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Volatiles, such as carbon and noble gases, are continuously degassed from Earth's mantle and crust to Earth's surface in continental rifting systems. Here, we present He-Ne-Ar abundances and isotopes as well as major gas chemistry (CO2, He, Ar and CH4) data for (n = 13) naturally-degassing seeps in the Rungwe Volcanic Province (RVP) of the East African Rift System (EARS) in southwestern Tanzania, Africa. Helium isotopes (3He/4He) suggest that trace gases are derived from mantle sources, with moderate crustal additions, in agreement with previous studies from RVP (e.g., Pik et al., 2006; Barry et al., 2013). Samples broadly fall into two groups: 1) dominantly mantle-like, with 3He/4He ranging from 5.2–6.3RA, which can be explained by release of magmatic volatiles, and 2) those with slightly lower 3He/4He (3.0–4.1RA), representing more 4He (i.e., crustal) additions. Furthermore, we report the first Ne and Ar isotopes from RVP, which show 20Ne/22Ne from 9.67–10.0 and 40Ar/36Ar from 301.2–412.2, respectively, which are broadly air-like. We employ a solubility degassing model to show that elemental ratios (4He/40Ar*, CO2/40Ar* and CO2/3He) in the gases can be explained by variable extents of open and closed system degassing from a melt with an initial mantle-like composition. However, we note that CO2/3He observations require additional carbon assimilation, likely derived from the thick nearby cratonic crust.

In regions where water resources are scarce and in high demand, it is important to safeguard against contamination of groundwater aquifers by oil-field fluids (water, gas, oil). In this context, the geochemical characterisation of these fluids is critical so that anthropogenic contaminants can be readily identified. The first step is characterising pre-development geochemical fluid signatures (i.e., those unmodified by hydrocarbon resource development) and understanding how these signatures may have been perturbed by resource production, particularly in the context of enhanced oil recovery (EOR) techniques. Here, we present noble gas isotope data in fluids produced from oil wells in several water-stressed regions in California, USA, where EOR is prevalent. In oil-field systems, only casing gases are typically collected and measured for their noble gas compositions, even when oil and/or water phases are present, due to the relative ease of gas analyses. However, this approach relies on a number of assumptions (e.g., equilibrium between phases, water-to-oil ratio (WOR) and gas-to-oil ratio (GOR) in order to reconstruct the multiphase subsurface compositions. Here, we adopt a novel, more rigorous approach, and measure noble gases in both casing gas and produced fluid (oil-water-gas mixtures) samples from the Lost Hills, Fruitvale, North and South Belridge (San Joaquin Basin, SJB) and Orcutt (Santa Maria Basin) Oil Fields. Using this method, we are able to fully characterise the distribution of noble gases within a multiphase hydrocarbon system. We find that measured concentrations in the casing gases agree with those in the gas phase in the produced fluids and thus the two sample types can be used essentially interchangeably. EOR signatures can readily be identified by their distinct air-derived noble gas elemental ratios (e.g., 20Ne/36Ar), which are elevated compared to pre-development oil-field fluids, and conspicuously trend towards air values with respect to elemental ratios and overall concentrations. We reconstruct reservoir 20Ne/36Ar values using both casing gas and produced fluids and show that noble gas ratios in the reservoir are strongly correlated (r2 = 0.88–0.98) to the amount of water injected within ~500 m of a well. We suggest that the 20Ne/36Ar increase resulting from injection is sensitive to the volume of fluid interacting with the injectate, the effective water-to-oil ratio, and the composition of the injectate. Defining both the pre-development and injection-modified hydrocarbon reservoir compositions are crucial for distinguishing the sources of hydrocarbons observed in proximal groundwaters, and for quantifying the transport mechanisms controlling this occurrence.

Carbon dioxide emissions from active subaerial volcanoes represent 20–50% of the annual global volcanic CO2 flux (Barry et al., 2014). Passive degassing of carbon from the flanks of volcanoes, and the associated accumulation of dissolved inorganic carbon (DIC) within nearby groundwater, also represents a potentially important, yet poorly constrained flux of carbon to the surface (Werner et al., 2019). Here we investigate sources and sinks of DIC in groundwaters in the Lassen Peak region of California. Specifically, we report and interpret the relative abundance and isotopic composition of helium (3He, 4He) and carbon (12C, 13C, 14C) in 37 groundwater samples, from 24 distinct wells, collected between 20 and 60 km from Lassen Peak. Measured groundwater samples have air-corrected 3He/4He values between 0.19 and 7.44 RA (where RA = air 3He/4He = 1.39 × 10−6), all in excess of the radiogenic production value (~0.05 RA), indicating pervasive mantle-derived helium additions to the groundwater system in the Lassen Peak region. Stable carbon isotope ratios of DIC (δ13C) vary between −12.6 and − 27.7‰ (vs. VPDB). Measured groundwater DIC/3He values fall in the range of 2.2 × 1010 to 1.1 × 1012. Using helium and carbon isotope data, we explore several conceptual models to estimate surface carbon contributions and to differentiate between DIC derived from soil CO2 versus DIC derived from external (slab and mantle) carbon sources. Specifically, if we use 14C to identify soil-derived DIC (assuming decadal-to-centennial groundwater ages and a soil CO2 14C activity equal to that of the atmosphere), we calculate that a hypothetical external carbon source would have an apparent δ13C signature between −10.3 and − 59.3‰ (vs. Vienna Pee Dee Belemnite (VPDB)) and an apparent C/3He between 7.0 × 109 and 1.0 × 1012. These apparent δ13C and C/3He values are substantially isotopically lighter than and greater than canonical MORB values, respectively. We suggest that >95% of any external (non-soil-derived) DIC in groundwater must thus be non-mantle in origin (i.e., slab derived or assimilated organic carbon). We further investigate possible sources of external DIC to groundwater using two idealized conceptual approaches: a pure (unfractionated) source mixing model (after Sano and Marty, 1995) and a scenario that invokes fractionation due to calcite precipitation. Because the former model requires carbon contributions from an organic source component with unrealistically low δ13C (~ − 60‰), we suggest that the second scenario is more plausible. Importantly, however, we caution that all conceptual models are dependent on assumptions about initial 14C activity. Thus, we cannot rule out the possibility that the true fraction of non-surface-derived DIC in these samples is lower or negligible, despite the pervasive mantle-derived He isotope signatures throughout the region. Following the 14C approach to deconvolving sources of DIC, we determine that the maximum passive carbon flux could be up to ~2.2 × 106 kg/yr, which is lower than previous magmatic carbon flux estimates from the Lassen region ( Rose and Davisson, 1996 ). We find that the passive dissolved carbon flux could represent a maximum of ~4–18% of the total Lassen geothermal CO2 degassing flux (estimated to be ~3.5 × 107 kg/yr Rose and Davisson, 1996 ; Gerlach et al., 2008 ), which is still more than an order of magnitude smaller than soil gas CO2 flux estimates (7.3–11 × 107 kg/yr) for nearby volcanoes ( Sorey et al., 1998 ; Gerlach et al., 1999 ; Evans et al., 2002 ; Werner et al., 2014 ). We conclude that passive dissolved carbon fluxes should be combined with geothermal fluxes and soil gas fluxes to obtain a complete picture of volcanic carbon emissions globally. Our approach highlights the utility of measuring helium isotopes in concert with the full suite of noble gas abundances, tritium, δ13C and 14C, which when interpreted together can be used to better elucidate the various sources of DIC in groundwater.

Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (

The Baja California Peninsula is located within a complex geodynamic setting, marked by continental rifting immediately to the east in the Gulf of California, and abundant geothermal manifestations. We report helium and carbon isotopic and relative abundance data sampled from gas and fluid localities (n = 12) across northern Baja California, Mexico. Samples were collected in three distinct regions of Baja: Pacific Borderlands (PB) in northwestern Baja, Peninsula Ranges and Inland Valleys (PRIV) in central Baja, and the Gulf Extensional Province (GEP) in northeastern Baja. Helium isotopes (3He/4He), carbon isotopes (δ13CO2), and CO2/3He values all are highest in GEP samples, and lower in samples from the PB and PRIV. In the GEP, helium isotopes reach moderately mantle-like values (1.74 RA) (where RA = air 3He/4He), while in the PRIV and PB provinces values are more radiogenic (~0.11 RA). 3He/4He values suggest mixing between mantle-derived and crustal-derived radiogenic components, with a maximum mantle contribution of ~21% in the GEP samples. High He isotopes correlate with low mantle velocity zones at a depth of 25–40 km, suggesting a mantle source of He in the GEP, due to extensional crustal thinning. Carbon isotopes (δ13CO2) vary widely between −17.6 and 4.5‰ (vs. PDVB) and CO2/3He values vary over several orders of magnitude (2.0 × 105 to 1.1 × 1013). Variable δ13CO2 and CO2/3He values in the PRIV and PB localities are consistent with calcite precipitation in shallow-level (upper 5 km) hydrothermal systems. The bimodal nature of the He-CO2 data suggests fundamentally different transfer mechanisms in each of the three distinct regions, indicating complex interplay between regional mantle degassing, faulting-assisted migration of crustal fluids, and volatile sequestration via secondary processes (e.g., calcite precipitation). These results, along with literature data, allow for systematic assessment of both mantle sources and secondary processes, providing new insights into how volatiles are transported through the Baja California peninsula.

Helium isotopes are useful for tracing terrestrial water movement and calculating residence times, which have important implications for radioactive waste disposal and carbon capture and storage projects. Helium concentrations are generally measured directly in water samples; however, this is not always possible. In this scenario, the He concentration equilibrium between pore waters and He accessible volumes (e.g., fluid inclusions, hereafter HAV) in minerals (e.g., quartz) can be utilized to determine He concentrations. In this study, the He partial pressure was measured in HAVs of quartz grains collected from cores of the Kola Super Deep Borehole (KSDB) using isothermal He extraction/saturation experiments. A large fraction of He was released from these samples during the first interval of isothermal heating, indicating a destruction of the unstable HAVs, likely due to decrepitation of fluid inclusions. We present a new approach to calculate He partial pressures using only the He fraction released from the stable HAVs. This approach gives a He partial pressure pHe of ≅ 1.5 atm at depths of ~ 10 km. When combined with solubility data, this value suggests a high He concentration in the deep pore waters nearby the KSDB, ≅ 1.3 × 10−6 mol (cc H2O)−1, indicating a long residence time of He atoms in a deep water – mineral system, close to the age of the regional metamorphism (≅1700 Ma). The 3He/4He in the deep KSDB quartz samples (and the pore waters) are ~ 3 × 10−8, typical of radiogenic He generated in 10 km deep Archean rocks of the KSDB. Importantly, external fluxes are not needed to explain He isotope abundances in rocks, minerals and pore waters at these depths.

To investigate the source of volatiles and their relationship to the San Andreas Fault System (SAFS), 18 groundwater samples were collected from wells near the Big Bend section of the SAFS in southern California and analyzed for helium and carbon abundance and isotopes. Concentrations of 4 He, corrected for air-bubble entrainment, vary from 4.15 to 62.7 (× 10 − 8 ) cm 3 STP g − 1 H 2 O. 3 He/ 4 He ratios vary from 0.09 to 3.52 R A (where R A = air 3 He/ 4 He), consistent with up to 44% mantle helium in samples. A subset of 10 samples was analyzed for the major volatile phase (CO 2 ) — the hypothesized carrier phase of the helium in the mantle–crust system: CO 2 / 3 He ratios vary from 0.614 to 142 (× 10 11 ), and δ 13 C (CO 2 ) values vary from − 21.5 to − 11.9‰ (vs. PDB). 3 He/ 4 He ratios and CO 2 concentrations are highest in the wells located in the Mil Potrero and Cuddy valleys adjacent to the SAFS. The elevated 3 He/ 4 He ratios are interpreted to be a consequence of a mantle volatile flux though the SAFS diluted by radiogenic He produced in the crust. Samples with the highest 3 He/ 4 He ratios also had the lowest CO 2 / 3 He ratios. The combined helium isotope, He–CO 2 elemental relationships, and δ 13 C (CO 2 ) values of the groundwater volatiles reveal a mixture of mantle and deep crustal (metamorphic) fluid origins. The flux of fluids into the seismogenic zone at high hydrostatic pressure may cause fault rupture, and transfer volatiles into the shallow crust. We calculate an upward fluid flow rate of 147 mm a − 1 along the SAFS, up to 37 times higher than previous estimates ( Kennedy et al., 1997 ). However, using newly identified characteristics of the SAFS, we calculate a total flux of 3 He along the SAFS of 7.4 × 10 3 cm 3 STP a − 1 (0.33 mol 3 He a − 1 ), and a CO 2 flux of 1.5 × 10 13 cm 3 STP a − 1 (6.6 × 10 8 mol a − 1 ), ~ 1% of previous estimates. Lower fluxes along the Big Bend section of the SAFS suggest that the flux of mantle volatiles alone is insufficient to cause the super hydrostatic pressure in the seismogenic zone; however, results identify crustal (metamorphic) fluids as a major component of the CO 2 volatile budget, which may represent the additional flux necessary for fault weakening pressure in the SAFS.

We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13 °C to 65 °C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13 °C and 36 °C and are located at higher elevation than the hot springs. The gas compositions are ~ 99% CO2, 0.0008 to 0.0078 mmol/mol H2, 0.0004 to 0.062 mmol/mol He, 0.08 to 0.77 mmol/mol Ar, 3.1 to 28.5 mmol/mol N2, 0.4 to 3.73 mmol/mol O2, 250 °C (from CO2–Ar) to ~ 60 °C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between + 2‰ and − 5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.

We report new helium and carbon isotope ( 3 He/ 4 He and δ 13 C) and relative abundance (CO 2 / 3 He) characteristics of a suite of 20 gases and fluids (cold mazuku-like CO 2 vents, bubbling mud-pots, hot and cold springs) from 11 different localities in Rungwe Volcanic Province (RVP), southern Tanzania and from 3 additional localities in northern Tanzania (Oldoinyo Lengai Volcano and Lake Natron). At RVP, fluids and gases are characterized by a large range in He-isotope compositions ( 3 He/ 4 He) from 0.97 R A to 7.18 R A (where R A = air 3 He/ 4 He), a narrow range in δ 13 C ratios from − 2.8 to − 6.5‰ (versus VPDB), and a large range in CO 2 / 3 He values spanning nearly four orders of magnitude (4 × 10 9 to 3.2 × 10 13 ). Oldoinyo Lengai possesses upper‐mantle-like He–CO 2 characteristics, as reported previously (Fischer et al., 2009), whereas hot springs at Lake Natron have low 3 He/ 4 He (~ 0.6 R A ), CO 2 / 3 He (~ 5–15 × 10 8 ) and intermediate δ 13 C (~−3.7 to − 4.9 ‰). At RVP, fluid phase samples have been modified by the complicating effects of hydrothermal phase-separation, producing CO 2 / 3 He and δ 13 C values higher than postulated starting compositions. In contrast, gas-phase samples have not been similarly affected and thus retain more mantle-like CO 2 / 3 He and δ 13 C values. However, the addition of crustal volatiles, particularly radiogenic helium from 4 He-rich reservoir rocks, has modified 3 He/ 4 He values at all but the three cold CO 2 gas vent (i.e., mazuku) localities (Ikama Village, Kibila Cold Vent and Kiejo Cold Vent) which retain pristine upper-mantle He-isotope (~ 7 R A ) and He–CO 2 characteristics. The extent of crustal contamination is controlled by the degree of interaction within the hydrothermal system, which increases with distance from each major volcanic center. In contrast, we propose that pristine cold CO 2 mazuku gases collected at stratigraphic contacts on the flanks of RVP volcanoes may potentially tap isolated gas pockets, which formed during previous eruptive events and have remained decoupled from the local hydrothermal system. Furthermore, by identifying and utilizing unmodified gas samples, we determine mantle versus crustal provenance of the CO 2 , which we use to estimate mantle-derived CO 2 fluxes at both Rungwe and Lake Natron. Finally, we investigate the origin of the apparent discrepancy in He isotopes between fluids/gases and mafic phenocrysts at RVP (from Hilton et al., 2011), and discuss the tectonic (i.e., rift zone dynamics) and petrogenic conditions that distinguish RVP from other plume-related subaerial rift zones.