Volatile metal emissions from volcanic degassing and lava-seawater interactions at Kīlauea Volcano, Hawai'i

Volcanoes are major natural sources of metals to Earth's surface, impacting the biosphere as pollutants or nutrients. Volcanic emissions, particularly during eruptions, can release significant amounts of metals like cadmium (Cd), copper (Cu), and zinc (Zn), sometimes exceeding daily anthropogenic emissions. Basaltic volcanoes often release plumes into the troposphere, affecting both proximal and distal populations. Ocean island volcanoes, like Kīlauea, experience additional emissions from lava-seawater interactions, creating acidic "laze" plumes with significant marine biosphere implications. The 2018 Kīlauea eruption provided a unique opportunity to study both magmatic and laze plume emissions. Before April 30, 2018, activity was stable at the summit and East Rift Zone (ERZ). However, the Pu'u 'O'o vent collapsed, and magma propagated down the lower ERZ, opening fissures and leading to lava flows reaching the coast on May 23, 2018, creating a substantial laze plume. The resulting SO2 emission rates were significantly higher than previous averages, increasing population exposure to poor air quality. This study aimed to quantify the major and trace element compositions of these plumes, model the speciation of elements during degassing, and determine the origin of the laze plume's unique composition.

Previous research has investigated volcanic metal emissions at various volcanoes globally, dating back to the 1960s and 70s. Studies have explored the abundance and speciation of metals in volcanic gases and aerosols, though this remains poorly understood. The impact of lava-seawater interactions on the biosphere has also been studied, with a focus on the production of HCl gas from laze plumes, yet the detailed chemistry of laze plumes remains under-researched compared to magmatic plumes. Prior research on Kilauea has provided some insights into volatile trace element emissions, but the 2018 eruption offered a unique scale of emissions for more in-depth analysis and comparison.

Ground-based and Unoccupied Aircraft System (UAS) platforms were used to collect samples from both the magmatic and laze plumes during the 2018 Kīlauea eruption. Ground-based Multi-GAS systems simultaneously measured CO2, SO2, H2S, pressure, temperature, and relative humidity. Filter packs and cascade impactors collected simultaneous gas and size-segregated particulate matter (PM) samples in both plumes. UAS platforms were employed for aerial sampling, with the filter pack and cascade impactor flown on separate flights to maximize data collection and minimize payload limitations. Background atmospheric measurements were also taken in 2019 after the eruption. Samples were processed in a clean-lab environment, with PM extracted using Milli-Q water and propan-2-ol, followed by acid reflux for analysis. Gas filters were extracted using MQ water and H2O2. IC and ICP-MS/OES were employed for analysis. SO2 emission rates were measured using a PiSpec instrument. Trace element fluxes were estimated using measured X/SO2 ratios and SO2 emission rates. An ash correction was applied to determine concentrations of elements in the non-silicate aerosol phase, employing a matrix glass composition. Emanation coefficients (ε) were calculated using element-to-sulfur ratios and an estimated total sulfur degassed during sub-aerial eruption. Equilibrium chemistry modeling using HSC Chemistry's Gibbs Energy Minimisation module was applied to model gas phase speciation of the magmatic plume under various atmospheric mixing ratios and initial HCl gas concentrations.

Emanation coefficients were calculated to distinguish between volatile (ε > 0.001%) and refractory elements (ε < 0.001%). Size-segregated concentrations in the magmatic plume indicated that S and Cl were primarily present as gases, with a small portion adsorbed onto ash. Refractory elements were largely associated with coarse particles (ash), while volatile elements were concentrated in smaller particles consistent with nucleation mode formation. The relative abundances of volatile trace elements in the 2018 magmatic plume agreed closely with those observed in previous Kilauea eruptions, indicating a consistent trace metal fingerprint across eruptive periods, with variations in the size distribution of elements. Speciation modeling of the magmatic plume highlighted the importance of the S2− ligand in highly volatile metal/metalloid degassing at the magmatic vent. The laze plume's composition differed significantly from the magmatic plume. While elements abundant in seawater had similar ratios in the laze plume, other volatile and refractory elements showed enrichments of 1–6 orders of magnitude. Correction for silicate contributions from ash partially explained some of the elevated refractory elements. Moderately to highly volatile elements (Cd, Zn, Ag, Cu, Bi, Re) remained elevated even after the correction, indicating degassing from ocean entry lavas, primarily facilitated by the formation of chloride complexes. Speciation modeling, with increased HCl gas concentrations to mimic the laze plume, showed that these elements form chloride complexes, particularly at high chloride concentrations, potentially explaining their enrichment in the laze plume. In contrast, elements that complex with sulfide were absent or at low concentrations in the laze plume, likely due to the prior degassing of sulfur during the sub-aerial eruption.

The contrasting compositions of the magmatic and laze plumes emphasize the impact of lava-seawater interactions on volcanic emissions. The Kīlauea 2018 eruption's data provide valuable analogues for understanding gas and PM emissions during major volcanic events in Earth's history, such as flood basalt eruptions. Scaling up the results to estimate volatile trace element emissions from large igneous provinces (LIPs), like the Deccan Traps, suggests potentially massive releases of metals and metalloids, exceeding present-day anthropogenic emissions in some cases. Laze plumes, through their direct transfer of elements to the marine biosphere, may have played a crucial role in transferring volcanogenic metals to the environment and biosphere during past large eruptions. The high Cu emission rate from the laze plume could potentially be greater than those from the magmatic plume. The mechanism for the enhanced degassing of metal chlorides at the ocean entry remains speculative. It might be attributed to the high chlorine concentrations from boiling seawater stabilizing metal chloride complexes at high temperatures.

This study demonstrates the contrasting compositions of Kīlauea's magmatic and laze plumes during the 2018 eruption. The findings highlight the importance of both magmatic degassing and lava-seawater interactions as sources of volatile metal emissions, with implications for past and future large-scale volcanic events. The unique metal signature of laze plumes underscores their potential impact on the marine biosphere. Future research could focus on improving the accuracy of speciation modeling, quantifying the effect of changes in the melt oxidation state on gas speciation, and further investigating the mechanisms by which late-stage degassing is enhanced at chlorine-rich ocean entries.

The study's reliance on limited non-saturated ground-based samples near Fissure 8 may affect the overall representation of the magmatic plume's composition. The ash correction relies on assumptions about the composition of the ash and the absence of elements in the aerosol phase. The simplification in the speciation model, assuming equilibrium conditions, might not fully account for kinetic effects. Further, some element concentrations were near the detection limits of the instrumentation, leading to potential uncertainties in the calculated values. Finally, the effect of rotor turbulence or thermal updrafts on the measurement of particle size distribution from UAS samples has not been fully constrained.