The Trans-Mexican Volcanic Belt (TMVB) and the Gulf Extensional Province (Baja California) are the regions in Mexico with the highest heat flow and where the main geothermal energy resources are currently exploited. Here, previously published and newly collected heat and volatile species (He, Ar and CO2) data from 132 fluid samples of four geothermal areas belonging to TMVB (Los Azufres, Los Humeros, Acoculco and Cerritos Colorados) and two in Baja California (Cerro Prieto and Las Tres Vírgenes) were analyzed. The goal was to identify heat sources, understand how heat is transported into the geothermal reservoirs and how the multiple fluid sources (meteoric, magmatic and crustal) in the reservoir might impact the heat in each field. Distinct R/Ra ratios (where R is the measured 3He/4He ratio and Ra is the atmospheric ratio) are observed in the TMVB and Baja California geothermal fields, with those in the TMVB displaying significantly higher R/Ra values (7.14–7.27) than those in Baja California (1.21–6.62), indicating a higher contribution of mantle helium from possibly younger, active magmatic heat sources. Lower R/Ra values in Baja California might reflect local tectonic features, where reservoirs are primarily affected by crustal contributions from an old, subducted fossil slab related to the Farallon Plate. The analysis of heat to 3He ratios (or Q/3He) together with information provided by 4He/36Ar ratios suggest that heat and volatiles are transferred into the hydrothermal system by advection. The magma bodies beneath the geothermal areas of interest are sufficiently voluminous to provide aqueous fluids by exsolution to sustain this advective flow. The release of He and Ar from magma, during its degassing, appears to be controlled by their diffusivities, fractionating the pristine mantle 4He/36Ar ratio toward higher values, while advection transfers volatiles more rapidly than heat, fractionating the pristine mantle Q/3He ratios toward lower values. The Q/3He vs. 4He/3He relationship shows that boiling and dilution by meteoric water led to further fractionation of heat and volatiles, particularly in Los Azufres and Los Humeros fields, while Cerro Prieto has Q/3He ratios close to those at mid-ocean ridges or fractionated by circulation of low-heat, 4He-rich fossil waters. Distinct sources of heat and volatiles are also reflected in the CO2 content, the major dry gas component in these geothermal fields. Main sources of CO2 are mantle carbon (1%-51%) and limestone (34%–100%) from local sources and the subducting slab, although expected fractionation of the C isotopic composition and CO2/3He during magma ascent and degassing introduce uncertainties on the precise estimation of each source contributing C to the studied geothermal reservoirs. Yet, contribution of carbon from sediments might be minor (0%–35%), and mainly found in Cerro Prieto and Las Tres Virgenes fields located in Baja California, which are affected by the release of volatiles from the subducted Farallon plate and presence of connate waters in the reservoir of Cerro Prieto.

Extraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40Ar-39Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1Md, 1M, and 2M1) was chosen for dating with the 40Ar-39Ar method. The results of 40Ar-39Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1Md) and inherited (1M + 2M1) components, respectively. The MODELAGE-based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1Md, 15 ± 37 Ma; 1M, 135 ± 57 Ma; 2M1, 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1Md polytype was probably formed during the most recent stage of fault activity, while 1M and 2M1 polytypes are of equal age, roughly.

The noble gas temperature climate proxy is an established tool that has previously been applied to determine the source of groundwater recharge, however, unanswered questions remain. In fractured media (e.g., volcanic islands) recharge can be so rapid that groundwater is significantly depleted in heavy noble gases, indicating that the water has retained noble gas concentrations from higher elevations. Previous studies of rain samples have confirmed a match to patterns seen in fractured‐rock groundwater for heavy noble gases along with a significant helium excess. Snow has been shown to be a credible source for both the helium excess and the observed heavy noble gas pattern. Here, liquid cloud water samples were collected at two mountainous sites and analyzed for noble gas concentrations. A pattern like that of rainwater was found. However, an analysis of diffusive uptake of noble gases into cloud water demonstrates that droplets of 1 mm diameter and smaller should be in constant solubility equilibrium with the atmosphere. To explain this, we present a novel hypothesis that relies on the assumption that liquid water consists of two types of water molecule clusters bounded by hydrogen bonds: a low‐density ice‐like structure and a high‐density condensed structure. In this model, the pressure gradient near the surface of a droplet resulting from surface tension could allow for the formation of a surface layer that is rich in ice‐like low density clusters. This can explain both the helium excess and the heavy noble gas depletion seen in the samples.

The Los Humeros Geothermal Field, discovered in the late 1970s, is the last geothermal field in Mexico to be significantly developed in recent years. Located in a nested caldera system of the Central Mexican Volcanic Belt, the field is within an andesitic reservoir, with low porosity and permeability, limiting the recharge of the system. Here, a new dataset of noble gas and stable isotopes (δ2H and δ 18O), tritium (3H), CO2 contents, halogens (Cl content and δ37Cl, δ81Br), and isotopes of Sr (87Sr/86Sr) were analyzed and interpreted along with previously published noble gas data. The goal was to identify the fluid sources present in the Los Humeros reservoir. This study highlights a complex hydrogeological scenario, with multiple generations of meteoric waters. Furthermore, some volatile species of mantle origin (He, Ar, C) appear to be partially decoupled from the aqueous phase, providing only partial information on the origin and evolution of the waters present in the system. The northern sector of the field contains meteoric waters having interacted with the basalt-andesitic reservoir or deeper melts (87Sr/86Sr = 0.7040) and containing mantle He and Ar. Together with this fluid, a meteoric component containing tritium confirms the presence of local and rapid meteoric recharge in this sector. The central and southern sectors of the field contain meteoric waters having interacted with carbonates from the basement. However, the relationship between 87Sr/86Sr and CO2/3He suggests a possible decoupling between mantle C and He and the aqueous phase, transporting Sr. Of particular interest, limestone C seems to be sourced from subducting slab components from the northern sector of the field, while the often-cited C contribution from local basement metacarbonates could be negligible. Finally, chlorine is sourced by magmatic HCl and partitioned in the steam phase, causing acidic waters.

The Watershed tungsten deposit (49.2 Mt avg 0.14% WO3) lies within the Mossman orogen, which comprises deformed Silurian-Ordovician metasedimentary rocks of the Hodgkinson Formation intruded by Carboniferous-Permian granites of the Kennedy Igneous Association. The Hodgkinson Formation in the Watershed area comprises skarn-altered conglomerate, psammite, and slate units that record four deformation events evolving from ductile, isoclinal, colinear folding with transposition (D-1-D-3) to brittle ductile shear zones (D-4). Multiple felsic to intermediate dikes cut across the metasedimentary rocks at Watershed including the following: (1) Carboniferous, monzonite dikes (zircon U/Pb age of 350 +/- 7 Ma) emplaced during D1-2; and (2) Permian granite plutons and dikes (zircon U/Pb ages of 291 +/- 6, 277 +/- 6, and 274 +/- 6 Ma) and diorite (zircon U/Pb age of 281 +/- 5 Ma) emplaced during D-4. Tungsten mineralization is largely restricted to skarn-altered conglomerate, which preserves a peak metamorphic mineralogy formed during ductile deformation and comprises garnet (Grt(40-87)Alm(0-35)Sp(s1-25)Adr(0-16)), actinolite, quartz, clinopyroxene (Di(36-59) Hd(39-61) Jhn(1-5)), and titanite. A first mineralization event corresponds to the crystallization of disseminated scheelite in monzonite dikes (pre-D-3) and adjacent units, with scheelite grains aligned in the S1-2 fabric and affected by D-3 folding. This event enriched the Hodgkinson Formation in tungsten. The bulk of the scheelite mineralization formed during a second event and is concentrated in multistaged, shear-related, quartz-oligoclase-bearing veins and vein halos (muscovite 40Ar-39Ar weighted average age of 276 +/- 6 Ma), which were emplaced during D-4. The multistage veins developed preferentially in competent, skarn-altered conglomerate units and formed synchronous with four retrograde alteration stages. The retrograde skarn minerals include clinozoisite after garnet, quartz, plagioclase, scheelite, and phlogopite with minor sodium-rich amphibole, which formed during retrograde stages 1 and 2, accompanied by later muscovite, calcite, and chlorite formed during retrograde stage 3. Retrograde stage 4 was a late-tectonic, noneconomic sulfide stage. The principal controls on scheelite mineralization at Watershed were the following: (1) early monzonite dikes enriched in scheelite; (2) D-4 shear zones that acted as fluid conduits transporting tungsten from source areas to traps; (3) skarn-altered conglomerate lenses that provide a competent host to facilitate vein formation and a source for calcium to form scheelite; and (4) an extensional depositional environment characterized by vein formation and normal faulting, which provide trapping structures for tungsten-bearing fluids, with decompression being a likely control on scheelite deposition. The coexistence of scheelite with oligoclase in monzonite dikes and veins suggests that tungsten was transported as NaHWO40. Exploration in the area should target Carboniferous monzonite, associated with later syn-D-4 shear zones cutting skarn-altered conglomerate.

Summary Helium as a noble gases can be successfully used as tracers of the different fluid components which make a geothermal water, namely magmatic, meteoric and fossil groundwater. Here, case histories from geothermal fields of Mexico are reported.

Chlorine (Cl) and bromine (Br) are rare elements when considering the whole Earth. However, being highly volatile elements, their delivery and retention processes during planetary accretion and chemical differentiation provide important clues about the formation of the Earth. Variations in Cl and Br isotopic systems (Cl-37/Cl-35 or delta Cl-37 and Br-81/Br-79 or delta Br-81) among terrestrial reservoirs could trace these processes. While the Br isotopic value of the mantle remains entirely unknown, the measurement of mantle Cl isotopic values is a controversial subject, with measured delta Cl-37 values ranging from -3 to 0 parts per thousand in midocean-ridge basalts (MORBs) and from -2 to +3 parts per thousand in oceanic island basalts (OIBs). Here, we report newly-determined delta Br-81 and delta Cl-37 values, together with noble gas He-3/He-4 (R) ratios, measured in geothermal fluids from production wells of three Mexican fields: Cerro Prieto, Las Tres Virgenes, and Los Azufres. Relationships between He-3/He-4 ratios and both delta Cl-37 and delta Br-81 suggest that geothermal fluid volatiles have three distinct sources: (1) a local crustal source, enriched in radiogenic He-4 (R = 1.7-1.9Ra, where Ra is the atmospheric He-3/He-4 ratio), and halogens from brines with delta Cl-37 and delta Br-81 of +0.1 and +0.3 parts per thousand respectively; (2) the mantle wedge, with He-3/He-4 ratios of 6-6.5Ra, typical of arc volcanism, and delta Cl-37 and delta Br-81 of -0.4 and -1.0 parts per thousand respectively, typical (for Cl) of fluids derived from the dehydration of serpentinite in the subducting slab; and (3) a mantle source, with He-3/He-4 ratios of 7.7-8.2Ra, typical of MORBs, and delta Cl-37 and delta Br-81 of +0.9 and +0.7 parts per thousand respectively. These results suggest that the primitive mantle Cl isotopic composition was positive - possibly >=+3 parts per thousand, as measured in some OIBs - and inherited during the Moon forming impact. The progressive subduction of isotopically lighter halogens over the last 2-3 Ga could have progressively lowered this initial value to those currently measured in the depleted mantle beneath Mexico. It is speculated that the different isotopic values measured in mantle rocks and fluids could reflect the heterogeneous regassing of subducted halogens and the inefficient homogenization of recycled material in the MORB source, as suggested in other studies by the heterogeneous isotopic compositions of the heavier Ar and Xe of the convective mantle. (C) 2020 Elsevier Ltd. All rights reserved.

Water resources management in Maui, Hawaii, remains a challenge due to uneven distribution of rainfall, water scarcity in populated areas, and an ill-defined hydrological system. Based on data collected in June 2014, Niu et al. (2017) suggested that noble gases in this rapid groundwater infiltration system record seasonality and may shed light on water sources, recharge locations, and groundwater flow paths. This study goes one step further by analyzing rainwater, basal and perched (springs) aquifer data collected in June 2014 and February 2016. It shows for the first time that both rain and spring water display sharp temporal variations which are related to either seasonal (summer/winter) variations or to the strong 2015–2016 El Niño year. These spring water temporal variations also suggest that perched aquifer groundwater is extremely young and likely unreliable as a long-term water supplier. In contrast, noble gas temporal variations are absent in the basal aquifer. This, in turn, is consistent with tritium derived groundwater ages on the order of a few decades for the basal aquifer. This study further demonstrates that Ne is an excellent candidate to estimate the source elevation of rain and thus, to distinguish between orographic and synoptic-scale rain.

Results of a survey of noble gases, water stable isotopes (delta D and delta O-18), and chlorine (Cl) and bromine (Br) contents measured in geothermal waters from the high-enthalpy, liquid-dominated Cerro Prieto Geothermal Field (CPGF) are reported. Helium isotopic ratios (He-3/He-4) help to identify two distinct fluids: a magmatic fluid, with He-3/He-4 (R) ratios of up to 7.3 times that of the atmospheric ratio (Ra = 1.384 x 10(-6)), and a He-4-rich fluid, with He-3/He-4 <= 3.5 Ra. The helium isotopic systematics point to limited modern freshwater recharge in the CPGF, and thus to the presence of a nearly fossil geothermal system. U-Th/He-4-estimated minimum residence times for the He-4-rich fluid component vary from 1.5 to 1.8 Ma, corresponding to the age of the Colorado River paleo-delta sediments, suggesting the presence of connate waters. Cl and Br concentrations indicate that this connate water component is seawater with minor halite dissolution. Together with Cl and Br, the isotopic signature of the water (delta D, delta O-18) indicates the presence of a third old fluid of meteoric origin, from Late Quaternary to Holocene, corresponding to recharge during colder and wetter periods than today. This is identified as being old groundwater from the Colorado Delta unconsolidated sediments, which cap the Cerro Prieto reservoir. Our results suggest little direct recharge to the CPGF. The spatial distribution of helium isotopes in the reservoir shows that connate waters dominate the central portion of the field and mix with the magmatic fluid containing mantle He, with both fluids rising at the center of the basin where the heat source is located. Deep magmatic fluids containing mantle helium enter laterally into the reservoir, through the main lateral strike-slip faults (i.e., the Cerro Prieto and Imperial faults). (C) 2019 Elsevier B.V. All rights reserved.

A geochemical survey of the Las Tres Virgenes geothermal field (LTVGF), Baja California Sur, Mexico, was carried out to determine the origin and evolution of its fluids, along with the influence of injection of local brines, and inhibitors intended to manage carbonate scaling issues. Major and minor cations, Cl, Br, Sr, O, H, and noble gas (He, Ne, Ar, Kr and Xe) isotopes were measured in fluids collected from four production wells, two injection wells, and one fumarole (El Azufre) in 2016 and 2018. Variations in the stable isotopes of water (delta D and delta O-18) and halogens (delta Cl-32, delta Br-81) over the years suggest that local brine reinjection might have started to modify the pristine chemistry of the fluids. However, relevant information on the original geothermal fluids has been preserved. The Na/Br and Cl/Br ratios suggest that LTVGF brines are seawater having leached evaporite deposits (halite). These deposits are absent in the area, but were likely present during the Miocene, prior to the breakup of the Gulf of California, suggesting that the saline fluid end-member at the LTVGF could be several million years old. Measured He-3/He-4 ratios (R) normalized to the atmospheric ratio Ra (= 1.384 x 10(-6)) plotted against the He-4/Ne-20 ratios suggest mixing between a mantle fluid with a R/Ra of 6.19, typical of the sub-continental litho spheric mantle and an atmospheric helium component. The R/Ra versus the Xe-132/Ar-36 ratio normalized to air suggests mixing between a mantle-rich pre-production fluid and air. The source of air appears to be linked to local gravity-controlled flow reinjection which causes the entrapment of air bubbles, leading to a fluid supersaturated in atmospheric noble gases. This pattern suggests that reinjection fluids are impacting the production area. (C) 2019 Elsevier B.V. All rights reserved.

A geochemical survey of fluids in the Las Tres Vírgenes geothermal field (LTVGF) in Baja California Sur, Mexico, was carried out to describe their origins and evolution within the reservoir. Major and minor elements and noble gas isotopes (He, Ne, Ar, Kr, and Xe) were measured in fluids from three production wells, one injection well, and one fumarole (El Azufre). Stable isotopes of water (δD and δ18O) show mixing between Quaternary recharge, lighter than present-day rainfall, and a deep fluid of andesitic origin. The Na/Br and Cl/Br ratios indicate that deep brines from the LTVGF are seawater having leached evaporite deposits (i.e., halite). These deposits are presently absent in the area, but likely occurred during the Miocene, prior to the breakup of the Gulf of California, suggesting that the saline fluid end-member of the LTVGF is several million year old. Measured 3He/4He ratios of greater than 6.5 Ra (where Ra is the atmospheric ratio of 1.384 x10-6) show that LTVGF fluids are a mixture of meteoric waters and deep magmatic fluids, with the hotter and more pristine mantle fluids found in the southern part of the exploitation zone.

Concern that hydraulic fracturing and natural gas production contaminates groundwater requires techniques to attribute and estimate methane flux. Although dissolved alkane and noble gas chemistry may distinguish thermogenic and microbial methane, low solubility and concentration of methane in atmosphere-equilibrated groundwater precludes the use of methane to differentiate locations affected by high and low flux of stray methane. We present a method to estimate stray gas infiltration into groundwater using dissolved nitrogen. Due to the high concentration of nitrogen in atmospheric-recharged groundwater and low concentration in natural gas, dissolved nitrogen in groundwater is much less sensitive to change than dissolved methane and may differentiate groundwater affected high and low flux of stray natural gas. We report alkane and nitrogen chemistry from shallow groundwater wells and eight natural gas production wells in the Barnett Shale footprint to attribute methane and estimate mixing ratios of thermogenic natural gas to groundwater. Most groundwater wells have trace to nondetect concentrations of methane. A cluster of groundwater wells have greater than 10 mg/L dissolved methane concentrations with alkane chemistries similar to natural gas from the Barnett Shale and/or shallower Strawn Group suggesting that localized migration of natural gas occurred. Two-component mixing models constructed with dissolved nitrogen concentrations and isotope values identify three wells that were likely affected by a large influx of natural gas with gas:water mixing ratios approaching 1:5. Most groundwater wells, even those with greater than 10-mg/L methane, have dissolved nitrogen chemistry typical of atmosphere-equilibrated groundwater suggesting natural gas:water mixing ratios smaller than 1:20. Plain Language Summary Hydraulic fracturing, horizontal drilling, and associated natural gas production have dramatically changed the energy landscape across America over the past 10 years. Along with this renaissance in the energy sector has come public concern that hydraulic fracturing may contaminate groundwater. In this study we measure the chemistry of dissolved gas from shallow groundwater wells located above the Barnett Shale natural gas play, a tight gas reservoir located west of the Dallas-Fort Worth Metroplex. We compare groundwater chemistry results to natural gas chemistry results from nearby production wells. Most groundwater wells have trace to nondetectible concentrations of methane, consistent with no measurable infiltration of natural gas into shallow groundwater. A cluster of groundwater wells have greater than 10 mg/L dissolved methane concentrations with alkane chemistries similar to natural gas. Using dissolved nitrogen and alkane concentrations and their stable isotope ratios in combination with chemical mixing models, we conclude that natural gas transported from the shallower Strawn Group affected these groundwater wells rather than natural gas from the deeper Barnett Shale, which is the target of hydraulic fracturing in this area. These results suggest that hydraulic fracturing has not affected shallow groundwater drinking sources in this area.

Noble gas concentrations in water are ideal probes to study surface and groundwater dynamics by providing indications of flow paths, connectivity between aquifers, and water residence times. Recent studies have pointed out anomalies in noble gas concentrations derived from groundwater in fractured systems, likely due to the presence of rapid infiltration and preferential flow paths. It has been suggested that such anomalies originate from conditions at high altitude when rainwater has had insufficient time to equilibrate with surface conditions. Potential sources also include snow, never previously investigated for its noble gas composition. In order to document the noble gas signature in snow, noble gas concentrations and isotopic ratios were measured in samples collected between 2013 and 2016. Here, we outline a methodology for measuring noble gases in collected snow samples that involves a two-step procedure where He and Ne are measured independently from Ar, Kr and Xe. Our results show that snow has elevated He concentrations with depleted concentrations of other noble gases with respect to air-saturated water (ASW). However, samples collected in 2013 show significant He and Ne depletion compared to those collected in 2014, 2015 and 2016. We suspect that, despite the well-controlled conditions of storage, the 2013 batch sample might have significantly re-crystalized, leading to a reduction in the characteristic diffusion length scale of the snow crystal structure. In addition, He and Ne concentrations display relatively low variability among all measured samples (< 14%), while Ar, Kr and Xe show large variability in their concentrations (> 40%). Our results confirm that He and Ne, which have small atomic radii, are likely dissolved within the ice/snow crystal lattice itself while the heavy noble gases (Ar, Kr and Xe) are likely accommodated by fluid inclusions, including air and quenched liquid water inclusions. Consequently, the smaller variability recorded in light noble gases may be due to the fact that He and Ne are hosted within plentiful host sites within the snow crystal lattice structure, whereas heavy noble gases rely on the presence of comparatively rare fluid inclusions.

Thirty geothermal wells and two hot springs were sampled for volume fraction and isotopic measurements of noble gases (He, Ne, Ar, Kr, Xe) and strontium in the Los Azufres Geothermal Field (LAGF), Mexico. The aim of this study was to understand the evolution of fluid circulation following three decades of exploitation and re-injection of used brines and to identify the heat source. The LAGF, divided into the Southern Production Zone (SPZ) and the Northern Production Zone (NPZ), is hosted in a Miocene to Pliocene andesitic volcanic complex covered by Quaternary rhyolitic-dacitic units. Air component corrected He-3/He-4 ratios (Rc) normalized to the atmospheric ratio (Ra = 1.384 x 10(-6)), range from 4.21 to 7.93 for most samples pointing to the occurrence of a MORB-type mantle helium component, with contributions of crustal helium up to 53% and 18% in NPZ and SPZ, respectively. Observations based on Rc/Ra and Sr-87/Sr-86 ratios point to mixing of three magmatic sources supplying mantle helium to the LAGF: (1) a pure mantle He (Rc/Ra = 8) and Sr (Sr-87/Sr-86 = 0.7035) source related to mafic magmas; (2) a pure mantle helium component (Rc/Ra = 7) with some radiogenic Sr (Sr-87/Sr-86=0.7049) source, possibly related to Quaternary rhyolitic magmas; and (3) a fossil mantle He component (Rc/Ra = 4.0) with some radiogenic Sr (Sr-87/Sr-86=0.7038), corresponding possibly to Miocene andesitic magmas. Parental magmas related to sources (1) and (2) emplaced <50 kyrs ago are likely responsible for the addition of mantle volatiles and heat (Q) to the hydrothermal system of Los Azufres. An observed He-4/Ar-36 vs. He-3/Q correlation suggests that heat is transferred by conduction and convection in both NPZ and SPZ. Atmospheric noble gas elemental ratios suggest that geothermal wells located closer to the western re-injection zone are dominated by re-injection of used brines (injectate). The area affected by boiling in LAGF has extended further to the north and west since the last noble gas sampling campaign in 2007-2009 (Pinti et al., 2013)