Hydrogen sulfide (H2S), an environmentally harmful pollutant, is a byproduct of geothermal energy production. To reduce the H2S emissions, H2S-charged water is injected into the basaltic subsurface, ...where it mineralizes to iron sulfides. Here, we couple geophysical induced polarization (IP) measurements in H2S injection wells and geochemical reactive transport models (RTM) to monitor the H2S storage efforts in the subsurface of Nesjavellir, one of Iceland’s most productive geothermal fields. An increase in the IP response after 40 days of injection indicates iron-sulfide formation near the injection well. Likewise, the RTM shows that iron sulfides readily form at circumneutral to alkaline pH conditions, and the iron supply from basalt dissolution limits its formation. Agreement in the trends of the magnitude and distribution of iron-sulfide formation between IP and RTM suggests that coupling the methods can improve the monitoring of H2S mineralization by providing insight into the parameters influencing iron-sulfide formation. In particular, accurate fluid flow parameters in RTMs are critical to validate the predictions of the spatial distribution of subsurface iron-sulfide formation over time obtained through IP observations. This work establishes a foundation for expanding H2S sequestration monitoring efforts and a framework for coupling geophysical and geochemical site evaluations in environmental studies.
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IJS, KILJ, NUK, PNG, UL, UM
Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport
, important characteristics of mid-ocean ridge magmatism
. A consequence ...of such shallow crustal processing of magmas
is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust
. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 10
-10
m
of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Mercury (Hg) is naturally released by volcanoes and geothermal systems, but the global flux from these natural sources is highly uncertain due to a lack of direct measurements and uncertainties with ...upscaling Hg/SO2 mass ratios to estimate Hg fluxes. The 2021 and 2022 eruptions of Fagradalsfjall volcano, southwest Iceland, provided an opportunity to measure Hg concentrations and fluxes from a hotspot/rift system using modern analytical techniques. We measured gaseous Hg and SO2 concentrations in the volcanic plume by near-source drone-based sampling and simultaneous downwind ground-based sampling. Mean Hg/SO2 was an order of magnitude higher at the downwind locations relative to near-source data. This was attributed to the elevated local background Hg at ground level (4.0 ng m−3) likely due to emissions from outgassing lava fields. The background-corrected plume Hg/SO2 mass ratio (5.6 × 10−8) therefore appeared conservative from the near-source to several hundred meters distant, which has important implications for the upscaling of volcanic Hg fluxes based on SO2 measurements. Using this ratio and the total SO2 flux from both eruptions, we estimate the total mass of gaseous Hg released from the 2021 and 2022 Fagradalsfjall eruptions was 46 ± 33 kg, equivalent to a flux of 0.23 ± 0.17 kg d−1. This is the lowest Hg flux estimate in the literature for active open-conduit volcanoes, which range from 0.6 to 12 kg d−1 for other hotspot/rift volcanoes, and 0.5–110 kg d−1 for arc volcanoes. Our results suggest that Icelandic volcanic systems are fed from an especially Hg-poor mantle. Furthermore, we demonstrate that the aerial near-source plume Hg measurement is feasible with a drone-based active sampling configuration that captures all gaseous and particulate Hg species, and recommend this as the preferred method for quantifying volcanic Hg emissions going forward.
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•Lowest mercury concentrations and fluxes reported for any active volcano on Earth•Iceland volcanic systems appear to be sourced from a particularly Hg-poor mantle.•Simultaneous near-vent and downwind data suggest Hg/SO2 is conserved over distance.•First drone-based measurements of plume gaseous mercury above an erupting volcano•Drone-based volcanic mercury sampling now feasible and should be the preferred method.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Reconstructing the original geometry of a high‐pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, ...nappe‐internal thrusts may disrupt it into several subunits. The Middle‐CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans‐Cycladic‐Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi‐petrological and geochronological data from the Middle‐CBU nappe from the Thera and Ios islands (Greece). Using Zr‐in‐rutile thermometry coupled with quartz‐in‐garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium‐Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle‐CBU nappe occurred during the Oligocene at different rates for different localities. The Middle‐CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle‐CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle‐CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•Mass changes of non-volatile elements upon hydrothermal alteration is overall isochemical.•Extent metasomatism was largely dependent on the alteration mineralogy, protolith and fluid composition and ...temperature of the system.•Large mass changes in volatile elements (C, S) resulted from degassing of a magmatic source leading to formation of carbonates and sulfides.•Active hydrothermal systems such as Nesjavellir could give insights on global volatile budgets from mantle degassing and natural sequestration.
The lithogeochemistry of altered basalts was investigated to assess elemental mass changes in the active meteoric water dominated geothermal system at Nesjavellir, SW Iceland. The study revealed that elemental mass changes upon hydrothermal alteration is rather limited and the extent of metasomatism is controlled by the type of alteration minerals. Large mass movement was detected in volatiles (C and S), likely derived from magma degassing. The geothermal system at Nesjavellir may thus provide an ideal locality to investigate the natural sequestration of deep-sourced volatiles and deliver insights in quantifying the input of mantle degassing on the budget of geothermal systems.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH4 in active ...hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO2 (2.60–184 mmol/kg), CH4 (2.39·10−4–0.325 mmol/kg), dissolved organic carbon (4.78·10−3–0.112 mmol/kg), and CO (1.89·10−6–4.16·10−4 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO2 (δ13C-CO2: −4.80 to −1.50 ‰, CO2/3He: 1.49·109–4.14·1010 14C-CO2: 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO2, CH4, CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH4 concentrations vary from lower, to significantly higher than those calculated using CO2 reduction kinetics. The isotope composition (δD-CH4: −172 to −138 ‰, δ13C-CH4: −32.0 to −24.6 ‰; 14C-CH4: 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH4 originates from a radiocarbon inactive source, i.e. mantle CH4, reduction of mantle CO2 and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH4 do not match those expected for mantle-derived CH4 as well as values generated from reduction of mantle-derived CO2. Instead, differences in δD-CH4 and δ13C-CH4 values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO2 source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH4 and δ13C-CH4 values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH4 in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH4 concentrations, may contain a significant portion of CH4 that may originate from CO2 reduction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
