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Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park

Deborah Bergfeld, Jennifer L. Lewicki, Sara E. Peek, Andrew G. Hunt, E.T. Allen, A.L. Day · U.S. Geological Survey

USGS Publications · Papers · License: Public Domain

center chemistry data filereport gas monitoring national park

usgs, geological survey, united states, science, government publication, usgs open-file report, 2025, 1056, gas, chemistry, isotope, data, volcano, monitoring, lassen, volcanic, center, national, park

Open-File Report 2025–1056

U.S. Department of the Interior U.S. Geological Survey

Cover: Steam rising from a super-heated fumarole at Upper Sulphur Works, Lassen Volcanic National Park, August 18, 2015. Photograph by Deborah Bergfeld, U.S. Geological Survey.

Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park By Deborah Bergfeld, Jennifer L. Lewicki, Sara E. Peek, and Andrew G. Hunt

Open-File Report 2025–1056

U.S. Department of the Interior U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2025

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov or call 1–888–392–8545. For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/ or contact the store at 1–888–275–8747. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Bergfeld, D., Lewicki, J.L., Peek, S.E., and Hunt, A.G., 2025, Gas chemistry and isotope data for volcano monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park: U.S. Geological Survey Open-File Report 2025–1056, 23 p., https://doi.org/10.3133/ofr20251056. Associated data for this publication: Lewicki, J.L., Bergfeld, D., Peek, S., Evans, W.C., Ingebritsen, S.E., Clor, L., and Hunt, A.G., 2024, Chemical and isotopic compositions of gases from volcanic and geothermal areas in California: U.S. Geological Survey data release, https://doi.org/10.5066/P9W29CON. ISSN 2331-1258 (online)

iii

Acknowledgments We thank Michael Magnuson, wildlife biologist at Lassen Volcanic National Park, for supporting our work. Without his help, many of the samples reported in this study would not have been collected. We thank Mike Clynne and Patrick Muffler of U.S. Geological Survey for their invaluable advice on historic sample locations. Joel Robinson of U.S. Geological Survey provided the base map used in figure 1. Recent samples from the Lassen Peak hot spot were part of a monitoring effort sustained through the work of Steve Ingebritsen (U.S Geological Survey). Helpful reviews were provided by William Evans and Patrick Muffler.

Contents Acknowledgments��iii Abstract��1 Introduction��1 Background��3 Field and Laboratory Methods��3 Results��4 Bulk Gas Contents��4 Carbon and Helium Isotope Compositions��5 Discussion��20 Sources of Carbon Dioxide and Helium��20 Overview of Circulation Cell 1 Gas and Implications for Monitoring��20 Summary��21 References Cited��21

Figures 1. 2.

4. 5.

Map showing generalized gas sample locations within Lassen Volcanic National Park and the Lassen Volcanic Center��2 Ternary diagram showing relative concentrations of nitrogen (N2), helium (He), and argon (Ar) for samples collected in Giggenbach-type bottles at Lassen Volcanic National Park between 2002 and 2014��14 Ternary diagram showing relative concentrations of carbon dioxide (CO2), hydrogen sulfide (H2S), and nitrogen (N2) for samples collected in Giggenbach-type bottles at Lassen Volcanic National Park between 2002 and 2019��15 Plot showing δ13C-CO2 values for gases collected in EE bottles at Lassen Volcanic National Park between 2004 and 2019��16 Plot showing air-corrected helium isotope values for gas collected at Lassen Volcanic National Park between 1974 and 2019��19

Tables 1.

Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in caustic solution from thermal features in Lassen Volcanic National Park, 2002–2019��6 2. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in empty evacuated bottles from thermal features in Lassen Volcanic National Park, 2004–2019��10 3. Field parameters, helium-3 (3He) to helium-4 (4He) (3He/4He) ratios, and carbon isotope results for gas samples collected from thermal features in Lassen Volcanic National Park, 1974–2019��17

Conversion Factors International System of Units to U.S. customary units

Multiply

To obtain

Length meter (m)

3.281

foot (ft)

kilometer (km)

0.6214

mile (mi)

meter (m)

1.094

yard (yd)

Area square kilometer (km2)

247.1

square kilometer (km2)

0.3861

gram (g)

0.03527

Acre square mile (mi2)

Mass ounce, avoirdupois (oz)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F = (1.8 × °C) + 32.

Datums Vertical coordinate information is referenced to the World Geodetic System of 1984 (WGS 84). Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84). Elevation, as used in this report, refers to distance above mean sea level.

Abbreviations ASMW

air-saturated meteoric water

CC1

circulation cell 1

CC2

circulation cell 2

empty evacuated

LVC

Lassen Volcanic Center

not determined

per mil

parts per thousand (‰)

R/RA

measured 3He/4He isotope ratio normalized to the ratio in air

RC/RA

R/RA value corrected for potential air contamination

USGS

U.S. Geological Survey

VPDB

Vienna Pee Dee Belemnite

mol fraction of dry gas relative to the mol fraction of dry gas plus steam

Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park By Deborah Bergfeld, Jennifer L. Lewicki, Sara E. Peek, and Andrew G. Hunt

Abstract

Introduction

This report presents chemical and isotopic compositions of volcanic gases collected from thermal areas within Lassen Volcanic National Park in northern California from 1974 through 2019. As the southernmost volcano in the Cascade Range and designated a very-high-threat volcano by the U.S. Geological Survey, the Lassen Volcanic Center (LVC) requires consistent monitoring to assess potential volcanic hazards. In 2014, the California Volcano Observatory established a gas geochemical monitoring program at LVC to provide baseline data to evaluate future changes. Results demonstrate consistent spatial patterns in bulk gas chemistry that support a two-circulation-cell hydrothermal model previously established for LVC. Gas samples from circulation cell 1 thermal areas have higher helium isotope ratios (6.59–7.50 times the air value) than those from circulation cell 2 (5.86–6.52 times the air value), indicating a stronger magmatic signature. The Sulphur Works and Pilot Pinnacle thermal areas within circulation cell 1 consistently emit gases with the highest magmatic helium contents, suggesting gas at these areas best represents conditions in the underlying volcanic system. A slight decrease in helium isotope values since 1974 may indicate progressive dilution of magmatic helium-3 (3He) by radiogenic helium-4 (4He) in the absence of recent magma intrusion. Carbon isotope compositions of carbon dioxide across all thermal areas are relatively uniform (−9.7–−7.3 per mil), falling within the range observed at other Cascade Range volcanoes. Based on gas geochemical characteristics and site accessibility, the Sulphur Works and Pilot Pinnacle thermal areas represent optimal targets for continued monitoring of the LVC magmatic-hydrothermal system. This study includes the most comprehensive helium isotope dataset collected at LVC currently available and establishes critical baseline data for future volcanic monitoring efforts.

The 825,000-year-old Lassen Volcanic Center (LVC) in northern California is at the southernmost part of the Cascade Range (fig. 1), where volcanism results from the oceanic Juan de Fuca plate subducting beneath the continental North American plate. The LVC includes an early caldera complex, a composite volcano, and a domefield, and is distributed over approximately 500 square kilometers. The LVC is the youngest of five intermediate-to-silicic volcanic centers in the region that have erupted over the past 3.5 million years and is the only active center (Clynne and Muffler, 2010). The LVC experienced three Holocene eruptions, with the most recent being a series of eruptions at Lassen Peak from 1914 through 1917 (Clynne and others, 2014; Muffler and Clynne, 2015). The Center hosts a long-lived hydrothermal system indicated by thermal areas south of Lassen Peak (fig. 1). Designated a very-high-threat volcano by the U.S. Geological Survey (USGS), LVC threatens the region with a variety of volcanic hazards (Ewert and others, 2005; Clynne and others, 2012; Ewert and others, 2018). As such, the USGS California Volcano Observatory initiated a regular gas geochemical monitoring program at LVC in 2014, designed to provide a baseline of chemical and isotopic data relevant to assessing change in the context of potential unrest. This report presents the chemical and isotopic compositions of gas samples collected from 1974 to 2019 from LVC’s major thermal areas, with focus on data collected as part of the California Volcano Observatory geochemical monitoring program and available in Lewicki and others (2024a). For completeness, we also include data from two samples collected in 2002 (Janik and Bergfeld, 2010). These data include the most comprehensive helium isotope record ever collected at LVC. We assess these data in the context of earlier work at LVC (Craig and others, 1978; Janik and McLaren, 2010), with a focus on the conceptual model of LVC’s hydrothermal system, temporal variations that could indicate changes in the magma-hydrothermal system, and implications for future monitoring strategies.

630000

6350000

EXPLANATION

Lassen Peak hot spot

CANADA

Southern border of Lassen Volcanic National Park Lassen Peak

s s

WASHINGTON

Highway 89

Trail

Monitoring site within the primary thermal areas 89 Reading Peak

Pilot Pinnacle

s ss s s

Monitoring site external to the primary thermal areas

OREGON

Cascade Range volcanoes

CALIFORNIA Lassen Peak

4480000

Bumpass Hell Sulphur Works

Little Hot Springs Valley

Devils Kitchen Boiling Springs Lake

Terminal Geyser 4475000

Base from U.S. Geological Survey digital data, 2020 and 2022 Universal Transverse Mercator, zone 10 north World Geodetic System of 1984

0 0

1 1

2 MILES 2 KILOMETERS

Figure 1. Map showing generalized gas sample locations within Lassen Volcanic National Park and the Lassen Volcanic Center. Red circles are monitoring sites within the primary thermal areas. Blue circles are other gas sample sites, external to the main thermal areas. Thick black line is highway 89, thin black lines are trails, dashed green line is the southern border of the park. Inset shows the location of Lassen Peak relative to other Cascade Range volcanoes (triangles).

2 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park

6250000

Field and Laboratory Methods 3

Background Early studies of the LVC’s thermal features were reported by Waring (1915) and Day and Allen (1925). Later advancements in analytical methods enhanced the understanding of noble-gas and stable-isotope systematics within the volcanic system. Deuterium and oxygen isotope data revealed that the thermal fluids originate from locally sourced meteoric water from high elevations, such as Lassen Peak (Janik and others, 1983). Carbon and sulfur isotope data indicated the presence of magmatic and metamorphic contributions to the hydrothermal system (Janik and others, 1983). These data enabled the development of a model describing fluid pathways and temperatures within the hydrothermal system. An early model of Lassen’s hydrothermal system, detailed by Muffler and others (1982), Janik and others (1983), and Truesdell and others (1983), described a single hydrothermal system recharged by cold meteoric water infiltrating the subsurface. The water, heated conductively by proximity to young silicic intrusions, reaches temperatures of about 235–240 degree Celsius (°C). This buoyant hot water rises and boils, creating a vapor cap above a liquid thermal reservoir. The main upflow of gas and steam occurs at Bumpass Hell with additional steam discharge at acid-sulfate thermal areas: Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, Devils Kitchen, Boiling Springs Lake, and Terminal Geyser (fig. 1). Low gas-to-steam ratios at Devils Kitchen, Boiling Springs Lake, and Terminal Geyser suggested a west-to-east direction of flow. An outflow plume of neutral-pH, high-chloride water discharges at the lower-elevation Growler and Morgan Hot Springs. Later work by Janik and McLaren (2010) integrated regional seismic data with isotope and compositional data on gas and steam collected from superheated fumaroles between 1976 and 2000. That research produced a refined model of the hydrothermal system that describes two distinct hydrothermal cells. The hottest circulation cell (circulation cell 1; CC1) is recharged from precipitation on the southwest flank of Lassen Peak and reaches temperatures of 235–270°C. The most vigorous upflow of gas supplies features at Bumpass Hell and Little Hot Springs Valley, whereas cooler, less-boiled gas discharges at Sulphur Works and Pilot Pinnacle. A degassed plume of thermal water from CC1 outflows and discharges at Growler and Morgan Hot Springs. A second circulation cell (circulation cell 2; CC2) is recharged from precipitation on Reading Peak and the southeast side of Lassen Peak. Reservoir temperatures in CC2 are between 220 and 240°C. This second reservoir is the source of gas and steam that discharges at Boiling Springs Lake and Devils Kitchen. The outflow of degassed water from CC2 boils a second time and discharges at Terminal Geyser. Throughout this report, sampled features are described as fumaroles, frying pans, steaming ground, hot springs, mud pots, and pools. For the purposes of this report fumaroles are vents that have a strong, focused outflow of gas and

temperatures greater than or equal to boiling (Bergfeld and others, 2014). Fumaroles are “superheated” when the vent temperature is greater than the local boiling point (Allen and Day, 1935). The term frying pan describes a shallow depression where the gas upflow causes a mixture of steam and water to splash above the ground surface (Clynne and others, 2003). Sites lacking discrete vents such that the gas and steam discharges over a broad area are considered steaming ground. Features where gas discharges through water are referred to as pools if there is no outflow channel, or hot springs otherwise. Mudpots are degassing features where cavities in the underlying rock are filled with muddy water and steam (Clynne and others, 2003). Our goal in this work was to revisit historical sample locations with a focus on collecting gas from superheated fumaroles to augment earlier research with helium isotope data. Superheated temperatures vary with elevation and exceed 94°C at lower elevations (for example, Boiling Springs Lake and Terminal Geyser) and 92°C at higher elevations (for example, Bumpass Hell and Pilot Pinnacle) because the boiling point of water decreases with decreasing atmospheric pressure at higher elevation. During this investigation, superheated fumaroles were sampled at Bumpass Hell, upper Sulphur Works, Little Hot Springs Valley, and Terminal Geyser. Resampling historical sites proved challenging, because some fumaroles described by Janik and McLaren (2010) were unsafe to access or were no longer present. Due to flooding, we were only able to sample Big Boiler fumarole at Bumpass Hell once, and the upflow was so forceful it repeatedly blew the fumarole tube out of the vent. In addition, an area of thermal ground that developed at Devils Kitchen between 2000 and 2002 is no longer active. We infer that two superheated fumaroles at Pilot Pinnacle and upper Sulphur Works collapsed and now are roiling pools of water.

Field and Laboratory Methods The primary targets of this study were superheated fumaroles; however, when they were not available, sample sites were selected based on high temperatures and strong gas upflows. When possible, we sampled the same locations on repeat visits, but due to changing surface conditions, this wasn’t always possible. For example, Big Boiler fumarole vents from the bottom of a depression and during most site visits was submerged beneath a violently roiling pool of water. Similarly, the superheated fumarole in Little Hot Springs Valley discharges from the bank of Sulphur Creek and at times was submerged. In addition, after 2007, ground collapse around the fumarole at Terminal Geyser created an enclosed depression that was not safe to enter. Samples were collected and analyzed using standard field and laboratory methods (Bergfeld and others, 2014). Location coordinates were recorded using a Garmin global positioning system. Temperature was measured using a K-type

4 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park thermocouple and a digital thermometer. The gas upflow was captured using one of several collection devices: a titanium tube inserted into discrete vents; a Pyrex-lined stainless-steel tube inserted into the ground at sites that lacked a discrete vent; or an inverted plastic funnel placed over gas bubbles at hot springs, pools, frying pans, or areas of steaming ground. Tygon tubing was used to connect the collection devices to preevacuated double-port glass collection bottles, and the entire setup was purged of atmospheric components before gas and steam collection. The same setup was used to collect gas for analysis of helium isotopes and other noble gases. For these samples, the gas was flowed through copper tubing for sufficient time to purge atmospheric gas before the tubing was crimped shut. Our standard sampling routine to determine bulk chemistry was to collect gas in two types of double-port glass bottles. Giggenbach-type bottles are glass bottles partially filled with a 4-normal sodium hydroxide (herein called caustic) solution (Giggenbach, 1975). The solution is gently heated, and the headspace is evacuated before use. Empty evacuated (EE) bottles, by contrast, contain no solution and are similarly evacuated before use. Samples collected in Giggenbach-type bottles provide several advantages over those collected in EE bottles. Because hydrogen sulfide dissolves in the caustic solution, this method prevents reaction with any oxygen in the headspace gases and provides the most robust analysis of hydrogen sulfide concentrations. Also, since steam and carbon dioxide dissolve in the caustic solution, the vacuum is retained for a long period of time and permits concentration of trace gas components in the headspace (the gas-filled space above the solution). In addition, we can determine ammonia concentrations in samples collected in a caustic solution. We collect gas in EE bottles to provide the most accurate analyses of certain components. Carbon monoxide concentrations can be directly measured in samples collected in EE bottles, whereas in Giggenbach bottles, carbon monoxide concentrations must be calculated due to partitioning between the headspace and caustic solution. Carbon dioxide collected in EE bottles is preferred for isotopic analysis to avoid carbon isotope fractionation that can occur when preparing the caustic solution. At sites with low gas upflows, where collection into liquid-filled bottles was impractical, only EE bottles were used. Bulk gas compositions were determined at the Volcano Hazards gas laboratory at the USGS in Menlo Park, California. Analysis of headspace components was performed using Varian gas chromatographs equipped with thermal conductivity, flame ionization, and pulsed discharge helium ionization detectors using argon and helium carrier gases. Gas concentrations reported for EE samples containing greater than 1 gram of condensed steam were corrected for dissolution using Henry’s law (Henry, 1803). Concentrations of the absorbed soluble gases in Giggenbach-type bottles were determined by wet-chemical techniques, including ion chromatography using a Dionex

ICS–2000 for hydrogen sulfide (after conversion to sulfate), an ion-selective electrode for ammonia, and direct measurement of carbon dioxide on a vacuum extraction line by acidifying an aliquot of the caustic solution. Analysis of caustic blanks provided details on background concentrations of carbon dioxide and sulfate that were used to correct measured values. Gas-to-steam ratios (Xg), the mole fraction of dry gas relative to gas plus steam, and ammonia concentrations are reported only for caustic samples and are not given for gas collected from hot springs and pools. The carbon monoxide concentrations reported for samples collected in Giggenbach-type bottles are calculated by multiplying the ratio of carbon monoxide to methane (or hydrogen) determined in the EE analysis by the measured methane (or hydrogen) concentration in the caustic analysis, assuming the same ratio is present in both samples. More details on analytical techniques for bulk gas compositions can be found in Bergfeld and others (2014). Isotope analyses were performed at other USGS laboratories. Stable isotope analysis of carbon dioxide gas was performed using standard mass spectrometry methods at the USGS Reston Stable Isotope Laboratory in Reston, Virginia. The purified carbon dioxide gas was analyzed on a dual inlet isotope ratio mass spectrometer (Coplen, 1973), and results are reported in δ13C notation (δ13C-CO2) expressed as per mil (‰) relative to the Vienna Pee Dee Belemnite (VPDB) standard. Noble gas abundances and isotopic ratios were analyzed by mass spectrometry at the USGS Noble Gas Laboratory in Denver, Colorado. The helium isotope ratios are reported as R/RA values where R is the 3He/4He ratio in the sample and RA is the atmospheric ratio. We also report RC/RA values that are R/RA values corrected for air contamination (if present). Details on noble gas analysis are given in Bergfeld and others (2020).

Results Bulk Gas Contents Tables 1 and 2 include field parameters and dry-gas (steam excluded) chemistry for Giggenbach-type and EE-type samples, respectively. Sites where samples were collected using both types of bottles are identified as having a replicate in table 1. The maximum fumarole temperature measured was 136.6°C at Little Hot Springs Valley in 2018. A similarly high temperature of 132.4°C was measured at Bumpass Hell in 2014, the only year we were able to collect gas from Big Boiler fumarole. Comparison of the gas compositions reveals large differences between the LVC thermal areas. Gas from upper Sulphur Works has high nitrogen to argon (N2/Ar) ratios (110–850) and Xg values (8–15), whereas gas from Terminal Geyser contains substantial steam with low Xg values (less than or equal to 1), and higher concentrations of carbon

Results 5 monoxide, hydrogen sulfide, and ammonia compared to Sulphur Works (table 1). Methane concentrations are relatively low (less than or equal to 0.1 mole percent) at all thermal areas, typical of gas from geothermal systems hosted in volcanic rocks (Goff and Janik, 2002), but are highest in gas associated with the CC2 reservoir (Devil’s Kitchen, Boiling Springs Lake, and Terminal Geyser). Ternary diagrams are used to illustrate differences in gas compositions associated with each thermal area (figs. 2 and 3). For these diagrams, we correct the data for the presence of atmospheric nitrogen (N2) and argon (Ar) by assuming any oxygen (O2) in the bulk gas is derived from air. We then use atmospheric N2/O2 and Ar/O2 ratios to calculate and subtract N2 and Ar associated with the O2. All data plotted on the ternary diagrams are from samples collected in Giggenbach-type bottles, and most are from samples collected at fumaroles, steaming ground, and frying pans. We exclude results from hot springs and pools, with one exception, because some components of the gas may partially dissolve into water, especially at slowly degassing sites. We include gas data from the roiling pool at Pilot Pinnacle because it is the major feature in the thermal area. A ternary N2-He-Ar diagram (fig. 2) allows comparison between gases with contributions from shallow meteoric sources (air and air-saturated meteoric water [ASMW]) and those derived from sources rich in helium (mantle or crust) or nitrogen-rich arc-type sources (Giggenbach, 1996). As a whole, LVC gases plot along a mixing trend between shallow meteoric and deep arc-type sources. Samples with the largest component of deeply derived gas were collected from Sulphur Works and Pilot Pinnacle, whereas samples with the largest meteoric components were collected from Terminal Geyser, Bumpass Hell, and Devils Kitchen. Three samples from Sulphur Works indicate mixing with an air endmember. Compared with gas from the steaming ground site, the three samples collected at the roiling pool at Pilot Pinnacle demonstrate a stronger shallow component. Gases from Bumpass Hell have a strong meteoric signature but are varied and distinct. The N2/Ar ratios in three samples are characterized by an air-like source, and three others are more similar to ASMW. Together, carbon dioxide, hydrogen sulfide, and nitrogen account for more than 95 percent of the total gas in each sample (table 1). A ternary diagram of the relative concentrations of these components reveals three groupings: (1) gases from Sulphur Works and Pilot Pinnacle, dominated by high carbon dioxide and relatively low hydrogen sulfide; (2) gases from Terminal Geyser, characterized by high hydrogen sulfide, and low carbon dioxide; and (3) a group consisting of samples from Little Hot Springs Valley, Devils

Kitchen, Boiling Springs Lake, and three Bumpass Hell sites, that contain moderate concentrations of both hydrogen sulfide and carbon dioxide (fig. 3). Three additional Bumpass Hell samples and a sample from Terminal Geyser trend toward the nitrogen vertex of the diagram.

Carbon and Helium Isotope Compositions Table 3 presents carbon and helium isotope results for gas collected from within and outside of the main LVC thermal areas. The external areas include the Lassen Peak hot spot (a persistent zone of steaming ground on the north side of Lassen Peak), two small travertine-depositing hot springs, and a warm pool (fig. 1). Most of the carbon dioxide isotope data in table 3 are from gas collected in EE bottles. When samples were not collected in EE bottles, we report results from carbon dioxide extracted from caustic in the Giggenbach bottle in table 3 for completeness but do not discuss further. Carbon isotope values of carbon dioxide from all locations in this study are similar, with only a 2.4‰ difference across all the sites (from −9.7 to −7.3‰). Within this range, the outlying values are carbon dioxide from Terminal Geyser (from −9.7 to −9.1‰), the external travertine-depositing hot springs (from −9.3 to −9.0‰), and the warm pool (−7.3‰) (table 3, fig. 4). The δ13C values of carbon dioxide within the major thermal areas vary only by 2‰. Carbon isotope values of gas from all CC1 sites and Devils Kitchen are between −8.6 and −7.7‰, and contrast with slightly lower isotope values for carbon dioxide from Boiling Springs Lake, from −9.0 to −8.7‰, and Terminal Geyser as mentioned above. The carbon isotope compositions of two carbon dioxide samples from the Lassen Peak hot spot (between −8.2 and −8.1‰) are consistent with those from CC1 gases. Comparison of helium isotope Rc/RA values reveals that gases collected from the CC1 thermal areas have higher values, and thus a stronger magmatic signature, than gas from CC2 (table 3; fig. 5). The Rc/RA values of gas from CC1 locations are between 6.59 and 7.50 with the highest values associated with Sulphur Works and Pilot Pinnacle (fig. 5). By comparison, most Rc/RA values for gas from CC2 sites are between 5.86 and 6.52. Gas from sites at Devils Kitchen has consistently higher Rc/RA values than gas from Boiling Springs Lake and Terminal Geyser. One sample from Terminal Geyser (not plotted) had an anomalous value of 3.11 (table 3), indicating strong dilution by air. Figure 5 includes helium isotope values of gas collected sporadically from the Lassen Peak hot spot, which had a maximum value of 8.05 Rc/RA in 1974. In other years, the Rc/RA values of the hot spot gas are within the range of gas from the CC1 sites.

6 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park Table 1. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in caustic solution from thermal features in Lassen Volcanic National Park, 2002–19. [Gas concentrations reported as volume percent. Samples are grouped by thermal area. Sites not directly related to thermal areas are grouped as “external.” CO concentrations are calculated using gas ratios from the EE-type bottles (see text). DK data from 2002 was previously reported in Janik and Bergfeld (2010). All other compositional data are from Lewicki and others (2024a). Date shown as month/day/year. Replicate, “yes” indicates samples were collected in caustic and EE bottles, otherwise are listed with a “no.” Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; Xg, mol fraction of dry gas relative to the mol fraction of gas plus steam expressed as %, percent; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; NH3, ammonia; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

Location

Date

Replicate

Temp.

Type

(°C)

Easting

Northing

Elevation

(WGS 84)

(m)

Xg (%)

CO2

H2S

Bumpass Hell BH082707B

BH frying pan

08/27/07

91.2

626927

4479636

2,496

0.4

88.5

6.6

LVNP14-04

BH steaming ground

07/31/14

LVNP14-05

BH Big Boiler fumarole

09/11/14

Yes

92.6

627179

4479613

2,503

0.5

85.0

5.8

132.4

627179

4479613

2,503

5.1

70.7

LVNP18-05

BH steaming ground

3.4

08/14/18

Yes

91.7

627015

4479642

2,500

0.6

80.9

LVNP18-06

6.2

BH dry frying pan

08/14/18

Yes

92.1

626966

4479633

2,491

0.5

91.1

6.7

LVNP19-03

BH steaming ground

08/28/19

Yes

92.1

627024

4479618

2,482

0.5

91.8

6.4

LVNP14-08

LHSV pool

09/13/14

Yes

84.5

625730

4479082

2,135

97.5

1.3

LVNP14-09

LHSV superheated fumarole

09/13/14

Yes

105.0

625754

4479026

2,116

1.6

93.8

4.8

LVNP16-01

LHSV frying pan

08/30/16

Yes

95.4

625777

4478977

2,122

0.6

93.1

4.0

LVNP16-02

LHSV steaming ground

08/30/16

Yes

93.7

625855

4479042

2,137

0.6

94.3

4.2

LVNP18-08

LHSV superheated fumarole

08/15/18

Yes

136.6

625754

4479026

2,122

0.5

91.6

5.7

Little Hot Springs Valley

Pilot Pinnacle LVNP15-05

PP main pool

08/19/15

Yes

83.0

624658

4480537

2,475

98.4

0.5

LVNP15-06

PP steaming ground

08/19/15

Yes

90.5

624694

4480593

2,486

5.0

97.8

1.4

LVNP17-05

PP main pool

07/19/17

Yes

44.1

624655

4480533

2,486

97.9

0.6

LVNP17-06

PP cold pool

07/19/17

Yes

21.2

624843

4480532

2,440

97.4

1.1

LVNP18-03

PP main pool

08/13/18

Yes

87.7

624657

4480536

2,480

97.9

1.0

624226

4478704

2,147

4.6

97.7

0.8

Lower Sulphur Works LVNP16-06

lower SW steaming ground

08/31/16

Yes

92.4

Upper Sulphur Works USW082707A

upper SW pool

08/27/07

86.2

624162

4479265

2,280

98.2

0.6

LVNP15-03

upper SW dry frying pan

08/18/15

Yes

91.4

624172

4479290

2,273

7.6

98.6

0.5

LVNP16-04

upper SW frying pan

08/31/16

Yes

90.3

624168

4479218

2,251

14.7

98.6

0.4

LVNP16-05

upper SW steaming ground

08/31/16

89.5

624108

4479180

2,241

12.7

98.5

0.4

LVNP17-03

upper SW frying pan

07/19/17

Yes

92.1

624184

4479257

2,272

7.6

97.1

0.5

LVNP17-04

upper SW pool

07/19/17

Yes

85.4

624160

4479213

2,246

97.8

0.4

LVNP18-01

upper SW superheated fumarole

08/13/18

Yes

103.4

624183

4479252

2,278

13.7

98.5

0.6

LVNP18-02

upper SW steaming ground

08/13/18

Yes

89.2

624097

4479188

2,242

13.1

98.7

0.4

LVNP19-04

upper SW superheated fumarole

08/28/19

Yes

99.9

624183

4479252

2,278

14.1

98.4

0.7

Boiling Springs Lake LVNP14-07

BSL steaming ground

09/12/14

Yes

94.6

635987

4477276

1,810

0.1

90.9

5.0

LVNP17-01

BSL steaming ground

07/18/17

Yes

94.9

635997

4477262

1,808

0.1

91.1

4.9

LVNP19-01

BSL steaming ground

08/27/19

Yes

93.9

635980

4477265

1,806

0.1

90.8

4.8

Devils Kitchen L02-09_DK-

DK fumarole

09/19/02

95.3

633053

4477608

1,858

0.5

93.1

3.4

DK frying pan

09/19/02

95.5

633161

4477685

1,833

0.4

92.7

3.2

DK fumarole

07/30/14

Yes

94.2

632925

4477814

1,868

0.5

94.4

3.7

VHZ-01 L02-10_DKVHZ-02 LVNP14-01

Results 7 Table 1. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in caustic solution from thermal features in Lassen Volcanic National Park, 2002–19.—Continued Gas concentrations reported as volume percent. Samples are grouped by thermal area. Sites not directly related to thermal areas are grouped as “external.” CO concentrations are calculated using gas ratios from the EE-type bottles (see text). DK data from 2002 was previously reported in Janik and Bergfeld (2010). All other compositional data are from Lewicki and others (2024a). Date shown as month/day/year. Replicate, “yes” indicates samples were collected in caustic and EE bottles, otherwise are listed with a “no.” Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; Xg, mol fraction of dry gas relative to the mol fraction of gas plus steam expressed as %, percent; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; NH3, ammonia; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

BH082707B

0.0006

0.97

0.082

0.0180

3.7

LVNP14-04

<0.001

0.44

0.100

0.0080

LVNP14-05

<0.009

1.10

0.260

0.1300

LVNP18-05

0.0009

0.87

0.150

0.0056

LVNP18-06

0.0006

0.83

0.023

LVNP19-03

0.0010

0.09

LVNP14-08

0.0004

LVNP14-09

NH3

CH4

C2H6

C3H8

C4H10

N2/Ar

<0.004

0.08

45.2

8.5

<0.005

0.10

0.0001

84.4

<0.03

0.09

<0.006

89.3

0.07

<0.001

0.001

80.7

0.0017

1.3

0.002

0.10

0.0001

0.001

0.00006

0.00002

56.5

0.030

0.0033

1.6

0.12

0.0001

0.001

0.00004

0.00001

54.3

0.00

0.030

0.0010

1.1

<0.0004

0.01

<0.001

44.0

0.0008

0.62

0.004

0.0025

0.76

0.02

0.0002

0.000002

190

LVNP16-01

0.0008

0.50

0.030

0.0008

2.4

0.002

0.01

0.0001

0.001

0.00004

0.00001

88.1

LVNP16-02

0.0008

0.38

0.010

0.0012

<0.001

0.01

0.0001

0.002

0.00010

0.00005

85.8

LVNP18-08

0.0015

0.98

0.018

0.0150

1.6

0.03

0.02

0.0001

0.004

0.0001

0.0002

90.6

Bumpass Hell

Little Hot Springs Valley

Pilot Pinnacle LVNP15-05

0.0010

0.16

0.005

0.0001

0.97

0.0005

0.01

0.00005

194

LVNP15-06

0.0008

0.01

0.001

0.0002

0.76

0.001

0.01

0.0003

760

LVNP17-05

0.0002

0.18

0.015

0.0003

1.4

<0.0002

0.01

<0.001

0.00003

91.3

LVNP17-06

0.0012

0.30

0.010

0.0013

1.2

0.0002

0.01

<0.001

0.00004

118

LVNP18-03

0.0009

0.17

0.005

0.0015

0.98

0.01

<0.002

0.0003

196

0.00

0.0001

0.0003

0.00002

0.00004

142

Lower Sulphur Works LVNP16-06

0.0007

0.06

0.010

0.0004

1.4

<0.001

USW082707A

0.0005

0.07

0.010

0.0250

1.1

<0.0004

0.003

110

LVNP15-03

0.0007

0.02

0.001

0.0003

0.8

0.001

0.003

0.0002

800

LVNP16-04

0.0007

0.04

0.002

0.0006

0.91

0.0006

0.002

0.0020

0.0001

455

LVNP16-05

0.0006

0.04

0.005

0.0008

1.1

0.002

0.00001

226

LVNP17-03

0.0006

0.07

0.019

0.0007

2.2

0.0005

0.003

<0.001

0.0001

117

LVNP17-04

0.0006

0.04

0.012

0.0038

1.8

<0.0002

0.002

<0.001

0.0002

147

LVNP18-01

0.0007

0.08

0.002

0.0007

0.82

0.002

0.003

<0.001

0.0002

410

LVNP18-02

0.0006

0.04

0.001

0.0004

0.83

0.002

<0.001

0.0002

830

LVNP19-04

0.0007

0.07

0.001

0.0010

0.85

0.002

0.003

0.00002

0.0001

0.00002

0.00001

850

Upper Sulphur Works

Boiling Springs Lake LVNP14-07

0.0021

0.76

0.036

0.0085

3.2

0.03

0.09

0.0001

88.3

LVNP17-01

0.0013

0.42

0.049

0.0056

3.4

0.09

<0.002

0.006

<0.00002

69.6

LVNP19-01

0.0039

0.44

0.053

0.0150

3.7

0.08

0.10

0.0005

0.020

0.0003

0.00004

69.2

Devils Kitchen L02-09_DK-

0.0007

0.48

0.051

0.0047

2.8

0.10

54.3

0.0010

0.47

0.060

3.5

0.01

0.10

57.8

0.0010

0.17

0.021

0.0054

1.6

<0.004

0.10

0.0001

0.00003

76.2

VHZ-01 L02-10_DKVHZ-02 LVNP14-01

8 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park Table 1. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in caustic solution from thermal features in Lassen Volcanic National Park, 2002–19.—Continued [Gas concentrations reported as volume percent. Samples are grouped by thermal area. Sites not directly related to thermal areas are grouped as “external.” CO concentrations are calculated using gas ratios from the EE-type bottles (see text). DK data from 2002 was previously reported in Janik and Bergfeld (2010). All other compositional data are from Lewicki and others (2024a). Date shown as month/day/year. Replicate, “yes” indicates samples were collected in caustic and EE bottles, otherwise are listed with a “no.” Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; Xg, mol fraction of dry gas relative to the mol fraction of gas plus steam expressed as %, percent; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; NH3, ammonia; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

Location

Date

Replicate

Temp. (°C)

Type

Easting

Northing

Elevation

(WGS 84)

(m)

Xg (%)

CO2

H2S

Devils Kitchen—Continued LVNP14-02

DK pool

07/30/14

Yes

92.4

632778

4477898

1,856

94.1

2.7

LVNP17-07

DK fumarole

07/20/17

Yes

94.1

632928

4477810

1,862

0.6

94.6

3.5

LVNP19-05

DK fumarole

08/29/19

Yes

94.1

632936

4477816

1,863

0.5

92.8

4.1

Terminal Geyser TG082807A

TG superheated fumarole

08/28/07

113.3

637664

4475782

1,808

0.03

52.7

7.5

LVNP14-06

TG steaming ground

09/12/14

Yes

95.5

637666

4475785

1,836

0.02

77.3

12.6

LVNP17-02

TG fumarole

07/18/17

Yes

94.7

637654

4475798

1,808

0.02

72.8

16.7

LVNP19-02

TG fumarole

08/27/19

Yes

95.0

637654

4475595

1,809

0.03

80.7

10.2

624001

4479149

2,231

98.8

0.3

External LVNP15-04

Warm pool, trail to upper Sulphur Works

08/18/15

Yes

41.9

Results 9 Table 1. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in caustic solution from thermal features in Lassen Volcanic National Park, 2002–19.—Continued Gas concentrations reported as volume percent. Samples are grouped by thermal area. Sites not directly related to thermal areas are grouped as “external.” CO concentrations are calculated using gas ratios from the EE-type bottles (see text). DK data from 2002 was previously reported in Janik and Bergfeld (2010). All other compositional data are from Lewicki and others (2024a). Date shown as month/day/year. Replicate, “yes” indicates samples were collected in caustic and EE bottles, otherwise are listed with a “no.” Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; Xg, mol fraction of dry gas relative to the mol fraction of gas plus steam expressed as %, percent; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; NH3, ammonia; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

LVNP14-02

0.0008

0.43

0.039

0.0061

2.6

LVNP17-07

0.0009

0.20

0.021

0.0031

1.6

LVNP19-05

0.0014

0.20

0.040

0.0035

2.7

NH3

CH4

C2H6

0.002

0.10

0.0005

0.00001

0.10

0.0001

0.001

0.06

0.14

0.0001

0.003

C3H8

C4H10

N2/Ar

66.7

0.00002

0.00001

74.3

0.0001

0.00002

67.0

Devils Kitchen—Continued

Terminal Geyser TG082807A

0.0034

0.25

0.800

3.30

0.50

0.05

0.0060

43.5

LVNP14-06

0.0056

0.75

0.170

0.064

8.9

0.10

0.0006

0.004

52.9

LVNP17-02

0.0015

0.52

0.180

0.047

9.4

0.20

0.14

0.0005

0.060

0.00040

0.00090

53.8

LVNP19-02

0.0015

0.22

0.180

0.016

8.4

0.20

0.07

0.0007

0.020

0.0002

0.00005

47.9

0.001

0.0001

470

External LVNP15-04

0.0007

0.001

0.002

0.0003

0.94

0.0005

10 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park Table 2. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in empty evacuated bottles from thermal features in Lassen Volcanic National Park, 2004–19. [Data are from Lewicki and others (2024a). Date shown as month/day/year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; Steam correlation, “yes” indicates that the measured concentration has been corrected for the dissolved component calculated using Henry’s law (Henry, 1803), otherwise “no.” BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample

Location

number

Date

Temp. (°C)

Type

Easting

Northing

Elevation

(WGS 84)

(m)

CO2

H2S

Bumpass Hell LVNP14-04

BH steaming ground

07/31/14

92.6

627179

4479613

2,503

83.1

5.4

0.0005

LVNP18-05

BH steaming ground

08/14/18

91.7

627015

4479642

2,500

89.5

6.8

0.0004

LVNP18-06

BH dry frying pan

08/14/18

92.1

626966

4479633

2,491

91.3

6.6

0.0006

LVNP19-03

BH steaming ground

08/28/19

92.1

627024

4479618

2,482

91.8

6.5

0.0008

Little Hot Springs Valley LVNP14-08

LHSV pool

09/13/14

84.5

625730

4479082

2,135

97.3

1.3

<0.0002

LVNP14-09_1

LHSV superheated fumarole

09/13/14

105.0

625754

4479026

2,116

93.3

4.9

0.0005

LVNP14-09_2

LHSV superheated fumarole

09/13/14

105.0

625754

4479026

2,116

93.4

4.8

0.0002

LVNP16-01

LHSV frying pan

08/30/16

95.4

625777

4478977

2,122

93.2

5.1

<0.0001

LVNP16-02

LHSV steaming ground

08/30/16

93.7

625855

4479042

2,137

93.3

4.5

<0.0001

LVNP18-08

LHSV superheated fumarole

08/15/18

136.6

625754

4479026

2,116

93.3

0.0007

LVNP18-09

LHSV steaming ground

08/15/18

92.9

625861

4479041

2,131

25.5

0.58

0.0011

Pilot Pinnacle LVNP15-05

PP main pool

08/19/15

83.0

624658

4480537

2,475

97.4

0.9

0.0005

LVNP15-06

PP steaming ground

08/19/15

90.5

624694

4480593

2,486

98.3

0.97

0.0005

LVNP17-05

PP main pool

07/19/17

44.1

624655

4480533

2,486

98.0

0.6

0.0010

LVNP17-06

PP cold pool

07/19/17

21.2

624843

4480532

2,440

97.6

1.1

0.0010

LVNP18-03

PP main pool

08/13/18

87.7

624657

4480536

2,480

89.6

0.84

0.0009

LVNP18-04

PP steaming ground

08/13/18

78.4

624776

4480579

2,470

97.5

1.4

0.0009

LDB09-01

lower SW frying pan

04/11/09

89.2

624199

4478726

2,150

98.5

0.48

0.0006

LDB09-02

lower SW pool

04/11/09

68.8

624196

4478710

2,142

98.2

0.51

0.0005

LVNP16-06

lower SW steaming ground

08/31/16

92.4

624226

4478704

2,147

98.3

0.78

<0.0001

Lower Sulphur Works

Upper Sulphur Works LVNP15-03

upper SW dry frying pan

08/18/15

91.4

624172

4479290

2,273

98.6

0.57

0.0003

LVNP16-04

upper SW frying pan

08/31/16

90.3

624168

4479218

2,251

98.7

0.51

<0.0001

LVNP17-03

upper SW frying pan

07/19/17

92.1

624184

4479257

2,272

96.7

0.55

0.0007

LVNP17-04

upper SW pool

07/19/17

85.4

624160

4479213

2,246

98.8

0.41

0.0005

LVNP18-01

upper SW superheated fumarole

08/13/18

103.4

624183

4479252

2,278

98.3

0.58

0.0010

LVNP18-02

upper SW steaming ground

08/13/18

89.2

624097

4479188

2,242

98.5

0.36

0.0005

LVNP19-04

upper SW superheated fumarole

08/28/19

99.9

624183

4479252

2,278

98.4

0.56

0.0004

Boiling Springs Lake LVNP14-07

BSL steaming ground

09/12/14

94.6

635987

4477276

1,810

90.9

6.1

0.0006

LVNP17-01

BSL steaming ground

07/18/17

94.9

635997

4477262

1,808

92.2

5.4

0.0010

LVNP19-01

BSL steaming ground

08/27/19

93.9

635980

4477265

1,806

91.4

5.4

0.0012

Devils Kitchen LVNP14-01

DK fumarole

07/30/14

94.2

632925

4477814

1,868

93.5

3.4

0.0008

LVNP14-02

DK pool

07/30/14

92.4

632778

4477898

1,856

86.4

2.9

0.0012

LVNP17-07

DK fumarole

07/20/17

94.1

632928

4477810

1,862

95.0

3.6

0.0008

LVNP19-05

DK fumarole

08/29/19

94.1

632936

4477816

1,863

94.2

3.8

0.0010

Results 11 Table 2. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in empty evacuated-type bottles from thermal features in Lassen Volcanic National Park, 2004–19.—Continued [Data are from Lewicki and others (2024a). Date shown as month/day/year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; Steam correlation, “yes” indicates that the measured concentration has been corrected for the dissolved component calculated using Henry’s law (Henry, 1803), otherwise “no.” BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

CH4

LVNP14-04

0.39

0.11

2.10

8.80

0.091

LVNP18-05

0.68

0.07

0.43

2.50

LVNP18-06

0.75

0.04

0.09

1.10

LVNP19-03

0.06

0.07

1.40

C2H6

Steam

C3H8

C4H10

N2/Ar

<0.0002

<0.001

<0.0005

80.0

0.058

0.0002

0.001

0.0001

35.7

Yes

0.074

0.001

27.5

Yes

0.085

<0.0004

0.001

<0.0001

20.0

Yes

correlation

Bumpass Hell

Little Hot Springs Valley LVNP14-08

0.004

0.025

0.12

1.30

0.005

<0.0002

<0.001

<0.0005

52.0

LVNP14-09_1

0.63

0.009

0.12

1.10

0.013

<0.0002

0.0002

<0.0005

122.2

LVNP14-09_2

0.61

0.008

0.10

1.10

0.013

<0.0002

0.0001

<0.0005

137.5

LVNP16-01

0.55

0.03

0.16

1.00

0.012

<0.0002

0.0007

<0.0001

<0.0002

33.3

Yes

LVNP16-02

0.35

0.03

0.27

1.60

0.013

<0.0002

0.001

<0.0001

<0.0002

53.3

Yes

LVNP18-08

0.58

0.03

0.12

0.93

0.005

0.0008

31.0

Yes

LVNP18-09

0.11

0.52

15.00

58.0

0.001

111.5

Yes

Pilot Pinnacle LVNP15-05

0.16

0.01

0.16

1.40

0.006

<0.0002

0.0001

<0.0005

155.6

LVNP15-06

0.01

0.002

0.021

0.74

0.006

<0.0002

0.0003

<0.0005

370.0

LVNP17-05

0.15

0.04

0.16

1.10

0.007

<0.0002

0.00003

27.5

LVNP17-06

0.22

<0.04

0.15

0.90

0.011

<0.0002

0.00004

LVNP18-03

0.14

0.08

1.80

7.50

0.002

0.0001

98.7

LVNP18-04

0.20

0.03

0.07

0.84

0.004

0.0002

31.1

Lower Sulphur Works LDB09-01

0.03

0.00

0.055

0.91

0.003

<0.0002

<0.001

<0.0005

303.3

LDB09-02

0.03

0.01

0.13

1.10

0.003

<0.0002

<0.001

<0.0005

157.1

LVNP16-06

0.06

0.05

0.79

0.004

<0.0002

0.0002

<0.0001

<0.0002

Yes

Upper Sulphur Works LVNP15-03

0.01

0.002

0.031

0.77

0.003

<0.0002

0.0002

<0.0005

385.0

LVNP16-04

0.05

<0.03

0.03

0.75

0.002

<0.0001

0.00004

<0.0001

<0.0003

LVNP17-03

0.05

0.10

0.58

2.00

0.003

<0.0002

0.0001

20.0

Yes

LVNP17-04

0.03

0.01

0.05

0.70

0.002

<0.0002

0.0001

70.0

Yes

LVNP18-01

0.07

0.02

0.07

0.95

0.002

0.0001

52.8

LVNP18-02

0.03

0.10

0.99

0.001

0.0001

33.0

Yes

LVNP19-04

0.07

<0.04

0.11

0.87

0.003

<0.0002

0.0001

<0.0001

0.002

<0.0001

100.0

Yes

Boiling Springs Lake LVNP14-07

0.59

0.02

0.13

2.20

0.078

LVNP17-01

0.25

0.03

0.11

2.00

0.073

0.00060

0.005

0.00060

0.00110

66.7

Yes

LVNP19-01

0.26

0.04

0.21

2.70

0.057

<0.0009

0.009

0.0003

0.0005

67.5

Yes

Devils Kitchen LVNP14-01

0.18

0.03

0.29

2.50

0.100

0.00018

0.002

<0.0005

80.6

LVNP14-02

0.42

0.11

1.70

8.50

0.090

<0.0002

0.0002

<0.0005

77.3

LVNP17-07

0.15

<0.04

0.04

1.10

0.099

<0.0002

0.001

Yes

LVNP19-05

0.14

<0.04

0.08

1.70

0.093

<0.0002

0.002

<0.0001

Yes

12 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park Table 2. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in empty evacuated bottles from thermal features in Lassen Volcanic National Park, 2004–19.—Continued [Data are from Lewicki and others (2024a). Date shown as month/day/year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; Steam correlation, “yes” indicates that the measured concentration has been corrected for the dissolved component calculated using Henry’s law (Henry, 1803), otherwise “no.” BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample

Location

number

Date

Temp. (°C)

Type

Easting

Northing

Elevation

(WGS 84)

(m)

CO2

H2S

Terminal Geyser LVNP14-06

TG steaming ground

09/12/14

95.5

637666

4475785

1,836

86.3

0.0001

LVNP17-02

TG fumarole

07/18/17

94.7

637654

4475798

1,808

86.2

0.0006

LVNP19-02

TG fumarole

08/27/19

95.0

637654

4475595

1,809

88.2

0.0009

External LM 4250

Small travertine spring, upper LHSV

08/19/04

56.3

625392

4479814

2,275

96.7

0.27

0.0012

LVNP16-03

Small travertine spring, upper LHSV

08/30/16

67.3

625274

4480026

2,298

97.8

0.27

<0.0002

LVNP15-07

Travertine spring below SW

08/19/15

85.2

626131

4483938

2,091

99.5

0.003

<0.0002

08/14/18

626131

4483938

2,098

99.0

08/18/15

41.9

624001

4479149

2,231

98.8

0.3

0.0004

parking lot LVNP18-07

Travertine spring below SW parking lot

LVNP15-04

Warm pool, trail to upper Sulphur Works

Results 13 Table 2. Field parameters and mole percent dry-gas compositions (steam removed) of gas collected in empty evacuated-type bottles from thermal features in Lassen Volcanic National Park, 2004–19.—Continued [Data are from Lewicki and others (2024a). Date shown as month/day/year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; CO2, carbon dioxide; H2S, hydrogen sulfide; He, helium; H2, hydrogen; Ar, argon; O2, oxygen; N2, nitrogen; CH4, methane; C2H6, ethane; CO, carbon monoxide; C3H8, propane; C4H10, butane; N2/Ar, nitrogen to argon ratio; Steam correlation, “yes” indicates that the measured concentration has been corrected for the dissolved component calculated using Henry’s law (Henry, 1803), otherwise “no.” BH, Bumpass Hell; BSL, Boiling Springs Lake; DK, Devils Kitchen; LHSV; Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; <, less than; bd, below detection limit; nd, not determined] Sample number

CH4

LVNP14-06

0.12

0.03

0.02

1.30

0.029

LVNP17-02

0.07

0.06

0.20

1.60

0.023

LVNP19-02

0.11

0.06

0.27

0.11

0.023

C2H6

Steam

C3H8

C4H10

N2/Ar

0.00033

0.006

<0.0001

50.0

Yes

0.00020

0.010

0.00020

26.7

Yes

0.0003

0.006

0.0002

0.0001

1.8

Yes

correlation

Terminal Geyser

External LM 4250

0.0011

0.05

0.04

2.90

0.011

<0.0002

<0.001

<0.0005

58.0

LVNP16-03

<0.001

0.05

0.09

1.80

<0.04

<0.11

36.0

0.0003

0.008

0.05

0.40

0.001

<0.0002

<0.001

<0.0005

50.0

0.0009

0.014

0.15

0.82

0.0004

0.0003

58.6

0.0008

0.001

0.014

0.86

0.001

<0.0002

0.0001

<0.0005

860.0

LVNP15-07 LVNP18-07 LVNP15-04

14 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park N 2 /100 100

r ce

BB p

r ce

c-t

y pe

Air (N 2 /Ar = 84)

60 ASMW (N 2 /Ar = 38)

Mantle or crust

10 0 0

10 0

10He

Percent He

EXPLANATION Circulation cell 1

Circulation cell 2

BSL

LHSV

Figure 2. Ternary diagram showing relative concentrations of nitrogen (N2), helium (He), and argon (Ar) for samples collected in Giggenbach-type bottles at Lassen Volcanic National Park between 2002 and 2014. Data from the BH (Bumpass Hell), LHSV (Little Hot Springs Valley), SW (Sulphur Works), and PP (Pilot Pinnacle) thermal areas associated with circulation cell 1 (CC1) are shown as green, blue, red, and black diamond symbols, respectively. Data from the BSL (Boiling Springs Lake), DK (Devils Kitchen), and TG (Terminal Geyser) thermal areas associated with circulation cell 2 (CC2) are shown with magenta, orange, and black squares, respectively). BB = Big Boiler fumarole, p indicates gas samples collected from a roiling pool, ASMW = air-saturated meteoric water. Note that gas concentrations have been corrected for the presence of air.

Results 15 CO 2 /10 100

r ce nt

r ce

BB 20

2007 fumarole

10 0

H 2S

Percent H 2S

EXPLANATION Circulation cell 1

Circulation cell 2

BSL

LHSV

Figure 3. Ternary diagram showing relative concentrations of carbon dioxide (CO2), hydrogen sulfide (H2S), and nitrogen (N2) for samples collected in Giggenbach-type bottles at Lassen Volcanic National Park between 2002 and 2019. Data from the BH (Bumpass Hell), LHSV (Little Hot Springs Valley), SW (Sulphur Works), and PP (Pilot Pinnacle) thermal areas associated with circulation cell 1 (CC1) are shown as green, blue, red, and black diamond symbols, respectively. Data from the BSL (Boiling Springs Lake), DK (Devils Kitchen), and TG (Terminal Geyser) thermal areas associated with circulation cell 2 (CC2) are shown with magenta, orange, and black squares, respectively. Symbols are as in figure 2. BB = Big Boiler fumarole, results from the 2007 fumarole at TG are distinct from TG samples collected in other years. Note that gas concentrations have been corrected for the presence of air.

16 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park –7.0 Circulation cell 2

Features outside main thermal areas Warm pool near SW

–7.5

SW –8.0 PP

LHSV

Lassen Peak hot spot

2009 2012

–8.5

Range for circulation cell 1 δ13C

Carbon isotope composition of carbon dioxide (δ13C-C02), in parts per mil

Circulation cell 1

BSL –9.0 Travertine-depositing hot springs TG

–9.5

–10.0

EXPLANATION Circulation cell 1 BH

Circulation cell 2 BSL

LHSV

Features outside main thermal areas Lassen Peak hot spot Hot springs or pool

Figure 4. Plot showing δ13C-CO2 values for gases collected in EE bottles at Lassen Volcanic National Park between 2004 and 2019. Data from thermal areas associated with circulation cell 1 (CC1), BH (Bumpass Hell), LHSV (Little Hot Springs Valley), SW (Sulphur Works), PP (Pilot Pinnacle) are shown as diamond symbols. Results from thermal areas associated with circulation cell 2 (CC2), BSL (Boiling Springs Lake), DK (Devils Kitchen), and TG (Terminal Geyser) are shown with squares. Data from outside the main thermal areas, the Lassen Peak hot spot, a warm pool near SW, and 4 travertine-depositing hot springs are shown as circles.

Results 17 Table 3. Field parameters, helium-3 (3He) to helium-4 (4He) (3He/4He) ratios, and carbon isotope results for gas samples collected from thermal features in Lassen Volcanic National Park, 1974–2019. [Data are from Lewicki and others (2024a). Carbon isotope values are reported as per mil relative to Vienna Pee Dee Belemnite. Date shown as month/day/ year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; R/RA, 3He/4He ratio in the gas relative to the ratio in air; RC/RA, the R/RA value corrected for air; δ13C-CO2, isotope value of carbon dioxide expressed in per mil, BH, Bumpass Hell; DK, Devils Kitchen; LHSV, Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; nd, not determined] Sample number

Location

Date

Temp.

Type

(°C)

Easting

Northing

Elevation

(WGS 84)

(m)

R/RA

Rc/RA

δ13C-CO2

Bumpass Hell BH082707B

BH frying pan

08/27/07

91.2

626927

4479636

2,496

6.81

7.07

1–9.2

LVNP14-04

BH steaming ground

07/31/14

92.6

627179

4479613

2,503

7.17

7.24

–8.4

LVNP18-05

BH steaming ground

08/14/18

91.7

627015

4479642

2,500

6.98

7.03

–8.5

LVNP18-06

BH dry frying pan

08/14/18

92.1

626966

4479633

2,491

7.06

7.11

–8.5

LVNP19-03

BH steaming ground

08/28/19

92.1

627024

4479618

2,482

6.68

6.73

–8.5

–8.4

Little Hot Springs Valley LM 4234-D

LHSV frying pan

08/19/04

94.2

625720

4479245

2,154

LVNP14-08

LHSV pool

LVNP14-09_1

LHSV superheated fumarole

09/13/14

84.5

625730

4479082

2,135

–8.2

09/13/14

105

625754

4479026

2,116

7.13

7.14

LVNP14-09_2

–8.5

LHSV superheated fumarole

09/13/14

105

625754

4479026

2,116

–8.4

LVNP16-01

LHSV frying pan

08/30/16

95.4

625777

4478977

2,122

7.17

7.20

–8.6

LVNP16-02

LHSV steaming ground

08/30/16

93.7

625855

4479042

2,137

6.97

6.99

–8.5

LVNP18-08

LHSV superheated fumarole

08/15/18

136.6

625754

4479026

2,116

7.13

7.14

–8.4

LVNP18-09

LHSV steaming ground

08/15/18

92.9

625861

4479041

2,131

6.42

6.59

LVNP15-05

PP main pool

08/19/15

83.0

624658

4480537

2,475

7.42

7.43

–7.9

LVNP15-06

PP steaming ground

08/19/15

90.5

624694

4480593

2,486

–8.0

LVNP17-05

PP main pool

07/19/17

44.1

624655

4480533

2,486

–8.0

LVNP17-06

PP cold pool

07/19/17

21.2

624843

4480532

2,440

7.38

7.39

–8.4

LVNP18-03

PP main pool

08/13/18

87.7

624657

4480536

2,480

7.44

7.45

–8.1

LVNP18-04

PP steaming ground

08/13/18

78.4

624776

4480579

2,470

–8.5

Pilot Pinnacle

Lower Sulphur Works LDB09-01

lower SW frying pan

04/11/09

89.2

624199

4478726

2,150

6.94

6.95

–7.9

LDB09-02

lower SW pool

04/11/09

68.8

624196

4478710

2,142

–7.8

LVNP16-06

lower SW steaming ground

08/31/16

92.4

624226

4478704

2,147

7.16

7.17

–8.1

Upper Sulphur Works USW082707A

upper SW pool

08/27/07

86.2

624162

4479265

2,280

7.47

7.50

1–8.2

LVNP15-03

upper SW dry frying pan

08/18/15

91.4

624172

4479290

2,273

7.44

7.45

–7.8

LVNP16-04

upper SW frying pan

08/31/16

90.3

624168

4479218

2,251

–7.9

LVNP16-05

upper SW steaming ground

08/31/16

89.5

624108

4479180

2,241

6.02

7.09

1–8.0

LVNP17-03

upper SW frying pan

07/19/17

92.1

624184

4479257

2,272

7.40

7.42

–8.1

LVNP17-04

upper SW frying pan

07/19/17

85.4

624160

4479213

2,246

–7.9

LVNP18-01

upper SW superheated fumarole

08/13/18

103.4

624183

4479252

2,278

7.28

7.29

–8.0

LVNP18-02

upper SW steaming ground

08/13/18

89.2

624097

4479188

2,242

–7.7

LVNP19-04

upper SW superheated fumarole

08/28/19

99.9

624183

4479252

2,278

7.06

7.07

–7.9

Boiling Springs Lake BSL082807B

BSL mud pot

08/28/07

93.5

Mud Pot

635942

4477272

1,810

6.06

6.18

LVNP14-07

BSL steaming ground

09/12/14

94.6

635987

4477276

1,810

5.88

5.99

–8.7

LVNP17-01

BSL steaming ground

07/18/17

94.9

635997

4477262

1,808

6.22

6.28

–9.0

LVNP19-01

BSL steaming ground

08/27/19

93.9

635980

4477265

1,806

5.90

5.95

–9.0

18 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park Table 3. Field parameters, helium-3 (3He) to helium-4 (4He) (3He/4He) ratios, and carbon isotope results for gas samples collected from thermal features in Lassen Volcanic National Park, 1974–2019.—Continued [Data are from Lewicki and others (2024a). Carbon isotope values are reported as per mil relative to Vienna Pee Dee Belemnite. Date shown as month/day/ year. Temp., temperature; °C, degree Celsius; Type = F, fumarole; FP, frying pan; HS, hot spring; P, pool; SG, steaming ground; WGS 84, World Geodetic System of 1984; m, meters; R/RA, 3He/4He ratio in the gas relative to the ratio in air; RC/RA, the R/RA value corrected for air; δ13C-CO2, isotope value of carbon dioxide expressed in per mil, BH, Bumpass Hell; DK, Devils Kitchen; LHSV, Little Hot Springs Valley; PP, Pilot Pinnacle; SW, Sulphur Works; TG, Terminal Geyser; nd, not determined] Sample number

Location

Date

Temp.

Type

(°C)

Easting

Northing

Elevation

(WGS 84)

(m)

R/RA

Rc/RA

δ13C-CO2

Devils Kitchen L02-09_DK-

DK fumarole

09/19/02

95.3

633161

4477685

1,833

1–8.8

DK frying pan

09/19/02

95.5

633053

4477608

1,858

1–9.3

LVNP14-01

DK fumarole

07/30/14

94.2

632925

4477814

1,868

6.32

6.36

–8.0

LVNP14-02

DK pool

07/30/14

92.4

632778

4477898

1,856

6.42

6.52

–8.2

LVNP17-07

DK fumarole

07/20/17

94.1

632928

4477810

1,862

6.47

6.50

–8.2

LVNP19-05

DK fumarole

08/29/19

94.1

632936

4477816

1,863

6.34

6.37

–8.2

VHZ-01 L02-10_DKVHZ-02

Terminal Geyser TG082807A

TG superheated fumarole

08/28/07

113.3

637664

4475782

1,808

2.25

3.11

LVNP14-06

TG steaming ground

09/12/14

95.5

637666

4475785

1,836

5.91

6.04

–9.7

LVNP17-02

TG fumarole

07/18/17

94.7

637654

4475798

1,808

6.11

6.28

–9.1

LVNP19-02

TG fumarole

08/27/19

637654

4475595

1,809

5.67

5.86

–9.2

External LM 4250

Small travertine spring, upper

08/19/04

56.3

625392

4479814

2,275

–9.3

08/30/16

67.3

625274

4480026

2,298

–9.2

08/19/15

85.2

624289

4478473

2,091

2.26

2.95

–9.0

08/14/18

624289

4478473

2,098

–9.1

08/18/15

41.9

624001

4479149

2,231

–7.3

91.5

626131

4483938

2,091

–8.1

6.90

LHSV LVNP16-03

Small travertine spring, upper LHSV

LVNP15-07

Travertine Spring below SW parking lot

LVNP18-07

Travertine Spring below SW parking lot

LVNP15-04

Warm pool, trail to upper Sulphur Works

VH16RM09

Lassen Peak hot spot

07/13/09

Lassen HS_2002

Lassen Peak hot spot

2002

Lassen HS_061512

Lassen Peak hot spot

06/15/12

89.2

626131

4483938

2,098

–8.2

Lassen HS_1974

Lassen Peak hot spot

1974

8.05

Lassen HS_1988

Lassen Peak hot spot

1988

7.45

1Five δ13C values determined from caustic extractions.

Results 19 8.5 Circulation cell 1

Circulation cell 2

Features outside main thermal areas

7.5

1988

Lassen Peak hot spot

around 1970

7.0 2002

LHSV

He/4He, as Rc/RA

Range for circulation cell 1 Rc/RA

1974

8.0

6.5 DK

BSL

6.0

5.5

EXPLANATION Circulation cell 1 BH

Circulation cell 2 BSL

LHSV

Features outside main thermal areas Lassen Peak hot spot

Figure 5. Plot showing air-corrected helium isotope values for gas collected at Lassen Volcanic National Park between 1974 and 2019. Data from thermal areas associated with circulation cell 1 (CC1), BH (Bumpass Hell), LHSV (Little Hot Springs Valley), SW (Sulphur Works), and PP (Pilot Pinnacle) are shown as diamond symbols. Data from thermal areas associated with circulation cell 2 (CC2), BSL (Boiling Springs Lake), DK (Devils Kitchen), and TG (Terminal Geyser) are shown with squares. Gas from the Lassen Peak hot spot is shown with circles, the R/RA value from about 1970 is from Welhan and others (1988).

20 Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park

Discussion Sources of Carbon Dioxide and Helium The δ13C values of carbon dioxide discharging from the main thermal areas at Lassen are slightly lower than carbon derived from a mantle source (from −8 to −3‰; Rollinson, 1993) but fall within the range observed at other Cascade Range volcanoes (from −11.8 to −6.5‰; Evans and others, 2010). A detailed study by Evans and others (2010) found that low δ13C-CO2 values predominate at nearly all the major Cascade Range volcanoes, across many tectonic settings. The authors concluded that the isotopically light carbon originates from parent magmas that incorporated large amounts of subducted, organic-rich sediments. The carbon isotope composition of carbon dioxide in two samples from the Lassen Peak hot spot falls within the range of δ13C-CO2 values measured at the CC1 sites and Devils Kitchen (table 3; fig. 4). Given the relatively recent volcanic activity at LVC and the similarity in isotopic composition with other Cascade Range volcanoes, the data indicate that carbon dioxide in these thermal areas is ultimately magmatic in origin. Lower δ13C-CO2 values at Terminal Geyser and Boiling Springs Lake may result from the addition of non-magmatic carbon dioxide or the effect of surficial processes. At Terminal Geyser, lower δ13C values can be explained by isotopic fractionation related to secondary boiling of CC2 fluids (Friedman and O’Neil, 1977). However, the cause of the shift in carbon isotope values at Boiling Springs Lake is less clear. There are no fumaroles at Boiling Springs Lake, and all samples were collected from steaming ground on the south shore of the lake. The shift could be related to carbon dioxide degassing in the lake or from carbon input from microorganisms residing in the thermal ground, but further study is needed to test these possibilities. The δ13C values of carbon dioxide collected from the small hot springs and pool outside the main LVC thermal areas are varied and likely result from processes unrelated to the magmatic system. Carbon dioxide from the small thermal springs has relatively low δ13C values between −9.3 and −9.0‰, which may result from isotope fractionation between carbon in the gas phase and dissolved in the spring water (Friedman and O’Neil, 1977; Bergfeld and others, 2017). The reason for the relatively heavy composition of the single sample of gas from the warm pool is unclear and would require additional sampling and analysis to understand the processes affecting the carbon isotope composition. Helium isotope data provide a key geochemical tracer for understanding subsurface conditions in volcanic settings. These data are important in volcano monitoring as increases in 3He/4He ratios may signal the presence of volatiles related to the injection of new magma (Hilton and others, 2002) Helium isotope analyses of LVC gases have been conducted since the 1970s (Craig and others, 1978; Welhan

and others, 1988), though existing records are sparse and often lack essential details such as location coordinates, vent temperatures, and feature descriptions. This report presents the most comprehensive dataset available on helium isotope compositions in the LVC’s thermal areas, establishing a baseline of background variability and defining optimal monitoring locations. The highest helium isotope values recorded at LVC were from samples collected in 1974, with Rc/RA values of 8.1 and 7.7 at Sulphur Works and Bumpass Hell, respectively (Craig and others, 1978), and 8.05 at Lassen Peak hot spot (Lewicki and others (2024a). The maximum Rc/RA value measured in this study was 7.50 at Sulphur Works in 2007 (table 3; fig. 5). Long-term temporal trends in helium isotopic compositions of LVC gases are difficult to discern due to sparse data between 1974 and 2014. However, the lower Rc/RA values observed in this study relative to 1974 likely indicate dilution of magmatic 3He by ingrowth of radiogenic 4He, in the absence of intrusion of new magma beneath the LVC. Regarding spatial patterns in helium isotopic compositions at Lassen, our results indicate that gas from Sulphur Works, Pilot Pinnacle, Little Hot Springs Valley, and Bumpass Hell has a stronger magmatic signature than those from Devils Kitchen or Boiling Springs Lake (fig. 5). These findings provide additional support for the two-circulation-cell model of Janik and McLaren (2010).

Overview of Circulation Cell 1 Gas and Implications for Monitoring The chemical compositions of gases collected from the CC1 thermal areas in this study are generally consistent with the findings of Janik and McLaren (2010). Gases from Sulphur Works and Pilot Pinnacle are distinct from other CC1 sites, characterized by relatively high carbon dioxide concentrations, gas-to-steam ratios, and N2/Ar ratios, relatively low hydrogen sulfide concentrations, and minimal dilution from shallow sources (air and air-saturated meteoric water) (table 1; figs. 2 and 3). Samples from Little Hot Springs Valley and Bumpass Hell contain carbon dioxide and hydrogen sulfide concentrations and gas-to-steam ratios similar to previous measurements. However, three Bumpass Hell samples from this study differ notably in helium, nitrogen, and argon contents compared to three other Bumpass Hell samples collected in this study and prior data, suggesting increased mixing with air at these sites (fig. 2). The current model of the Lassen hydrothermal system suggests that Bumpass Hell and Little Hot Springs Valley are above a hotter part of the CC1 reservoir, where upflow of steam is strongest. Gases from these sites have undergone greater degrees of boiling, as demonstrated by lower gas-to-steam ratios and lower carbon dioxide concentrations (Janik and McLaren, 2010). In contrast, gas emitted from the Sulphur Works and Pilot Pinnacle thermal areas is less boiled and considered to be the most representative of conditions in the underlying volcanic system. Helium isotope data from this

References Cited 21 study further support this interpretation, as gases from Sulphur Works and Pilot Pinnacle have the highest magmatic helium contributions. Based on gas geochemical characteristics and overall accessibility, the Sulphur Works and Pilot Pinnacle thermal areas are optimal targets for monitoring the Lassen volcanic system. The data presented here provide a baseline to assess future geochemical changes associated with volcanic unrest, should they occur. Implementation of a data collection strategy aligned as proposed for Level 4 volcanoes by Lewicki and others (2024b) should support long-term monitoring and early detection of unrest at Lassen. Following these guidelines, full geochemical analyses, including carbon and helium isotope data, would be collected at 1- to 2-year intervals from superheated fumaroles when available, or other accessible sites when not. If conditions change such that a period of unrest initiates, the monitoring scheme would be modified to increase areal coverage and shorten the interval between sample collections.

Carbon isotope values of carbon dioxide at the main thermal areas fall within a narrow range (from −9.7 to −7.7 per mil) and are consistent with incorporation of isotopically light carbon from subducted organic sediments into parent magmas, as observed at other Cascade Range volcanoes. Helium isotope data further support the two-circulation-cell model, with higher Rc/RA values in circulation cell 1 gases (6.59–7.50) than in circulation cell 2 (5.86–6.52), indicating a stronger magmatic signature. The maximum Rc/RA value in this study is lower than three 1974 samples (7.7–8.1), possibly indicating dilution of magmatic 3He by radiogenic 4He over time. Gases from the Sulphur Works and Pilot Pinnacle thermal areas have the strongest magmatic signatures and are relatively accessible, making them ideal targets for monitoring the Lassen volcanic system. Regular sampling at these locations can enhance the long-term dataset, may improve detection of geochemical changes in the underlying volcanic system, and could strengthen early warning capabilities for potential volcanic unrest.

Summary Designated a very-high-threat volcano by the U.S. Geological Survey, the Lassen Volcanic Center (LVC) hosts a long-lived hydrothermal system indicated by thermal areas to the south and southeast of Lassen Peak. This report presents chemical and isotopic compositions of gas collected from LVC's thermal areas from 2014 to 2019 as part of the California Volcano Observatory geochemical monitoring program, supplemented by earlier U.S. Geological Survey datasets. These results represent the most comprehensive helium isotope record compiled for LVC and are assessed in the context of the two-circulation-cell model of LVC's hydrothermal system, temporal geochemical variations, and implications for monitoring strategies. Consistent with earlier findings, we observed large compositional differences among the main thermal areas. Sulphur Works and Pilot Pinnacle thermal area gases are characterized by high carbon dioxide concentrations, low hydrogen sulfide concentrations, and high gas-to-steam and nitrogen-to-argon ratios. Notably, gas compositions at Sulphur Works have been stable since the 1980s. Bumpass Hell and Little Hot Springs Valley gases have lower carbon dioxide, and gas-to-steam ratios but higher hydrogen sulfide and methane concentrations. Three samples of Bumpass Hell gas have air-like nitrogen-to-argon ratios, suggesting increased subsurface mixing with air. Devils Kitchen and Boiling Springs Lake gases are moderately similar to Bumpass Hell gas, but gas-to-steam ratios at Boiling Springs Lake are notably lower than gas at Devils Kitchen. Gases discharging at Terminal Geyser are unique, with very high concentrations of hydrogen sulfide, ammonia, methane, and carbon monoxide and very low gas-to-steam ratios.

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For more information about this report, contact: Director, Volcano Disaster Assistance Program U.S. Geological Survey, Volcano Hazards Program 1300 SE Cardinal Court Suite 100 Vancouver, WA 98683 or visit our website at https://www.usgs.gov/programs/VHP/connect Publishing support provided by the U.S. Geological Survey, Science Publishing Network, Baltimore Publishing Service Center.

Bergfeld and others—Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park—OFR 2025–1056

ISSN 2331-1258 (online) https://doi.org/10.3133/ofr20251056

Gas Chemistry and Isotope Data for Volcano Monitoring at the Lassen Volcanic Center, Lassen Volcanic National Park

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