The initial budgets and subsequent exchange of volatiles (e.g., water, carbon, nitrogen, and noble gases) among surface and deep reservoirs govern planetary surface conditions and long-term habitability. Varieties of modern terrestrial basalts-mid-ocean ridge basalts (MORBs), which derive primarily from melting of Earth's upper mantle, and plume-influenced basalts that sample the lower mantle-provide access to two distinct volatile reservoirs within Earth's interior. Their heavy noble gas (Ne, Ar, Kr, and Xe) isotope systematics offer powerful insights into volatile accretion, outgassing, and recycling histories of different terrestrial mantle reservoirs. However, existing high-precision heavy noble gas datasets are limited due to challenges related to sample access and analytical methods. Basaltic glasses from the Central Indian Ridge (CIR), influenced by the nearby R & eacute;union hotspot, provide an opportunity to expand the global heavy noble gas record. Here, we present high precision He, Ne, Ar, and Xe elemental and isotopic data measured from four submarine basalts collected from the CIR axis between 16.7 degrees S and 20.6 degrees S. The resulting heavy noble gas dataset confirms R & eacute;union plume influence at the CIR axis near similar to 20 degrees S and provides constraints on the origins of the region's compositional variations. The mantle source Xe composition of the R & eacute;union-influenced CIR basalt D14-1 is consistent with observations from other plume-influenced localities, suggesting globally uniform plume-source volatile origins, with early-formed isotopic signatures preserved from accretion to the present-day.

Several ocean island basalts (OIBs) exhibit less radiogenic He isotope ratios (i.e., higher 3He/4He) than mid-ocean ridge basalts (MORBs), as well as distinctive primordial and short-lived Ne and Xe isotope signatures. These isotopic features are generally considered to originate from long-term isolation of the deep-mantle source of these OIBs. However, global seismic tomography models have clarified that material exchange occurs between the upper and lower mantle via oceanic lithosphere subduction. This is consistent with the presence of subducted material in the deep-mantle sources of OIBs indicated by the radiogenic isotope compositions of lithophile elements (Sr, Nd, Hf, Pb). The origin of He isotope variations in mantle-derived magmas and especially the occurrence of highly unradiogenic He in some plume-related lavas remains, therefore, unclear. Considering negligible He recycling in the mantle via subduction, previous explanations for these ancient He isotopic signatures include melting of highly depleted sources, storage of primordial He in dense Fe-rich piles resulting from magma ocean crystallization, migration of solar-like He from the core to the lowermost mantle, or preservation of minimally processed, undegassed material in the deep convecting mantle. Here we show that high 3He/4He lavas from oceanic and continental settings predominantly derive from high degrees of melting as indicated by tholeiitic major element compositions, low concentrations of incompatible trace elements (Nb, Th) and low La/Sm ratios. Unradiogenic Sr and Pb combined with supra-chondritic Nd isotope compositions indicate that these lavas were derived from sources that have experienced minimal addition of recycled crustal material. However, these sources do not match ancient mantle domains (similar to FOZO) that are scattered throughout the convective mantle. This inference, combined with no clear relationship between He isotopes and 142Nd/144Nd anomalies, rules out the involvement of old and minimally processed convective mantle in the source of high 3He/4He lavas. The association of high 3He/4He lavas with seismically resolved conduits rooted in the lowermost mantle requires that unradiogenic He is stored in domains at the core-mantle boundary, possibly ULVZ (ultra-low velocity zones), which are exclusively tapped by the hottest and most buoyant plumes. Coupled with a correlation between He isotopes and 182W/184W anomalies, these observations suggest that unradiogenic He is delivered to peridotites in the lowermost mantle from either the core or dense residues of a basal magma ocean. In both cases, this high 3He/4He component exerts a negligible influence on the major and trace element compositions of the mantle source of oceanic and continental lavas with high 3He/4He, which is demonstrably peridotitic and moderately depleted in incompatible elements.

Rift volcanoes are sites of intense volatile emissions. However, major uncertainties remain about the magnitude of rift volatile fluxes, particularly for greenhouse gases (e.g. CO2) and whether along-rift variations in degassing relate to changing mantle sources and/or rifting processes (i.e. melt production and lithospheric thinning). Here, we investigate CO2 flux and gas isotopes in Ethiopia; a mature, plume-influenced continental rift. We focus on one of the largest caldera volcanoes, Bora-Baricha-Tulu Moye (BBTM), which is situated in a region of high mantle melt production in the Central Main Ethiopian rift (similar to 500 km south-west the putative plume head). BBTM gases are characterized by plume-like He-3/He-4 values up to 17 R-a (the highest ever observed in Ethiopian Rift fumaroles) and we calculate total magmatic CO2 flux of 757-901 t d(-1) (making BBTM the largest volcanic CO2 emitter in the Ethiopian Rift). Using our CO2 flux measurements, we estimate total CO2 emissions from Ethiopia's volcanic systems to be 2500-9200 kt yr(-1) and reveal important along-rift variation in CO2 flux. High CO2 flux sectors are found in Central Afar, as well as the Northern and Central Main Ethiopian Rift. These sectors are all regions of high partial melt content (evidenced by low seismic velocity in the underlying mantle) and also show the greatest He-3/He-4 values (>14 R-a). Cooccurrence of high mantle melt production, elevated CO2 emissions and high He-3/He-4 demonstrates that in mature continental rifts carbon emissions and plume volatile contributions are particularly elevated in regions of greatest lithospheric thinning and mantle melting.

Subduction zone forearcs represent the first major devolatilization window of the downgoing slab and play a critical, yet poorly constrained, role in regulating volatile recycling between Earth’s surface and interior. Here we present the first coupled dataset of dissolved noble gases (He–Ne–Ar–Kr–Xe), halogens (Cl–Br–I), and iodine isotopes from Mariana forearc borehole fluids to better constrain the sources, transport, and modification of slab-derived volatiles. The fluids reflect mixing between seawater and deep mantle and slab-derived components. Halogen systematics identify altered oceanic crust containing carbonates and iron oxyhydroxides as the dominant slab fluid source, with volatile release linked to decarbonation and mineral breakdown. Iodine isotopes indicate that these fluids tap ancient reservoirs isolated within the slab for tens to hundreds of millions of years prior to subduction. Ultra-high-precision (sub-permil) xenon isotope measurements provide the first identification of resolvable Xe isotope anomalies in forearc fluids, with heavy Xe excesses uniquely fingerprinting contributions from both mantle and U-rich slab lithologies. In contrast, elemental noble gas systematics reveal strong fractionation during slab processing, with preferential loss of light noble gases relative to heavier species. This is expressed as systematic neon depletion and decoupling of helium from heavier noble gases, consistent with diffusive loss during progressive burial and heating of the slab. Together, these results demonstrate that the Mariana forearc system acts as an efficient filter of slab-derived volatiles, selectively removing light noble gases and limiting their recycling to the deeper mantle.

Abstract The subsurface of our planet hosts 15% of Earth’s biomass and plays a key role in mediating the exchange of volatiles and elements between deep, long-residence-time geological reservoirs and rapidly cycling surface environments, influencing planetary climate and habitability. While a significant fraction of subsurface microorganisms rely on surface-derived organic carbon, an unknown portion is sustained through chemolithotrophic carbon fixation. Despite its importance, the global diversity and distribution of microbial carbon fixation pathways in the subsurface, and the environmental drivers shaping them, remain poorly constrained. Here we systematically characterise carbon fixation pathways for 412 subsurface metagenomes, including 242 new metagenomes, and compare them to surface-derived datasets. We find that subsurface environments span a broader physicochemical space than surface systems and support a higher abundance and diversity of carbon fixation strategies. Using colocated geochemical data spanning >50 variables, we show that the reductive tricarboxylic acid cycle and the reductive acetyl-CoA pathway are enriched in reducing, geochemically evolved fluids. We use the metagenomic results together with previously published carbon fixation rates in the subsurface to derive a global continental subsurface carbon fixation rate of ∼2.65 Pg C yr −1 (range: 0.31–2.99). This represents ∼2% of terrestrial photosynthetic primary production, and is an order of magnitude higher than geological fluxes between the surface and the subsurface. These results identify the subsurface as a reservoir of autotrophic strategies organized along geochemical gradients, contributing substantially to the global carbon cycle. One Sentence Summary The subsurface is a widespread, environmentally and functionally diverse reservoir of autotrophic carbon fixation pathways that can contribute substantially to the global carbon cycle, fixing ∼2.65 Pg C yr −1 in continental settings alone.

The Callaqui volcano (south-central Chile) currently exhibits significant fumarolic activity, and its remains indicate that it has had substantial eruptive activity in the past. However, there is no prior knowledge about the fluids discharged by this volcano, which is critical for volcanic monitoring and subsequent understanding of associated volcanic hazards. This work presents the chemical and isotopic compositions of fumarolic gases from the Callaqui volcano (period 2017–2024) to identify the fluid source(s) and evaluate the physicochemical conditions operating at depth. Gases were collected by direct sampling, using Giggenbach bottles and condensation procedures. The temperature of fumarolic gases ranged from 86.3 to 215 °C, with a composition dominated by water vapor, CO2, and minor amounts of acid species (SO2, HCl, HF). Helium isotope ratios (5.13 to 6.69 Ra) suggest a significant contribution from MORB-like fluids, while δ13C-CO2 isotopes (−10.8 to −9.11‰ vs. V–PDB) suggest assimilation of carbon-bearing crustal compounds. These features suggest a magmatic-hydrothermal origin of Callaqui emissions. The magmatic source corresponds to a degassing magma chamber likely hosted in an intermediate portion of the crust, whereas the hydrothermal system is fed mainly by meteoric waters. Despite the apparent large size of the hydrothermal system, since the volcano is in a rainy and snowy area, magmatically derived species still reach the surface. As a result, partial scrubbing of magma-derived species is assumed at this volcano. Following geothermometric approaches, fumarolic gases indicate the presence of high-temperature magmatic-hydrothermal fluids, with a vapor phase equilibrated at 375–415 °C, as well as super-heated vapors up to 350 °C. Callaqui gases show an increase in the magmatic signal by the end of the study period, consistent with incandescent volcanic episodes and anomalous LP earthquakes reported in late 2021 and early 2022.

The ISL23 expedition, conducted in Iceland during 2023, aimed to investigate microbial diversity and biogeochemical processes in Icelandic geothermal systems. These environments, characterized by extreme physicochemical gradients, provide unique insights into microbial life under extreme conditions. During this expedition we sampled 30 localities in geothermal systems located within neovolcanic zones and older segments of the Icelandic crust, in geothermal systems located within neovolcanic zones and older segments of the Icelandic crust,, including a variety of geothermal features like shallow-sea chimneys, and terrestrial hot springs. We collected fluids, sediments, and gases, alongside environmental data such as temperature, pH, and conductivity, to better understand the interactions between geology, geochemistry, and microbial communities. The samples will contribute to our understanding of microbial life in extreme environments, and offer potential insight into biogeochemical cycling, astrobiology, and biotechnological applications. Ultimately, this expedition aims at expanding our comprehension of how geological activity shapes microbial ecosystems, providing a foundation for future research in Earth’s extreme environments and their potential analogs on other planetary bodies.

We investigated geothermal gases from Homa Hills, a carbonatitic complex situated along an adjacent branch of the Kenyan rift system, using neon, argon, krypton, xenon and nitrogen isotopes. Large quantities of gas were sampled in Giggenbach-type bottles (Giggenbach, 1975) and analyzed by dynamic mass spectrometry to resolve isotopic variations at high precision (0.01-0.1 parts per thousand; Seltzer and Bekaert, 2022; Bekaert et al., 2023; 2024). Neon and nitrogen isotope compositions are consistent with parental magmas being derived from the convecting mantle. Xenon isotopic data present ubiquitous enrichments (relative to air) of 129Xe from the decay of extinct 129I (T1/2 = 15.7 Myr) and 131-136Xef from fissions of 238U (T1/2 = 4.468 Myr) and/or 244Pu (T1/2 = 82 Myr). We also find slight excesses of 128Xe (relative to 130Xe and air), which could be due to subsurface isotopic fractionation during e.g., diffusive transport fractionation (DTF) and gravitational settling. However, the 128Xe excesses are not accompanied by correlated Kr isotope excesses and plot off the empirical fractionation line defined from several other locations worldwide (Bekaert et al., 2023). Instead, a detailed isotope deconvolution suggests the occurrence of either chondritic Xe (with mantle 130Xe consisting of up to 22 % of chondritic 130Xe) or recycled Xe from the Archean atmosphere could explain the observed Xe isotope signatures. The latter possibility would have profound implications for models of mantle-surface exchange throughout Earth history. The fission spectra indicate a predominantly 238U origin for fissiogenic Xe, with contribution of 244Pu-derived Xe being negligible within uncertainties, implying extensive mantle degassing during the Hadean and Archean eons. The 129Xe*/136Xe* ratio (where * indicates non-atmospheric excesses of Xe isotopes) of Homa Hills samples correlates with other tracers of mantle/crust contributions such as He, Ar and N isotopes. Variations in 129Xe*/136Xe* among the different gases sampled at Homa Hills is mainly the result of contribution from fissiogenic Xe produced in uranium-rich crustal material. Therefore, this ratio may constitute a robust tracer of mantle-crust interactions. Given available high precision data (Bekaert et al., 2023; 2024; this work) together with mantle-derived rock data, 129Xe*/136Xe* appears homogenous in the convecting mantle, and comparable to values observed at mantle plumes. Such homogeneity is in sharp contrast with light noble gas systematics and may call for whole mantle convection and a core origin for He and Ne..

The Water Circuit of Minas Gerais, situated in the southeastern region of the Brazilian state of Minas Gerais, has garnered significant interest since the 19th century due to the abundance of mineral springs with heightened mineral compositions and high levels of CO2. However, despite decades of research, the origins of this natural CO2 enrichment phenomenon in these springs remain unclear. Here, we present the first noble gas isotope data from these spring waters combined with hydrochemical data. Calcium bicarbonate waters predominate in the spa town parks of Caxambu, Aguas de Contendas and Lambari, whereas sodium bicarbonate and "mixed type" is found in the Cambuquira park. All samples display high He concentrations and elevated 4He/20Ne values with respect to atmospheric values, indicating a deep origin and the gradual accumulation of radiogenic 4He in the waters. Spring samples show air-corrected 3He/4He (Rc/Ra) values between 0.55 and 3.39 Ra, suggesting an admixture of crustal and mantle volatile contributions. A consistent 3He/4He decrease with increasing distance from the Caxambu shear zone is observed, indicating that the release of deep fluids is fault controlled. We suggest that major fault segments within the Caxambu shear zone serve as continuous pathways for the ascent of mantlederived CO2 and He, enriching the springs of the Water Circuit region. Carbon isotope data indicates a significant contribution of carbonate-derived CO2. We combined discharge estimates with carbon concentrations to quantify a CO2 flux of 4.26 x 106 mol year-1 into the surface system from all springs in the four spa parks.

The fragmentation of continents results in microplates that rotate to accommodate the lateral propagation of bounding rifts. Yet, the relationships between microplate rotation rates, fault slip, and kinematics at propagating rift tips remain unknown. Here, we analyze new Global Navigation Satellite System (GNSS) data and structural geology data from the northern Western Branch of the East African Rift System that defines part of the boundary between the Nubian plate and the Victoria microplate. We resolve 0.0583 +/- 0.0293 degrees/Myr (6.48 +/- 3.26 mm/yr) counterclockwise rotation of the Victoria microplate, consistent with previous studies, but with significant northwestward shift in the Euler pole relative to earlier work. Strain is largely localized on microplate-bounding faults with 1.8-2.2 mm/yr slip rates, 7.2 x 10(-8)-1.28 x 10(-7) y(-1) strain rates, NE-directed extension, and oblique-normal fault kinematics. Most GNSS velocities are consistent with block rigidity, but three sites in the NW region of the Victoria microplate indicate possible internal deformation.

In subduction zones, thermal springs release deeply-sourced volatiles from Earth's mantle, crust, and/or subducted slab-derived material. The origin and apparent ages of these volatiles are important for understanding the deep volatile cycle, which in turn affects the distribution of microbial life in the subsurface. Here, we report carbon (C-13, C-14), noble gas (He, Ne, Ar, Kr and Xe), and clumped nitrogen isotope data in gas and water samples from thermal springs within the Central Volcanic Zone (CVZ) of the Andean Convergent Margin (ACM). He isotopes show that CVZ gases are predominantly sourced from the crust (similar to 77 %), with smaller mantle contributions (similar to 23 %), consistent with previous studies from the CVZ. Thermal spring samples with non-atmospheric He-Ne characteristics have low C-14 activities, and are deeply derived (i.e., from the mantle and crust) and old (>22,000 years). To gain additional constraints on volatile sources, a gas sample from Pirquitas Argentina was analyzed using a new high-precision technique to reveal significant geogenic anomalies in argon (Ar-40/Ar-36 = 492), fissiogenic xenon (88 % crustal), and helium (84 % crustal) isotopes. Clumped N-2 isotopologue results also indicate that the N-2-rich Pirquitas sample is dominated by crustal and magmatic N-2, which was unambiguously released at high temperatures (indicated by Delta(30) of similar to 0 parts per thousand). When taken together, all carbon, noble gas and clumped N-2 isotope data from CVZ thermal springs point toward a predominantly crustal source of volatile elements, which is consistent with the thick crust beneath the arc. We conclude that thermal springs with noble gas isotopic evidence for minimal air contributions are old, suggesting that any microbial communities entrained in them are also supported by deeply-derived and old organic carbon.

Billions of people rely upon groundwater for drinking water and agriculture, yet predicting how climate change may affect aquifer storage remains challenging. To gain insight beyond the short historical record, we reconstruct changes in groundwater levels in western North America during the last glacial termination (LGT, ~20 to 11 thousand years ago) using noble gas isotopes. Our reconstructions indicate remarkable stability of water table depth in a Pacific Northwest aquifer throughout the LGT despite increasing precipitation, closely matching independent Earth system model (ESM) simulations. In the American Southwest, ESM simulations and noble gas isotopes both suggest a pronounced LGT decline in water table depth in in response to decreasing precipitation, indicating distinct regional groundwater responses to climate. Despite the hydrologic simplicity of ESMs, their agreement with proxy reconstructions of past water table depth suggests that these models hold value in understanding groundwater dynamics and projecting large-scale aquifer responses to climate forcing.

Sources of nitrogen (N) in the plume mantle source remains hotly debated between a primordial origin (i.e., acquired during Earth's formation) and a recycled origin associated with subduction of surficial material. Although N isotope data for plume-derived magmas are limited, the available data show clear differences in N isotope compositions between plume (delta N-15 > -2 parts per thousand) and the depleted mid-ocean-ridge basalt mantle sources (i.e., DMM; delta N-15 = -5 +/- 2 parts per thousand). Here we present N-isotope and noble gas isotope data from two suites of well-characterized plume-influenced submarine basaltic glasses with high He-3/He-4: 1) the Rochambeau Bank in the northeastern Lau backarc basin (up to similar to 23 RA, where RA refers to the atmospheric He-3/He-4 ratio), and 2) the Reykjanes Ridge (up to similar to 18 R-A) south of Iceland. These sample suites are associated with different tectonic settings: the Tonga subducting slab interacts with the Samoan plume beneath the Lau backarc basin, whereas the Mid-Atlantic Ridge interacts with the Icelandic plume at the Reykjanes Ridge. The contrasting tectonic settings provide a unique opportunity to decipher both the origin of N in plume mantle sources and the interaction of the plume mantle with other mantle components, including recycled material from subducting slabs and the DMM. Our results show that Rochambeau Bank (delta N-15 from +1.3 parts per thousand to +2.8 parts per thousand) and Reykjanes Ridge samples (delta N-15 from -2.3 parts per thousand to +0.1 parts per thousand) are both characterized by delta N-15 values that are enriched relative to the DMM. Rochambeau Bank data are consistent with ternary mixing between the DMM (delta N-15 = -5 +/- 2 parts per thousand; N-2/He-3 = 3.7 +/- 1.2 x 10(6); N-2/Ar-40* = 138 +/- 65), a subduction component from the adjacent Tonga slab with recycled N (delta N-15 = similar to+3 parts per thousand; N-2/He-3 = similar to 10(9); N-2/Ar-40* = similar to 5 x 10(6)), and a third component with delta N-15 = similar to 0 %, N-2/He-3 = similar to 2 x 10(5) and N-2/Ar-40* = similar to 40, which we attribute to the Samoan plume component enriched in primordial N. In contrast, Reykjanes Ridge data, combined with Iceland data, are consistent with ternary mixing among the DMM, plume components with recycled N (delta N-15 from 0 parts per thousand to +6 parts per thousand; N-2/He-3 = similar to 10(9); N-2/Ar-40* = similar to 5 x 10(6)) and primordial N (delta N-15 from -2 parts per thousand to +2 %; N-2/He-3 = similar to 2 x 10(5); N-2/Ar-40* = similar to 40) endmembers in deep Icelandic mantle plumes. This is consistent with the presence of both recycled and primordial N being intrinsic to the Icelandic mantle plume. By integrating N data from other global plume-influenced samples (i.e., Society, Hawaii, and Central Indian Ridge), we show that the global dataset is consistent with ternary mixing among the DMM, and plume components entrained with both recycled N and primordial N. Nitrogen in deep plume sources is therefore likely hybrid in composition, containing both recycled N from subducting slab and primordial N retained from Earth's early stages of formation. The heavier delta N-15 of an apparent primordial N component within plume mantle sources (-2 parts per thousand to +2 parts per thousand) relative to that of the convecting MORB mantle (similar to-5 parts per thousand) potentially requires an addition of N-15-rich carbonaceous material from the outer solar system to the early accreting Earth.

The source of volcanism in the Cameroon volcanic line remains unresolved. To better constrain the mantle components involved, we measure light noble gas (He-Ne-Ar) isotopes and, for the first time, heavy noble gas (Kr-Xe) isotopes in mantle-derived gases from São Tomé, using high-precision dynamic mass spectrometry (DMS). Our data show that the source of São Tomé volcanism is a plume-like mantle reservoir, which is enriched in 22Ne relative to nucleogenic 21Ne when compared to samples originating from the convecting upper mantle, as represented by Mid-Ocean Ridge Basalts (MORBs). In contrast, the isotopic compositions of helium and xenon appear similar to those of the MORB mantle. DMS analysis of Xe isotopes reveals that volcanic gases are affected by diffusive transport fractionation in the subsurface, causing small enrichment in the light isotopes (e.g., 128Xe). After correction for this secondary fractionation, we observe a slight, yet discernible, 136Xe excess relative to the MORB mantle, attributable to spontaneous fission of 238U. We hypothesize that this excess fissiogenic Xe may arise from the 238U associated with recycled crustal component(s) in the São Tomé mantle source, potentially related to the influence of the HIMU-type mantle. The consistent 129Xe/136Xe (relative to air) observed in both MORB and plume-influenced mantle components is difficult to reconcile with the commonly accepted view that 136Xe excesses originate from two distinct sources238U and 244Pu fission, respectivelyin these separate mantle reservoirs. Further investigation is needed to determine whether the apparent homogeneity in 129Xe/136Xe excess across the mantle, which contrasts with the distinct sources and evolutionary histories of other volatile elements (e.g., He, Ne, and N2), is coincidental or indicative of an underlying process.

The genesis of kimberlites is unclear despite the economic and scientific interest surrounding these diamond-bearing magmas. One critical question is whether they tap ancient, deep mantle domains or the shallow convecting mantle with partial melting triggered by plumes or plate tectonics. To address this question, we report the He-Ne-Ar isotopic compositions of magmatic fluids trapped in olivine from kimberlites worldwide. The kimberlites which have been least affected by addition of deeply subducted or metasomatic components have Ne isotopes less nucleogenic than the upper mantle, hence requiring a deep-mantle origin. This is corroborated by previous evidence of small negative W isotope anomalies and kimberlite location along age-progressive hot-spot tracks. The lack of strong primordial He isotope signatures indicates overprinting by lithospheric and crustal components, which suggests that Ne isotopes are more robust tracers of deep-mantle contributions in intraplate continental magmas. The most geochemically depleted kimberlites may preserve deep remnants of early-Earth heterogeneities.

We present geochemical data from gas samples from similar to 1200 km of arc in the Central Volcanic Zone of the Andes (CVZA), the volcanic arc with the thickest (similar to 70 km) continental crust globally. The primary goals of this study are to characterize and understand how magmatic gases interact with hydrothermal systems, assess the origins of the major gas species, and constrain gas emission rates. To this end, we use gas chemistry, isotope compositions of H, O, He, C, and S, and SO2 fluxes from the CVZA. Gas and isotope ratios (CO2/S-T, CO2/CH4, H2O/S-T, delta C-13, delta S-34, He-3/He-4) vary dramatically as magmatic gases are progressively affected by hydrothermal processes, reflecting removal and crustal sequestration of reactive species (e.g., S) and addition of less reactive meteoric and crustal components (e.g., He). The observed variations are similar in magnitude to those expected during the magmatic reactivation of volcanoes with hydrothermal systems. Carbon and sulfur isotope compositions of the highest temperature emissions (97-408 degrees C) are typical of arc magmatic gases. Helium isotope compositions reach values similar to upper mantle in some volcanic gases indicating that transcustal magma systems are effective conduits for volatiles, even through very thick continental crust. However, He isotopes are highly sensitive to even low degrees of hydrothermal interaction and radiogenic overprinting. Previous work has significantly underestimated volatile fluxes from the CVZA; however, emission rates from this study also appear to be lower than typical arcs, which may be related to crustal thickness.

Nitrogen plays a critical role in maintaining Earth's hospitable surface environment over geological time. Despite our atmosphere being dominated by nitrogen, our understanding of how nitrogen was delivered to Earth and how subsequent planetary processes modified Earth's nitrogen budget through time is currently lacking. Here, we report measurements of isotopologues of N2 (Δ30), along with ultrahigh precision measurements of Ar, Kr, and Xe isotopes, of hydrothermal gas samples from Yellowstone National Park. We show that δ15N variations are correlated with nonradiogenic Ar, Kr, and Xe isotope ratios, indicating that groundwater-derived nitrogen and noble gases in hydrothermal samples are fractionated by the same process as they diffuse through a rising column of magmatic CO2. Notably, a similar correlation exists regardless of the degree of atmospheric contamination, suggesting that the δ15N of the Yellowstone mantle source is similar to the atmosphere (i.e., ∼0‰). Two component mixing models between Δ30 and noble gases demonstrate that N2/36Ar (5.3 ± 0.7 × 105) and 36Ar/130Xe (1611 ± 212) in the Yellowstone mantle source are lower and greater than the MORB mantle source, respectively, suggesting that contrary to previous findings, the plume mantle source has not been more efficiently overprinted by the addition of N2- and Xe-rich recycled material. Conversely, we suggest that the similarity in δ15N and N2/36Ar between the Yellowstone mantle source and chondritic meteorites indicates that nitrogen and noble gases in the deep mantle reflect the composition of the material that initially formed Earth.

Helium (He), which is an irreplaceable resource in low-carbon technologies, medical applications, and various science and engineering sectors, is currently being explored as a primary resource. The correlation between crustal helium (He) release and heat flow in certain geologic environments (e.g., mid-ocean ridge and continental hydrothermal systems) is well established, but few studies have evaluated how past igneous processes influence current gas release/storage from crustal rocks. Here, we report bulk gas and noble gas geochemistry data (n = 43) gathered from thermal springs in and around the Yellowstone National Park (YNP), USA. Samples outside the YNP (near the plume track) are dominantly N2-rich, while most samples collected within the active caldera area are CO2-rich. Samples outside the YNP typically have much lower 3He/4He than those within the caldera (i.e., near the current plume head). We explore the relationship between thermal aureoles and He isotopic signatures using heat flow data coupled with bulk gas and noble gas geochemistry data. Data are used to determine gas origins, to understand fluid flow in a regional context, and to assess how different environments impact He release from crustal minerals. Models indicate that advection is the dominant process controlling heat and volatile loss from mantle to crustal systems from the Yellowstone Caldera. In contrast, the influence of conduction/boiling of crustal hydrothermal fluids is more substantial for samples outside of the Yellowstone Caldera. Helium-4 is enriched in the samples which are frontal and near the eruptive center, likely due to recent crustal degassing of 4He accumulated over long periods in the underlying craton. Ultimately, He and other volatiles are released due to tectonic activity and/or they are enriched as other gases partition out of groundwater (i.e., gas stripping from groundwater). However, elevated heat flow zones likely constitute poor He retention zones. We propose a twofold approach to help identify preferential zones of He release: 1) focusing on areas that are distal from active igneous zones (i.e., areas that have not been fully degassed) with localized moderate heat flow to release trapped crustal He, and 2) utilizing isotope models to constrain groundwater interactions (i.e., migration and accumulation potential).

The chemical and isotopic composition of the gases emitted by subduction zone volcanoes can provide insights into the origin of magmatic volatiles. In volcanic arcs, magmatic volatiles and therefore emitted gases can be supplied from the mantle, the subducting slab, or the rocks of the arc crust. Determining the relative contributions of these distinct sources is important for understanding the transfer of volatiles between Earth’s interior and exterior reservoirs, which has implications for the physical and chemical evolution of both the mantle and the atmosphere. Each subduction zone is a different experiment in recycling efficiency according to the composition of the slab and the pressure-temperature path it experiences on subduction, and accordingly all volcanic arc emissions can be characterised by their particular chemical and isotopic compositions. In this study, we analyse the composition of volcanic gases from Rabaul caldera in the New Britain subduction zone, Papua New Guinea, and show that the emissions are substantially influenced by slab recycling of carbon and nitrogen. We find helium emissions are dominated by a mantle contribution, with little influence from the arc crust. Carbon isotopes point to a mixture of mantle, carbonate and organic sediment-derived contributions, with carbonate dominant. This may be of sedimentary origin, seafloor calcareous muds, or altered basalts of the subducting oceanic crust. Nitrogen isotopes also indicate a significant influence of organic sediments. Our study is the first comprehensive investigation of volatile sources in this subduction and our results and interpretation are consistent with previous studies of element recycling based on New Britain arc lavas.

Subduction of the Cocos and Nazca oceanic plates beneath the Caribbean plate drives the upward movement of deep fluids enriched in carbon, nitrogen, sulfur, and iron along the Central American Volcanic Arc (CAVA). These compounds fuel diverse subsurface microbial communities that in turn alter the distribution, redox state, and isotopic composition of these compounds. Microbial community structure and functions vary according to deep fluid delivery across the arc, but less is known about how microbial communities differ along the axis of a convergent margin as geological features (e.g., extent of volcanism and subduction geometry) shift. Here, we investigate changes in bacterial 16S rRNA gene amplicons and geochemical analysis of deeply-sourced seeps along the southern CAVA, where subduction of the Cocos Ridge alters the geological setting. We find shifts in community composition along the convergent margin, with communities in similar geological settings clustering together independently of the proximity of sample sites. Microbial community composition correlates with geological variables such as host rock type, maturity of hydrothermal fluid and slab depth along different segments of the CAVA. This reveals tight coupling between deep Earth processes and subsurface microbial activity, controlling community distribution, structure and composition along a convergent margin.