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Kilauea Volcano's 2018 lower East Rift Zone (LERZ) eruption produced exceptionally high lava effusion rates and record-setting SO2 emissions. The eruption involved a diverse range of magmas, including primitive basalts sourced from Kilauea's summit reservoirs. We analyzed LERZ matrix glasses, melt inclusions, and host minerals to identify melt volatile contents and magma storage depths. The LERZ glasses and melt inclusions span nearly the entire compositional range previously recognized at Kilauea. Melt inclusions in Fo(86-89) olivine from the main eruptive vent (fissure 8) underwent 70-170 degrees C cooling during transport in LERZ carrier melts, causing extensive post-entrapment crystallization and sulfide precipitation. Many of these melt inclusions have low sulfur (400-900 ppm) even after correction for sulfide formation. CO2 and H2O vapor saturation pressures indicate shallow melt inclusion trapping depths (1-5 km), consistent with formation within Kilauea's Halema'uma'u and South Caldera reservoirs. Many of these inclusions also have degassed delta S-34 values (1.5 to -0.5%). Collectively, these results indicate that some primitive melts experienced near-surface degassing before being trapped into melt inclusions. We propose that decades-to-centuries of repeated lava lake activity and lava drain-back during eruptions (e.g., 1959 Kilauea Iki) recycled substantial volumes of degassed magma into Kilauea's shallow reservoir system. Degassing and magma recycling from the 2008-2018 Halema'uma'u lava lake likely reduced the volatile contents of LERZ fissure 8 magmas, resulting in lower fountain heights compared to many prior Kilauea eruptions. The eruption's extreme SO2 emissions were due to high lava effusion rates rather than particularly volatile-rich melts. U.S. Geological Survey (USGS) Volcano Science Center; Hawaiian Volcano Observatory (HVO); University of Hawaii-Hilo; HVO volunteer program; Department of Earth Sciences at the University of Oregon; Mineralogical Society of America; Geological Society of America; Mazamas student research grant program; National Science Foundation (NSF) Graduate Research Fellowship ProgramNational Science Foundation (NSF); NSF Graduate Research Internship Program (GRIP) Published version The authors would like to thank Matthew Loewen, Nicole Metrich, and Matt Patrick for constructive input that significantly improved this manuscript. The authors also thank the U.S. Geological Survey (USGS) Volcano Science Center, Hawaiian Volcano Observatory (HVO), University of Hawaii-Hilo, partner agencies, and the residents of Hawaii for support, field access, data sharing, and for their great care in documenting and responding to the 2018 LERZ eruption crisis. AHL thanks Tina Neal and the HVO volunteer program for support, Mike Zoeller for map assistance, and Carolyn Parcheta for collecting and sharing samples. Geochemical analyses were conducted with the help of John Donovan and Julie Chouinard (EPMA), and Brian Monteleone and Glenn Gaetani (SIMS and MI rehomogenization). AHL thanks Michelle Muth and Madison Myers for discussions on methodology and melt inclusion interpretation. AHL acknowledges funding support from Department of Earth Sciences at the University of Oregon, the Mineralogical Society of America, the Geological Society of America, the Mazamas student research grant program, the National Science Foundation (NSF) Graduate Research Fellowship Program, and the NSF Graduate Research Internship Program (GRIP). Coordination of GRIP at the USGS is through the Youth and Education in Science programs within the Office of Science Quality and Integrity. Public domain – authored by a U.S. government employee

Starting on 3 May 2018, a series of eruptive fissures opened in Kīlauea Volcano’s lower East Rift Zone (LERZ). Over the course of the next 3 months, intense degassing accompanied lava effusion from these fissures. Here, we report on ground-based observations of the gas emissions associated with Kīlauea’s 2018 eruption. Visual observations combined with radiative transfer modeling show that ultraviolet light could not efficiently penetrate the gas and aerosol plume in the LERZ, complicating SO2 measurements by differential optical absorption spectroscopy (DOAS). By applying a statistical method that integrates a radiative transfer model with the DOAS retrievals, we were able to calculate sulfur dioxide (SO2) emission rates along with estimates of their uncertainty. We find that sustained SO2 emissions were highest in June and early July, when approximately 200 kt SO2 were emitted daily. At the 68% confidence interval, we estimate that 7.1–13.6 Mt SO2 were released from the LERZ during the entire May to September eruptive episode. Scaling our results with in situ measurements of plume composition, we calculate that 11–21 Mt H2O and 1.5–2.8 Mt CO2 were also emitted. The gas and aerosol emissions caused hazardous conditions in areas proximal to the active vents, but plume dispersion modeling shows that the eruption also significantly impacted air quality hundreds of kilometers downwind. Combined with petrologic studies of the erupted lavas, our measurements indicate that 1.1–2.3 km3 dense-rock equivalent of lava were erupted from the LERZ, which is approximately twice the concomitant collapse volume of the volcano’s summit.

From 2009 to 2015 the U.S. Geological Survey (USGS) systematically monitored hydrothermal behavior at selected Cascade Range volcanoes in order to define baseline hydrothermal and geochemical conditions. Gas and water data were collected regularly at 25 sites on 10 of the highest-risk volcanoes in the Cascade Range. These sites include near-summit fumarole groups and springs/streams that show clear evidence of magmatic influence (high 3He/4He ratios and/or large fluxes of magmatic CO2 or heat). Site records consist mainly of hourly temperature and hydrothermal-flux data. Having established baseline conditions during a multiyear quiescent period, the USGS reduced monitoring frequency from 2015 to present. The archived monitoring data are housed at (doi:10.5066/F72N5088). These data (1) are suitable for retrospective comparison with other continuous geophysical monitoring data and (2) will provide context during future episodes of volcanic unrest, such that unrest-related variations at these thoroughly characterized sites will be more clearly recognizable. Relatively high-frequency year-round data are essential to achieve these objectives, because many of the time series reveal significant diurnal, seasonal, and inter-annual variability that would tend to mask unrest signals in the absence of baseline data. Here we characterize normal variability for each site, suggest strategies to detect future volcanic unrest, and explore deviations from background associated with recent unrest.

Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2-δ13C of −4.2‰ to −4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice-wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.

The active Lassen hydrothermal system includes a central vapor-dominated zone or zones beneath the Lassen highlands underlain by ~240 °C high-chloride waters that discharge at lower elevations. It is the best-exposed and largest hydrothermal system in the Cascade Range, discharging 41 ± 10 kg/s of steam (~115 MW) and 23 ± 2 kg/s of high-chloride waters (~27 MW). The Lassen system accounts for a full 1/3 of the total high-temperature hydrothermal heat discharge in the U.S. Cascades (140/400 MW). Hydrothermal heat discharge of ~140 MW can be supported by crystallization and cooling of silicic magma at a rate of ~2400 km3/Ma, and the ongoing rates of heat and magmatic CO2 discharge are broadly consistent with a petrologic model for basalt-driven magmatic evolution. The clustering of observed seismicity at ~4–5 km depth may define zones of thermal cracking where the hydrothermal system mines heat from near-plastic rock. If so, the combined areal extent of the primary heat-transfer zones is ~5 km2, the average conductive heat flux over that area is >25 W/m2, and the conductive-boundary length

Magma movement, fault structures and hydrothermal systems influence volatile emissions at rift volcanoes. Longonot is a Quaternary caldera volcano located in the southern Kenyan Rift, where regional extension controls recent shallow magma ascent. Here we report the results of a soil carbon dioxide (CO2) survey in the vicinity of Longonot volcano, as well as fumarolic gas compositions and carbon isotope data. The total non-biogenic CO2 degassing is estimated at − 1, and is largely controlled by crater faults and fractures close to the summit. Thus, recent volcanic structures, rather than regional tectonics, control fluid pathways and degassing. Fumarolic gases are characterised by a narrow range in carbon isotope ratios (δ13C), from − 4.7‰ to − 6.4‰ (vs. PDB) suggesting a magmatic origin with minor contributions from biogenic CO2. Comparison with other degassing measurements in the East African Rift shows that records of historical eruptions or unrest do not correspond directly to the magnitude of CO2 flux from volcanic centres, which may instead reflect the current size and characteristics of the subsurface magma reservoir. Interestingly, the integrated CO2 flux from faulted rift basins is reported to be an order of magnitude higher than that from any of the volcanic centres for which CO2 surveys have so far been reported.

The thermal output from the Yellowstone magma chamber can be estimated from the Cl flux in the major rivers in Yellowstone National Park; and by utilizing continuous discharge and electrical conductivity measurements the Cl flux can be calculated. The relationship between electrical conductivity and concentrations of Cl and other geothermal solutes (Na, SO4, F, HCO3, SiO2, K, Li, B, and As) was quantified at monitoring sites along the Madison, Gibbon, and Firehole Rivers, which receive discharge from some of the largest and most active geothermal areas in Yellowstone. Except for some trace elements, most solutes behave conservatively and the ratios between geothermal solute concentrations are constant in the Madison, Gibbon, and Firehole Rivers. Hence, dissolved concentrations of Cl, Na, SO4, F, HCO3, SiO2, K, Li, Ca, B and As correlate well with conductivity (R2 > 0.9 for most solutes) and most exhibit linear trends. The 2011 flux for Cl, SO4, F and HCO3 determined using automated conductivity sensors and discharge data from nearby USGS gaging stations is in good agreement with those of previous years (1983–1994 and 1997–2008) at each of the monitoring sites. Continuous conductivity monitoring provides a cost- and labor-effective alternative to existing protocols whereby flux is estimated through manual collection of numerous water samples and subsequent chemical analysis. Electrical conductivity data also yield insights into a variety of topics of research interest at Yellowstone and elsewhere: (1) Geyser eruptions are easily identified and the solute flux quantified with conductivity data. (2) Short-term heavy rain events can produce conductivity anomalies due to dissolution of efflorescent salts that are temporarily trapped in and around geyser basins during low-flow periods. During a major rain event in October 2010, 180,000 kg of additional solute was measured in the Madison River. (3) The output of thermal water from the Gibbon River appears to have increased by about 0.2%/a in recent years, while the output of thermal water for the Firehole River shows a decrease of about 10% from 1983 to 2011. Confirmation of these trends will require continuing Cl flux monitoring over the coming decades.

[1] Volcanic gases are sensitive indicators of subduction processes and are used to evaluate the contributions from various source components. Nitrogen isotope systematics in particular are a valuable tool for determining the fate of organic matter in subduction zones. We present the first arc-wide survey of trace gas chemistry and nitrogen isotope variations from the Sangihe Arc of northeastern Indonesia, where the narrow Molucca Sea Plate subducts beneath the Sangihe Arc to the west and the Halmahera Arc to the east. Relative volatile abundances and N isotopic compositions of volcanic gases show systematic along-arc variations. Northern volcanoes exhibit low N2/He ratios and δ15N values (northern minima 542 and −7.3‰, respectively), indicating minimal addition of sediment to source magmas. In contrast, the southern part of the arc is characterized by high N2/He and δ15N values (southern maxima 2000 and +2.1‰, respectively), consistent with greater sediment contributions in the formation of the magmas. These observations can be correlated with the complex tectonic setting of the region whereby oblique collision between the two arcs has caused sediment obduction, decoupling the accretionary wedges from the underlying oceanic plate. In the north, where the collision is more developed, the lack of trace gas and N isotope evidence of sedimentary inputs to the source of arc magmas is consistent with enhanced sediment decoupling. In the south, where collision and accretionary wedge decoupling are not yet taking place, sediments would presumably subduct normally, in agreement with higher N2/He and δ15N values. Awu volcano, at the northernmost extension of the arc, is anomalous and exhibits high N2/He (2852) coupled with low δ15N (−3.3‰). These values are suggestive of increased slab contribution in the northernmost arc, possibly by slab melting as collision stalls the progress of the subducting plate and allows it to become superheated.