Logo: to the web site of Uppsala University

Ocean island basalt (OIB) magmas are generally water poor and usually contain less than 1 wt.% of H2O. Explosive eruption styles are therefore rare. When explosive eruptions occur, they are thought to be driven by either volatile-enriched mantle sources or by gas segregation processes during magma differentiation. Here we report on crystal- and water-rich porphyritic basanites and ankaramites from El Hierro in the Canary Islands, Spain, that erupted inside the El Golfo giant landslide collapse embayment that formed at >= 39 ka. Using rock and mineral chemistry in combination with H2O contents of nominally anhydrous minerals (olivine and clinopyroxene), we show that despite their relatively primitive composition, the post-collapse ankaramites are not primary mantle melts. Instead, they record high crystal contents as well as unusually high water contents of up to 3.20 +/- 0.64 wt.% H2O, and likely represent a normally inaccessible snapshot of dense crystal-rich magma compositions that reside in the sub-island underplating zone. We hypothesize that their eruption was facilitated by sudden decompression from crustal unloading, implying that the El Golfo landslide may have affected the deeper portions of the plumbing system and triggered the ascent of volatile-rich, crystal-laden magmas from the underplating zone. We propose that some "wet" and explosive ocean island eruptions might result from the ascent of deep-seated water-rich magmas in the aftermath of vertical unloading and associated decompression.

Liquid immiscibility is one of the viable genetic models to generate carbonatites. Experimental studies have demonstrated that lighter Fe isotopes are enriched in carbonatite melts, whereas heavier Fe isotopes preferentially enter silicate melts during liquid immiscibility. However, this observation has not been substantiated by natural samples, and the mechanism behind Fe isotope fractionation during silicate-carbonatite immiscibility remains unclear. Here, we present high-precision Fe isotope data, combined with petrography, whole-rock elemental and Sr-Nd isotopic compositions, for ultramafic lamprophyres (UML) and carbonatites from the Alno<spacing diaeresis> complex in central Sweden, to elucidate the Fe isotope fractionation during silicate-carbonatite immiscibility processes. The presence of various carbonate spherules in UML, coupled with enrichments in Sr and Ba and depletion in high field strength elements in carbonatites, as well as their overlapping Sr-Nd isotope compositions, supports a petrogenetic relationship involving liquid immiscibility between the UML and carbonatites. The mean δ⁵⁷Fe of UML (0.16 ± 0.08 ‰) is higher than that of carbonatites (0.03 ± 0.04 ‰), with ∆⁵⁷Fe_sil−carb of 0.13 ‰ (± 0.05, 2SD By excluding the effects of low temperature alteration and magmatic processes, we conclude that silicate-carbonatite immiscibility imparts significant Fe isotope fractionation. This fractionation may be influenced by different Fe bond strengths provided by the distinct polymer networks of silicate and carbonatite melts, as well as the varying degrees of Fe enrichment in minerals and melts. This leads to light Fe isotopes being preferentially enriched in the carbonate melt, while heavy Fe isotopes become enriched in the coexisting silicate melt.

The study of recent eruptions in Ocean Islands (OIs) provides a unique window into the magma dynamics governing their plumbing systems and the mechanisms leading to eruptions. Here we present an integrated approach to unravel the dynamics of magmatic plumbing systems through detailed spatial, petrological, and geochemical characterisation of volcanic products ranging from crystal-rich ankaramitic lavas to trachytic tephras. We focus on the textural and geochemical spatial variations of 42 Holocene subaerial eruptions at the OI of El Hierro (Canary Islands), as well as on their petrogenetic significance for magmatic evolution and plumbing system architecture. Integrating geochemical data within fractional crystallisation modelling and mass balance calculations reveals that ankaramitic and porphyritic lavas with phenocryst modal abundances > 10 vol% result from melt extraction and crystal accumulation. Aphyric to sub-aphyric eruption products and porphyritic lavas with phenocryst modal abundances < 10 vol% usually follow fractional crystallisation trajectories that start at similar to 10 wt% MgO. Periodic extraction of evolved melt from crystal mushes likely led to the occurrence of minor trachytic eruptions, which are difficult to reconcile with simple closed system fractional crystallisation trends. A complex, heterogeneous crustal mush system beneath El Hierro is, in fact, the most reliable scenario to explain the wide range of textures, whole-rock and mineral compositions, and the overall surface distribution of vents and eruptive styles displayed by the Holocene volcanism on the island. Our integrated findings highlight the importance of a combined field, petrological, and geochemical study to decipher plumbing system dynamics of OI magmatism. The results allow us to put forward an updated conceptual model of the current plumbing architecture of El Hierro's volcanic system during the Holocene.

The Katla volcano is a bimodal caldera complex within Iceland's basalt-dominated Eastern Volcanic Zone. To unravel the petrogenesis of silica-rich rocks from Katla, we provide new δ¹⁸O values for almost 60 basaltic, intermediate, and high-silica eruptive rocks, including a number of partially melted felsic xenoliths. The basaltic samples display a range in bulk-rock δ¹⁸O values from +4.3 to +8.5‰ (n = 17) and the sparse intermediate samples from +4.1 to +5.9‰ (n = 3). In turn, silicic rock samples and feldspar separates range from +2.7 to +6.4‰ (n = 38), whereas felsic xenoliths yield the lowest values from −4.9 to −2.3‰ (n = 4). The majority (95%) of the Katla silicic volcanics have δ¹⁸O values below typical MORB (i.e., ≤5.0‰), ruling out an origin via closed-system fractional crystallization from the basaltic magmas. We utilized the new δ¹⁸O values to model possible assimilation and fractional crystallization (AFC) scenarios. The results indicate an early stage of FC/AFC at deep- to mid-crustal levels, followed by assimilation of low-δ¹⁸O hydrothermally altered sub-volcanic materials similar to the low-δ¹⁸O felsic xenoliths at shallow crustal levels. Such a two-stage magma evolution is consistent with available geophysical and geobarometry studies at Katla, indicating mid- to deep-crustal and shallow-crustal magma domains. Importantly, mafic rocks dominantly show MORB-like δ¹⁸O values, whereas low δ¹⁸O values occur essentially in silicic rocks only. This implies that the low-δ¹⁸O values at Katla are imposed by interaction with the Icelandic crust rather than reflecting low δ¹⁸O mantle sources.

Hydrothermal alteration and mineralization processes can affect the physical and chemical properties of volcanic rocks. Aggressive acidic degassing and fluid flow often also lead to changes in the appearance of a rock, such as changes in surface coloration or intense bleaching. Although hydrothermal alteration can have far-reaching consequences for rock stability and permeability, limited knowledge exists on the detailed structures, extent, and dynamic changes that take place near the surface of hydrothermal venting systems. By integrating drone-based photogrammetry with mineralogical and chemical analyses of rock samples and surface gas flux, we investigate the structure of the evolving volcanic degassing and alteration system at the La Fossa cone on the island of Vulcano, Italy. Our image analysis combines principal component analysis (PCA) with image classification and thermal analysis through which we identify an area of approximately 70 000 m(2) that outlines the maximum extent of hydrothermal alteration effects at the surface, represented by a shift in rock color from reddish to gray. Within this area, we identify distinct gradients of surface coloration and temperature that indicate a local variability in the degassing and alteration intensity and define several structural units within the fumarole field. At least seven such larger units of increased activity could be constrained. Through mineralogical and geochemical analysis of samples from the different alteration units, we define a relationship between surface appearance in drone imagery and the mineralogical and chemical composition. Gradients in surface color from reddish to gray correlate with a reduction in Fe2O3 from up to 3.2 % in the unaltered regime to 0.3 % in the altered regime, and the latter coincides with the area of increased diffuse acid gas flux. As the pixel brightness increases towards higher alteration gradients, we note a loss of the initial (igneous) mineral fraction and a change in the bulk chemical composition with a concomitant increase in sulfur content from close to 0 % in the unaltered samples to up to 60 % in samples from the altered domains. Using this approach of combined remote-sensing and in situ analyses, we define and spatially constrain several alteration units and compare them to the present-day thermally active surface and degassing pattern over the main crater area. The combined results permit us to present a detailed anatomy of the La Fossa fumarole field, including high-temperature fumaroles and seven larger units of increased alteration intensity, surface temperature, and variably intense surface degassing. Importantly, we also identify apparently sealed surface domains that prevent degassing, likely as a consequence of mineral precipitation from degassing and alteration processes. By assessing the thermal energy release of the identified spatial units quantitatively, we show that thermal radiation of high-temperature fumaroles accounts for < 50 % of the total thermal energy release only and that the larger part is emitted by diffuse degassing units. The integrated use of methods presented here has proven to be a useful combination for a detailed characterization of alteration and activity patterns of volcanic degassing sites and has the potential for application in alteration research and for the monitoring of volcanic degassing systems.

Many continental large igneous provinces coincide with climate perturbations and mass extinctions. When basaltic plumbing systems traverse carbon-rich sedimentary rocks, large volumes of greenhouse gases may be generated. We document how intrusive sills of the Mesozoic High Arctic Large Igneous Province affected surrounding fine-grained, organic-rich siliciclastic rocks of the Sverdrup Basin in the Canadian Arctic Archipelago. Petrographic and X-ray diffraction data from samples located near sills show the presence of high-temperature metamorphic phases (diopside, andalusite, garnet, and cordierite). Raman thermometry on organic matter yields peak temperatures of 385-400 degrees C near sill contacts, tailing off to far-field temperatures of <= 230 degrees C. Samples located >20 m from sills show no systematic change in vitrinite reflectance and have a VRo eq% value of similar to 2.5%, which indicates a temperature of similar to 210 degrees C. The finite element thermal modeling tool SUTRAHEAT was applied to the 17-m-thick Hare Sill, emplaced at 3 km depth at 1105 degrees C. SUTRAHEAT results show that contact-proximal rocks attain temperatures of >700 degrees C for a brief period (similar to 1 year). By 5 years, the Hare Sill is completely solidified (<730 degrees C), and the temperature anomaly collapses rapidly thereafter as the thermal pulse propagates outward. By 10 years, all rocks within 10 m of the Hare Sill are between 450 degrees C and 400 degrees C, rocks at 20 m from the contact attain 200 degrees C, yet far-field temperatures (>50 m) have barely changed. When multiple sills are emplaced between 4 km and 6 km depth, all rocks between sills reach similar to 250 degrees C after 100 years, showing that it is possible to raise regional-scale background temperatures by similar to 150 degrees C for the observed High Arctic Large Igneous Province sill density. Vitrinite reflectance data and pyrolysis results, together with SILLi thermal modeling, indicate that much of the hydrocarbon-generating potential was eliminated by High Arctic Large Igneous Province intrusions. The SILLi model yields similar to 20 tonnes/m(2) of organic equivalent CO2 (all carbon gas is reported as CO2) from the Hare Sill alone when emplaced into Murray Harbour Formation rocks with 5.7 wt% organic carbon, and similar to 226 tonnes/m(2) by emplacement of multiple sills throughout the 2-km-thick Blaa Mountain Group with 3 wt% organic carbon. On a basin scale, this yields a total of similar to 2550 Gt CO2 from the Hare Sill, with similar to 13,000 Gt CO2 being generated by the multiple sill scenario, similar to estimates from other large igneous provinces. Much of the Blaa Mountain Group rocks now have organic carbon contents of <1 wt%, which is consistent with large volumes of carbon-species gas having been generated, likely a mixture of CO2, CH4, and other species. However, organic-rich Murray Harbour Formation rocks show no obvious reduction in organic carbon content toward the Hare Sill intrusive contacts, which suggests that not all of the carbon was lost from the sedimentary package hosting High Arctic Large Igneous Province magmas. We suggest that some of the gas generated by contact metamorphism failed to drain out for lack of high-permeability conduits, and then back-reacted to form calcite cements and pyrobitumen during cooling.

Active basaltic eruptions enable time-series analysis of geochemical and geophysical properties, providing constraints on mantle composition and eruption processes(1-4). The continuing Fagradalsfjall and Sundhnukur fires on Iceland's Reykjanes Peninsula, beginning in 2021, enable such an approach(5,6). Earliest lavas of this volcanic episode have been interpreted to exclusively reflect a change from shallow to deeper mantle source processes(7). Here we show using osmium (Os) isotopes that the 2021 Fagradalsfjall lavas are both fractionally crystallized and strongly crustally contaminated, probably by mid-ocean-ridge gabbros and older basalts underlying the Reykjanes Peninsula. Earliest eruptive products (¹⁸⁷Os/¹⁸⁸Os ≤ 0.188, platinum (Pt)/iridium (Ir)≤ 76) are highly anomalous for Icelandic lavas or global oceanic basalts and Os isotope ratios remain elevated throughout the 2021 eruption, indicating a continued but diluted presence of contaminants. The 2022 lavas show no evidence for contamination (¹⁸⁷Os/¹⁸⁸Os=0.131, Pt/Ir=30), being typical of Icelandic basalts (0.1320±007). Initiation of the Fagradalsfjall Fires in 2021 involved pre-eruptive stalling, fractional crystallization and crustal assimilation of earliest lavas. An established magmatic conduit system in 2022 enabled efficient magma transit to the surface without crustal assimilation.

The 2021 La Palma eruption (Tajogaite) was unprecedented in magnitude, duration, and degree of monitoring compared to historical volcanism on La Palma. Here, we provide data on melt inclusions in samples from the beginning and end of the eruption to compare the utility of both melt and fluid inclusions as recorders of magma storage. We also investigated compositional heterogeneities within the magmatic plumbing system. We found two populations of olivine crystals: a low Mg# (78–82) population present at the beginning and end of eruption, recording the maximum volatile contents (2.5 wt % H₂O, 1,800 ppm F, 700 ppm Cl, 3,800 ppm S) and a higher Mg# (83–86) population sampled toward the end of the eruption, with lower volatile contents. Despite their host composition, melt inclusions share the same maximum range of CO₂ concentrations (1.2–1.4 wt %), indicating olivine growth and inclusion capture at similar depths. Overall, both melt and fluid inclusions record similar pressures (450–850 MPa, ∼15–30 km), and when hosted in the same olivine crystal pressures are indistinguishable within error. At these mantle pressures, CO₂ is expected to be an exsolved phase explaining the similar range of CO₂ between the two samples, but other volatile species (F, Cl, S) behave incompatibly, and thus, the increase between the two olivine populations can be explained by fractional crystallization prior to eruption. Finally, based on our new data, we provide estimates on the total volatile emission of the eruption.

The Reykjanes Peninsula (RP) hosts several volcanic lineaments that have been periodically active over the last 4000 years. Since 2021, following a ca. 800-year quiescence, eight eruptions have occurred on the RP, with more expected in the future. To better understand the origins of this renewed volcanism and help forecast future eruptions, we examine (i) if the ongoing volcanism is fed from a single or multiple magma storage zone(s) or from several smaller reservoirs and; (ii) where the zone(s) are located (i.e. mantle or lower or upper crustal depths). Using major and trace element geochemistry, oxygen isotopes, and seismic tomography we rule out a single, RP-scale, deep-seated magma storage zone. Instead we propose the presence of a ca. 10-km-wide region of crustal-level (9–12 km) magma accumulation beneath the Fagradalsfjall volcanic lineament that fed both the 2021–23 eruptions of the Fagradalsfjall Fires and the 2023–24 eruptions of the Sundhnúkur Fires.

The 2021 La Palma eruption provided an unpreceded opportunity to test the relationship between earthquake hypocenters and the location of magma reservoirs. We performed density measurements on CO2-rich fluid in-clusions (FIs) hosted in olivine crystals that are highly sensitive to pressure via calibrated Raman spectroscopy. This technique can revolutionize our knowledge of magma storage and transport during an ongoing eruption, given that it can produce precise magma storage depth constraints in near real time with minimal sample prep-aration. Our FIs have CO2 recorded densities from 0.73 to 0.98 g/cm3, translating into depths of 15 to 27 km, which falls within the reported deep seismic zone recording the main melt storage reservoir.