Effective eruption forecasting and volcanic hazard management depend heavily on our ability to detect when a volcanic system switches from a state of unrest into a state of eruption. The 2021 eruption at Fagradalsfjall in SW Iceland, the first deep-sourced eruption on a mid-ocean ridge system monitored with modern instrumentation, presents an ideal opportunity to compare geophysical and petrological datasets to explore processes of deep magma mobilisation and eruption priming. Here we use diffusion chronometry to show that deep magmatic unrest in the roots of volcanic systems can precede apparent geophysical eruption precursors by a few years.  Early phases of magma accumulation and reorganisation in the near-Moho plumbing system, part of the priming for eruption, can occur in the absence of significant increases in shallow seismicity (<7 km depth) or rapid geodetic changes. In contrast, geophysical signals of unrest and crystal records of changing magmatic conditions both show significant increases in intensity in the months and days prior to eruption. This correlation may signal a rapid transition from a state of priming to full scale mobilisation in which magma begins to traverse the upper/ brittle crust. Our findings provide new insights into the dynamics of near-Moho magma storage and mobilisation. 

How to cite: Kahl, M., Mutch, E. J. F., Maclennan, J., Morgan, D., Couperthwaite, F., Bali, E., Thordarson, T., Guðfinnsson, G. H., Walshaw, R., Buisman, I., Buhre, S., van der Meer, Q. H. A., Caracciolo, A., Marshall, E. W., Rasmussen, M. B., Gallagher, C. R., Moreland, W. M., Höskuldsson, Á., and Askew, R. A.: Years of deep magmatic upheaval preceding the 2021 eruption at Fagradalsfjall, Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2742, https://doi.org/10.5194/egusphere-egu23-2742, 2023.

The Reykjanes Peninsula in SW Iceland is a part of the Mid-Atlantic plate boundary. It forms its transtensional segment with several volcanic and faulting systems. We focus on the 2017 seismicity that occurred in the central part of Reykjanes at the place of Fagradalsfjall volcano prior to its eruption on March 19, 2021. We invert well-determined focal mechanisms of the 2017 seismicity and provide mapping of tectonic stress in space and time. Our results disclose heterogeneous stress field manifested by mix of shear, tensile and compressive fracturing.  Although the fracturing was diverse, directions of the principal stress axes were stable and consistent with the processes at the transtensional divergent plate boundary. The prominent stress direction was in the azimuth of 120°±8°, which represents the overall extension related to rifting in the Reykjanes Peninsula. The activity initiated on the transform fault segment with predominantly shear strike-slip events. The non-shear fractures occurred later being associated with normal dip-slips and corresponding to the opening of volcanic fissures trending in the azimuth of 30-35°, perpendicular to the extension. The dip-slips were mainly located above an aseismic dike detected in the centre of the 2017 swarm. This dike represents a zone of crustal weakening during a preparatory phase of future 2021 Fagradalsfjall volcanic eruption located at the same place. Moreover, we detected local variation of stress when the stress axes abruptly interchanged their directions in the individual stress domains. These stress changes are interpreted in a consequence of plate spreading and upcoming fluid flow during a preparatory phase of a rifting episode.

How to cite: Hrubcová, P. and Vavryčuk, V.: Tectonic stress changes related to plate spreading prior to the 2021 Fagradalsfjall eruption in SW Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13373, https://doi.org/10.5194/egusphere-egu23-13373, 2023.

To get more data concerning de geochemistry and volcanology of Reykjanes Peninsula (Iceland) lavas, more high-precision analytical methods such as Instrumental Neutron Activation Analysis (INAA), Electron Microprobe Analysis (EMPA), X-ray Fluorescence (XRF), X-ray Diffraction (XRD), ICP-MS, X-ray microtomography (XRMT) and magnetism, coupled with mineralogical investigations were used. INAA, EPMA, XRF and ICP-MS were used to determine both major and trace element mass fractions. In the case of major elements, despite some differences inherent utilization of different analytical techniques, all analysis suggested a tholeiitic composition. Several discriminating diagrams clearly emphasize the subalkaline and tholeiitic trend, while the tectonic discrimination diagram assigned a “continental affinity”, as well as the existence of a minor crustal contamination. At their turn, the distribution of incompatible trace elements, represented into several discriminating diagrams, in agreement with PetDB database on Reykjanes Peninsula, as well as Hawaii and St Helen volcanic rocks, confirming the previous hypothesis based on major elements distribution on the tholeiitic and evolved character of the Reykjanes Peninsula lava, with an affinity towards ocean island basalts with traces crustal contamination. The results of mineralogical as well as BSE images analysis evidenced an abundance of plagioclase (albite), pyroxene (augite and pigeonite), as well as Fe-Ti oxides, while minerals such as olivine and spinel were less present. XRMT images revealed the presence of a multitude of vesicles showing preferred orientations, most probable due to lava flow, as the XRMT images loaded into stacks and analyzed by appropriate image analyzing software suggested. This particular features could suggest the existence of an important amount of volatiles, which lowering lava viscosity make them visible among larger vesicles. Raman spectroscopy results concerning the phases of each mineral, compared with literature and RRUFF^TM database confirmed previuous finding concerning the geochemistry of investigated Reykjaned Peninsula basalt sample. A magnetic analysis, performed by means of FORC diagrams as well as magnetic susceptibility dependence on temperature and the magnetic field, evidenced the presence of titanomagnetite as a main magnetic present in the sample.Therefore, all analyses suggested that the investigated basaltic lava present a tholeiitic composition, with an evolved continental affinity, but not related to rifting. The structural features suggests the presence of an important amount of volatiles existed prior the eruption.

How to cite: Stoicescu, D., Dumitras, D., Duliu, O., Panaiotu, C., Costin, G., Zinicovscaia, I., Dinca, G., Necula, C., Porosnicu, I., and Culicov, O.: Multi-analytical characterization of a Reykjanes Peninsula (Iceland) basalt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4209, https://doi.org/10.5194/egusphere-egu23-4209, 2023.

Volcanic gases are a major concern, especially when eruptions take place in inhabited or touristic areas. Several studies have revealed that during basaltic eruptions, toxic metals such as Pb, Cd, As and Zn are efficiently outgassed, carried by the major gas species, mainly sulfur and halogens. However, part of the degassing occurs after the eruption, while the lava flow is solidifying, and the composition of this secondary gas is virtually unknown.

After the primary (syn-eruptive) degassing, the lava is depleted in sulfur, leading to relative enrichment in halogens in secondary (post-eruptive) gas emission. This change in major species concentration could impact the volatility of metals and thus the toxicity of the gas emitted.

To investigate this subject, we collected, using filter packs, gas samples of both the primary and the secondary gas phases of the Geldingadalir and Meradalir eruptions. The filters were then leached in diluted acid and the resulting solution analyzed for trace element composition.

Results show syn-eruptive gas samples with very homogeneous trace volatile element composition and distinct from all the post-eruptive gas. Conversely, the secondary gas is more diverse, with distinct composition in samples collected around the main Geldingadalir crater and those collected on the lava flow.

To compare our gas samples (having different air dilution factors), we normalized each element to Cu (well measured and moderately volatile). Overall, the lava flow post-eruptive gas appears enriched in Zn, Sb and Pb with respect to syn-eruptive (10-100 times higher normalized enrichment factor). These elements are known to form chloride species and could thus have an enhanced volatility due to higher Cl concentration in the secondary gas phase. The Sulfur-loving (chalcophile) element Te has, on the other hand, a 10 times lower normalized enrichment factor in the lava flow gas, which is consistent with a sulfur depletion.

It thus seems that volcanic gas emission changes radically between primary and secondary degassing. Increase volatility of some metals such as Lead or Zinc might lead to higher toxicity, with important hazard for the local population and environment.

How to cite: Levillayer, N. and Sigmarsson, O.: Primary versus secondary degassing during basaltic eruptions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5120, https://doi.org/10.5194/egusphere-egu23-5120, 2023.

Fractures and tectonic structures have been related to dyke emplacements, eruption location or dynamics in several volcanic areas around the world. Mapping of active faults is therefore key for assessing the potential tectonic and volcanic hazard within a region. The 2021 eruption in the Fagradalsfjall volcanic area (Reykjanes Peninsula, SW Iceland) was preceded by two years of volcanic unrest, including four non-eruptive unrests in the Svartsengi and Krýsuvík volcanic areas and a dyke intrusion in the Fagradalsfjall volcanic segment. Nine earthquakes of magnitudes M 5–5.6 were recorded during this time period and were widely felt by the surrounding population. Using interferometric synthetic aperture radar (InSAR) applied to TerraSAR-X data collected over 2019–2021, we mapped fracture movements over the Reykjanes Peninsula. We identified ~1250 active structures across 54 interferograms during this time period, complementing previously mapped structures. Our study reveals extensive fracture movements across most of the Peninsula, extending from Reykjanes to NE Krýsuvík volcanic areas. We particularly highlight previously undetected structures beneath the town of Grindavík as well as a N45°E striking structure in the Fagradalsfjall volcanic area, active during summer-autumn 2020, prior to the 2021 dyke intrusion. We propose that this structure influenced the location of the longest lasting vent of the 2021 eruption. The observations presented in this study have important implications for improving our understanding of volcano-tectonic interactions and hazard assessments in Iceland and worldwide.

How to cite: Ducrocq, C., Árnadóttir, T., Einarsson, P., Jónsson, S., Drouin, V., Geirsson, H., and Hjartardóttir, Á. R.: Widespread fracture movements during the 2019–2021 volcano-tectonic unrest on the Reykjanes Peninsula from TerraSAR-X interferometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10732, https://doi.org/10.5194/egusphere-egu23-10732, 2023.

The 6-month long fissure eruption that started in Geldingadalir valley within Mt. Fagradalsfjall, Reykjanes Peninsula, SW Iceland, on 19March 2021 was preceded by three weeks of intense seismic activity associated with a ~10 km long NE-SW oriented dyke intrusion, along the Fagradalsfjall volcanic system. This was the first eruption in over 800 years on the Peninsula. A multi-institutional seismic network, installed prior to the dyke intrusion, comprises 27, 3-component instruments (25 broadband and 2 short-period instruments) covering the whole Reykjanes Peninsula. Here we focus on the Fagradalsfjall area (~12x10 km) with 4 instruments located within a 2.5 km radius of the observed dyke seismicity. Accurate automatic earthquake locations using a new detection and location algorithm QuakeMigrate[1] obtain an order of magnitude higher number of earthquakes than conventional location methods. For high precision locations, events are cross-correlated and then relatively relocated using GrowClust[2]. Here we present detailed earthquake location results from 18 September 2021 to 30 September 2022. This period comprises i) the 2021 post-eruptive seismicity along the 10 km long 2021 dyke path; ii) an earthquake swarm about 5 km NE of the eruption site at 5-7 km depth in October; iii) a 5 day-long dyke intrusion in December 2021 that failed to breach the surface; iv) a 5-day-long dyke intrusion that breached the surface on 3 August 2022, and led to a 6 week-long fissure eruption in Meradalir, located about 0.5 km NE of the 2021 eruption site.

We find that the failed dyke in December 2021 and the 2022 dyke that successfully breached the surface share many of the same features. They both propagated at similar depths of 3-6 km, in the pathway of the initial 2021 dyke and both show some sparser seismicity closer to the surface. The time span of their propagation is almost identical; both are propagating for around 5 days, with similar lengths of about 6 km, which is considerably shorter than the 10 km long 3-week 2021 dyke propagation. They differ, however, in their location with respect to the 2021 eruption site. The failed 2021 dyke intrusion propagated mainly SW of the 2021 eruption site, whereas the successful 2022 dyke propagated NE of it. Interestingly, our results suggest that during the initial phases of the 2022 dyke intrusion, two dykelets propagate in opposite directions simultaneously.

How to cite: Ágústsdóttir, T., Gudnason, E. Á., Magnússon, R. L., Fischer, T., Winder, T., Eibl, E. P. S., Glastonbury-Southern, E., Hersir, G. P., Horálek, J., Doubravová, J., Vlček, J., Hrubcová, P., Málek, J., Fojtíková, L., and Brandsdóttir, B.: Relative earthquake relocations and detailed evolution of failed and successful lateral dyke intrusions during the 2021-2022 Fagradalsfjall volcano-tectonic rifting event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13310, https://doi.org/10.5194/egusphere-egu23-13310, 2023.

Detailed investigation into local seismicity and geochemical analysis of erupted products from the 2021-22 Fagradalsfjall eruption has already provided new insights into the deep magma plumbing system beneath the Reykjanes Peninsular. Here we focus on producing a detailed regional-scale shear wave velocity model of the Reykjanes to provide wider scale crustal context for these results. Utilising seismic data from 105 stations operated by numerous groups on the peninsular from 2013 to present day, we use recordings of distant teleseismic earthquakes to observe P to s converted phases that provide insight into crustal structure through receiver function (RF) analysis. The total data set of nearly 3000 RFs is computed in several frequency bands. Small subsets of RFs from common backazimuths and epicentral distances displaying high waveform similarity are jointly inverted with surface wave dispersion measurements to produce approximately 300 individual velocity models across the area. These are migrated to depth within a 3D volume to define a single regional velocity model. Major interfaces such as the Moho and base of the upper crust are extracted to produce maps of peninsular wide variation. Computed velocity model inversion results are compared to  RF waveforms combined in multi-phase common conversion point stacks. We compare the velocity structure and interface depths extracted beneath Fagradalsfjall to magma depth estimates from geochemistry and potential structural changes hypothesised from local seismicity linked to the 2021-22 eruption.

How to cite: Jenkins, J., Greenfield, T., Rawlinson, N., Agustdottir, T., Hersir, G. P., Gudnason, E. Á., Horálek, J., Obermann, A., Dahm, T., and Milkerei, C.: Crustal context of the Fagradalsfjall eruption: shear-wave velocity structure of the Reykjanes Peninsular from receiver function analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3542, https://doi.org/10.5194/egusphere-egu23-3542, 2023.