rocks over long distances via thermal stress propagation and the opening of new pathways.
Most of the current ash transport and dispersion models neglect particle-fluid (two-way) and particle-fluid plus particle-particle (four-way) reciprocal interactions during particle fallout from volcanic plumes. These interactions, a function of particle concentration in the plume, could play an important role, explaining, for example, discrepancies between observed and modelled ash deposits. Aiming at a more accurate prediction of volcanic ash dispersal and sedimentation, the settling of ash particles at particle volume fractions (Ïp) ranging 10-7-10-3 was performed in laboratory experiments and reproduced by numerical simulations that take into account first the two-way and then the four-way coupling. Results show that the velocity of particles settling together can exceed the velocity of particles settling individually by up to 4 times for Ïp ~ 10-3. Comparisons between experimental and simulation results reveal that, during the sedimentation process, the settling velocity is largely enhanced by particle-fluid interactions but partly hindered by particle-particle interactions with increasing Ïp. Combining the experimental and numerical results, we provide an empirical model allowing correction of the settling velocity of particles of any size, density, and shape, as a function of Ïp. These corrections will impact volcanic plume modelling results as well as remote sensing retrieval techniques for plume parameters.
