Infiltration of river water to a shallow aquifer investigated with H/He, noble gases and CFCs

Noble gas isotopes (³He, ⁴He, Ne, Ar, Kr, Xe), tritium (³H), chlorofluorocarbons (CFCs) and dissolved oxygen (O₂) were seasonally measured in a small groundwater system recharged by infiltration of river water at Linsental, northeastern Switzerland. All Groundwater samples contained an excess of atmospheric noble gases (‘excess air’) usually with an elemental composition equal to air. The concentrations of atmospheric noble gases in the groundwater were used to calculate the excess air component and the water temperature at recharge. The noble gas temperatures (NGTs) in the boreholes close to the river vary seasonally, however, the average NGT of all samples lies close to the mean annual temperature of the river water. Groundwater ages were calculated using the tritium/helium-3 (³H/³He) dating method. The water ages of the samples obtained near the river depend on the amount of recently infiltrated river water and are young during times of active river discharge. In contrast, the mean water age of about 3 years of the deep aquifer remained nearly constant over the sampling period. The observed CFC-11 (CFCl₃) and CFC-12 (CF₂Cl₂) concentrations are significantly higher than the atmospheric equilibrium concentrations and therefore CFCs do not provide any direct information on the residence time of the groundwater. Nevertheless, the CFC excess in the groundwater shows a linear increase with the ³H/³He age. Additionally, both accumulation of radiogenic He (⁴He_rad) and O₂ consumption are strongly correlated with residence time. All these correlations can be interpreted either in terms of mixing of recently infiltrated river water with older groundwater or in terms of accumulation/consumption rates.     

Noble gas tracers for characterisation of flow dynamics and origin of groundwater: A case study in Switzerland

The Grenchen aquifer system in the Swiss Plateau was extensively investigated in order to determine the extent of groundwater contamination and to assess the natural attenuation capacity. Environmental tracer data were applied to estimate groundwater travel times, mixing ratios, and evaluate groundwater origin. Recharge is basically possible in two distinct topographical areas, the immediate vicinity of the town of Grenchen and the elevated plateau of the first Jura Mountain ridge. Groundwater dating was performed with the ³H/³He dating method and supplemented by ⁸⁵Kr measurements. Stable isotope data (δ¹⁸O, δ²H) and dissolved noble gas concentrations allow the determination of the recharge temperature, which is correlated to the recharge elevation. Noble gas temperatures (NGT) decrease in the direction of groundwater flow and range from 10 to 13 °C in the upstream area of the town to 7–9 °C in the downstream river plain. This trend could suggest the admixture of water from the underlying limestone aquifer recharged under cooler infiltration conditions, e.g. at higher recharge elevations. However, it is shown in this study that the difference in NGT does not require such a recharge. Rather, increasing air temperatures over the last 40 years and the urban heat island effect could possibly explain most of the observed temperature shift. Furthermore, it is concluded that the downstream river plain is hydrologically disconnected from the upstream town area. Consequently most water from the town area is drained by the creek Witibach and recharge in the river plain is higher than previously assumed.     

Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source.There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases.Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The³He/⁴He in the East Pacific Rise shows a remarkably uniform ratio of (1.21±0.07)×10^?5, while the⁴⁰Ar/³⁶Ar ranges from 700 to 5600.     

Helium flux through marine sediments of the northeast Pacific Ocean

Quantitative determinations of helium and other noble gas concentrations in pore waters of marine sediments permit the calculation of helium fluxes out of the sedimentary column. Values of 1.1 and 6.4 × 10⁴ atoms/cm² sec were found for two locations in the northeast Pacific. These fluxes are one to two orders of magnitude less than those based on theoretical degassing models of the earth or on measurements of excess helium in the deep ocean water.     

Applications of a total dissolved gas pressure probe in ground water studies

Measurements of dissolved gases have numerous applications in ground water hydrology, and it is now possible to measure the total dissolved gas pressure in situ using a probe. Dissolved gas pressure is measured by submerging a headspace volume with a gas-permeable membrane, allowing dissolved gases in the water to equilibrate with gases in the headspace, then measuring the pressure in the headspace with a pressure transducer. Total dissolved gas pressure (TGP) probes have many potential uses in ground water studies employing dissolved gases, including: (1) determining approximate excess air levels, which may provide information about the time and location of recharge; (2) screening wells for air contamination, which can compromise the accuracy of dissolved gas tracer techniques: (3) detecting a trapped gas phase, which can significantly reduce hydraulic conductivity and impede the transport of dissolved solutes and gases; (4) enabling the use of gas-filled passive diffusion samplers for determining accurate dissolved gas concentrations; and (5) determining relative concentrations of CH4 and CO2 when they are known to be highly abundant. Although TGP probes designed for surface water have been available for several years, TGP probes suitable for ground water applications have only recently become available. Herein we present what are, to our knowledge, the first reported ground water dissolved gas data collected using a TGP probe. We also explain the basic operating principles of these probes and discuss the potential applications listed.     

Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

Past studies of entrapped air dissolution have focused on one‐dimensional laboratory columns. Here the multidimensional nature of entrapped air dissolution was investigated using an indoor tank (180 × 240 × 600 cm³) simulating an unconfined sand aquifer with horizontal flow. Time domain reflectometry (TDR) probes directly measured entrapped air contents, while dissolved gas conditions were monitored with total dissolved gas pressure (P_TDG) probes. Dissolution occurred as a diffuse wedge‐shaped front from the inlet downgradient, with preferential dissolution at depth. This pattern was mainly attributed to increased gas solubility, as shown by P_TDG measurements. However, compression of entrapped air at greater depths, captured by TDR and leading to lower quasi‐saturated hydraulic conductivities and thus greater velocities, also played a small role. Linear propagation of the dissolution front downgradient was observed at each depth, with both TDR and P_TDG, with increasing rates with depth (e.g, 4.1 to 5.7× slower at 15 cm vs. 165 cm depth). P_TDG values revealed equilibrium with the entrapped gas initially, being higher at greater depth and fluctuating with the barometric pressure, before declining concurrently with entrapped air contents to the lower P_TDG of the source water. The observed dissolution pattern has long‐term implications for a wide variety of groundwater management issues, from recharge to contaminant transport and remediation strategies, due to the persistence of entrapped air near the water table (potential timescale of years). This study also demonstrated the utility of P_TDG probes for simple in situ measurements to detect entrapped air and monitor its dissolution.     

Noble gas elemental and isotopic abundances in deep-sea trenches in the western Pacific

Noble gas elemental and isotopic abundances were measured in seven deep-sea water samples from five different sampling sites in the Nankai Trough, the Japan Trench and the Kuril Trench. The samples were obtained by the manned submersible “Nautile”. Most of the sampling sites are associated with clam colonies and/or fluid venting. Excesses both in³He/⁴He ratio and He concentration are observed in a seawater sample collected a few kilometers off the clam colonies which were found at a depth of 3830 m at the mouth of the Tenryu Canyon. Concentrations of noble gases (Ne, Ar, Kr and Xe) in this sample show progressive depletion from Ne to Xe relative to those in 1°C air-saturated seawater, which can be attributed to mixing of hot water ( 15°C) with cold ambient water ( 1°C). Isotopic compositions of Ne, Ar, Kr and Xe in this sample are atmospheric. These observations may reflect venting of hot pore water around the Tenryu Canyon. All the other samples show a significant excess in concentration of all noble gases relative to 1°C air-saturated seawater and the isotopic compositions are atmospheric. This excess of noble gas concentrations may appear to be air contamination in the samples. However, results of hydrocarbon analyses of the Kaiko samples imply that such large amount of air contamination is improbable. Decomposition of gas hydrate in deep-sea sediments is a more likely explanation for the observed excess of noble gas concentration.     

Importance of river water recharge to the San Joaquin Valley groundwater system

Groundwater is not a sustainable resource, unless abstraction is balanced by recharge. Identifying the sources of recharge in a groundwater basin is critical for sustainable groundwater management. We studied the importance of river water recharge to groundwater in the south‐eastern San Joaquin Valley (24,000 km², population 4 million). We combined dissolved noble gas concentrations, stable isotopes, tritium, and carbon‐14 analyses to analyse the sources, mechanisms, and timescales of groundwater recharge. Area‐representative groundwater sampling and numerical model input data enabled a stable isotope mass balance and quantitative estimates of river and local recharge. River recharge, identified by a lighter stable isotope signature, represents 47 ± 4% of modern groundwater in the San Joaquin Valley (recharged after 1950) but only 26 ± 4% of premodern groundwater (recharged before 1950). This implies that the importance of river water recharge in the San Joaquin valley has nearly doubled and is likely the result of a 40% increase in total recharge, caused by river water irrigation return flows and increased stream depletion and river recharge due to groundwater pumping. Compared with the large and long‐duration capacity for water storage in the subsurface, storage of water in rivers is limited in time and volume, as evidenced by cold river recharge temperatures resulting from fast infiltration and recharge. Groundwater banking of seasonal surface water flows and expansion of managed aquifer recharge practices therefore appear to be a natural and promising method for increasing the resilience of the San Joaquin Valley water supply system.     

Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates

Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO₂), radon-222 (²²²Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO₂ and ²²²Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO₂ and O₂ in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly-exposed, wetted bedrock. Deficits of expected CO₂ relative to O₂ concentrations indicate some respired CO₂ is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.     

Effects of Airflow Induced by Rainfall on Shallow Groundwater Table Fluctuations

An investigation of groundwater table fluctuations induced by rainfall should consider interactions between the liquid and gas phases in soils. In this study, a water‐air two‐phase flow model was initially verified by simulating an infiltration experiment. It was then employed to model the interactions between liquid and gas phases regarding actions of airflow on the groundwater table and the fluctuations of the phreatic level and water level in the well induced by rainfall. The effects of airflo7w caused by rainfall on phreatic level fluctuations were also studied quantitatively by comparing the results obtained using the proposed model with those obtained from a water single‐phase flow model. The simulation results show that in addition to actual recharge, compressed airflow in unsaturated zones causes the phreatic level to increase, but the rise in the phreatic level is lower than that in the pore‐air pressure head in unsaturated zones due to the mitigation of capillary fringe. The existence of airflow enhances the phreatic level rise during and after rainfall. In addition, the water level in the well, pushed by the phreatic level fluctuations, varies similarly to the phreatic level, but it experiences somewhat delayed and slightly attenuated. The Lisse effect precisely reflects the phreatic level fluctuations before actual recharge. Furthermore, the fluctuations in the phreatic level and water level in the well and the contributions of airflow to phreatic level fluctuations are affected by many factors: rain intensity, initial moisture, overlying aquitard, groundwater table depths, and screen depths of the well.     

Erta'ale lava lake: heat and gas transfer to the atmosphere

Data on uncontaminated samples of volcanic gases can be counted on the fingers of one hand, yet estimation of total volcanic gas flow cannot be made without such data. In this paper the flux of gas from the lava lake to the atmosphere is calculated by a heat budget based on the excess heat loss caused by combustion of H₂ and CO and by the mass rate of loss of other gases on the basis of their ratios to H₂ and CO in the unoxidized gas samples. The estimated rates of loss of H₂O, CO₂, SO₂ and HCl are consistent with the rate of loss of heat if this heat is generated by crystallization and if the initial magma contains concentrations of gas appropriate for submarine basalt from oceanic ridges. The moderate activity of permanent degassing from the two active lava ponds studied gives a lower flux than that of other volcanoes.     

Atmospheric noble gas signatures in deep Michigan Basin brines as indicators of a past thermal event

Atmospheric noble gases (e.g., ²²Ne, ³⁶Ar, ⁸⁴Kr, ¹³⁰Xe) in crustal fluids are only sensitive to subsurface physical processes. In particular, depletion of atmospheric noble gases in groundwater due to boiling and steam separation is indicative of the occurrence of a thermal event and can thus be used to trace the thermal history of stable tectonic regions. We present noble gas concentrations of 38 deep brines (~ 0.5–3.6 km) from the Michigan Basin. The atmospheric noble gas component shows a strong depletion pattern with respect to air saturated water. Depletion of lighter gases (²²Ne and ³⁶Ar) is stronger compared to the heavier ones (⁸⁴Kr and ¹³⁰Xe). To understand the mechanisms responsible for this overall atmospheric noble gas depletion, phase interaction models were tested. We show that this atmospheric noble gas depletion pattern is best explained by a model involving subsurface boiling and steam separation, and thus, consistent with the occurrence of a past thermal event of mantle origin as previously indicated by both high ⁴He/heat flux ratios and the presence of primordial mantle He and Ne signatures in the basin. Such a conceptual model is also consistent with the presence of past elevated temperatures in the Michigan Basin (e.g., ~ 80–260 °C) at shallow depths as suggested by previous thermal studies in the basin. We suggest that recent reactivation of the ancient mid-continent rift system underneath the Michigan Basin is likely responsible for the release of both heat and mantle noble gases into the basin via deep-seated faults and fracture zones. Relative enrichment of atmospheric Kr and Xe with respect to Ar is also observed, and is interpreted as reflecting the addition of sedimentary Kr and Xe from associated hydrocarbons, following the hydrothermal event. This study pioneers the use of atmospheric noble gases in subsurface fluids to trace the thermal history of stable tectonic regions.     

Reliability of recharge rates estimated from groundwater age with a simplified analytical approach

Groundwater age is often used to estimate groundwater recharge through a simplified analytical approach. This estimated recharge is thought to be representative of the mean recharge between the point of entry and the sampling point. However, given the complexity in actual recharge, whether the mean recharge is reasonable is still unclear. This study examined the validity of the method to estimate long-term average groundwater recharge and the possibility of obtaining reasonable spatial recharge pattern. We first validated our model in producing reasonable age distributions using a constant flux boundary condition. We then generated different flow fields and age patterns by using various spatially varying flux boundary conditions with different magnitudes and wavelengths. Groundwater recharge was estimated and analysed afterwards using the method at the spatial scale. We illustrated the main findings with a field example in the end. Our results suggest that we can estimate long-term average groundwater recharge with 10% error in many parts of an aquifer. The size of these areas decreases with the increase in both the amplitude and the wavelength. The chance of obtaining a reasonable groundwater recharge is higher if an age sample is collected from the middle of an aquifer and at downstream areas. Our study also indicates that the method can also be used to estimate local groundwater recharge if age samples are collected close to the water table. However, care must be taken to determine groundwater age regardless of conditions.     

Quasi-Saturated Layer: Implications for Estimating Recharge and Groundwater Modeling

This study presents an extension of the concept of “quasi-saturation” to a quasi-saturated layer, defined as the uppermost dynamic portion of the saturated zone subject to water table fluctuations. Entrapped air here may cause substantial reductions in the hydraulic conductivity (K) and fillable pore water. Air entrapment is caused by a rising water table, usually as a result of groundwater recharge. The most significant effects of entrapped air are recharge overestimation based on methods that use specific yield (S_y), such as the water table fluctuation method (WTF), and reductions in K values. These effects impact estimation of fluid flow velocities and contaminant migration rates in groundwater. In order to quantify actual groundwater recharge rates and the effects of entrapped air, numerical simulations with the FEFLOW (Version 7.0) groundwater flow model were carried out using a quasi-saturated layer for a pilot area in Rio Claro, Brazil. The calculated recharge rate represented 16% of the average precipitation over an 8-year period, approximately half of estimates using the WTF method. Air entrapment amounted to a fillable porosity of 0.07, significant lower that the value of 0.17 obtained experimentally for S_y. Numerical results showed that the entrapped air volume in the quasi-saturated layer can be very significant (0.58 of the air fraction) and hence can significantly affect estimates of groundwater recharge and groundwater flow rates near the water table.     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

Noble gas abundance patterns in deep-sea basalts — Primordial gases from the mantle

Rapidly cooled portions of eleven samples of mid-ocean ridge tholeiitic basalt pillows have noble gas abundance patterns which resemble the solar rare gas pattern rather than the noble gas pattern of the terrestrial atmosphere. We conclude that these samples contain primordial noble gases. In contrast, holocrystalline samples and a sample from the interior of a basalt pillow have noble gas abundance patterns which resemble the sea water pattern. Whereas the quenched glossy margins of basalt pillows record a non-atmospheric gas reservoir, these slowly cooled samples apparently have undergone exchange of their noble gases with those dissolved in sea water.     

Noble gases in CO2 well gas,Harding County,New Mexico

The abundance and isotopic composition of noble gases were determined in samples of CO₂ well gas from Harding County, New Mexico. Our results confirm the presence of radiogenic¹²⁹Xe and fissiogenic^131–136Xe. Relative to noble gases in air, the CO₂ gas is selectively depleted in the lighter weight, nonradiogenic noble gases, except at neon. It is suggested that loss of atmospheric neon into space could account for an apparent excess of neon in juvenile gases.     

Mapping of H2S fluxes from the ground using copper passive samplers: An application study at the Zolforata di Pomezia degassing area (Alban Hills,Central Italy)

A new method for measuring H₂S mass flux from the ground, based on the digital analysis of the interference colours produced by the sulphidation of copper passive samplers (CPS), is proposed and discussed in this article. CPS sulphidation has a wide range of linear responses to H₂S doses and can be used together the accumulation chamber method to estimate gas fluxes from natural degassing areas. These are often characterized by the presence of vent centred degassing areas (VCDAs), which are recognizable from the absence or rarefaction of vegetation due to high acid gas concentrations in the soil pores and in the air at ground level. A reference emission curve, accounting for the advective and diffusive components of the flux, can be modelled and used to estimate the total H₂S mass released from each VCDA. The application of this method can be supported by remote sensing analysis that helps identify VCDAs in the field in perivolcanic H₂S degassing areas.As an illustrative application, H₂S gas fluxes from the ground were measured in spring 2007 at the Zolforata di Pomezia degassing area (ZPDA, Alban Hills, Central Italy) using an accumulation chamber internally equipped with CPS. H₂S peak fluxes were measured over the vents after remote sensing assisted identification of the VCDAs. Further measurements were carried out in two ponds and one artificial channel bordering the study area. The total atmospheric flux released at the ZPDA, estimated to be about 1207.6 kg day^? 1, was calculated as the summation of the fluxes from all the H₂S sources, the background flux being negligible.     

Influence of Fluctuating Groundwater Table on Volatile Organic Chemical Emission Flux at a Dissolved Chlorinated-Solvent Plume Site

Groundwater elevation fluctuation has been recognized as one mechanism causing temporal indoor air volatile organic chemical (VOC) impacts in vapor intrusion risk assessment guidance. For dissolved VOC sources, groundwater table fluctuation shortens/lengthens the transport pathway, and delivers dissolved contaminants to soils that are alternating between water saturated and variably saturated conditions, thereby enhancing volatilization potential. To date, this mechanism has not been assessed with field data, but enhanced VOC emission flux has been observed in lab-scale and modeling studies. This work evaluates the impact of groundwater elevation changes on VOC emission flux from a dissolved VOC plume into a house, supplemented with modeling results for cyclic groundwater elevation changes. Indoor air concentrations, air exchange rates, and depth to groundwater (DTW) were collected at the study house during an 86-d constant building underpressurization test. These data were used to calculate changes in trichloroethylene (TCE) emission flux to indoor air, during a period when DTW varied daily and seasonally from about 3.1 to 3.4 m below the building foundation (BF). Overall, TCE flux to indoor air varied by about 50% of the average, without any clear correlation to changes in DTW or its change rate. To complement the field study, TCE surface emission fluxes were simulated using a one-dimensional model (HYDRUS 1D) for conditions similar to the field site. Simulation results showed time-averaged surface TCE fluxes for cyclic water-table elevations were greater than for stationary water-table conditions at an equivalent time-averaged water-table position. The magnitudes of temporal TCE emission flux changes were generally less than 50% of the time-averaged flux, consistent with the field site observations. Simulation results also suggested that TCE emission flux changes due to groundwater fluctuation are likely to be significant at sites with shallow groundwater (e.g., < 0.5 m BF) and permeable soil types (e.g., sand).     

Brine history indicated by argon, krypton, chlorine, bromine, and iodine analyses of fluid inclusions from the Mississippi Valley type lead-fluorite-barite deposits at Hansonburg, New Mexico

Argon, krypton, chlorine, bromine, and iodine were measured in a homogeneous population of high-salinity hydrothermal fluid inclusions from the Tertiary-age Mississippi Valley-type (MVT) lead-fluorite-barite deposits at Hansonburg, New Mexico to establish new types of evidence for the history of both the fluid and the major dissolved salts. Noble gases and halogens in fluid inclusions containing 10⁻¹⁰–10⁻⁹ L of brine (Cl= 3 molal) were analyzed by laser microprobe noble-gas mass spectrometry (lmngms) on neutron-irradiated samples.

The concentrations of³⁶Ar (4.7 × 10⁻⁸ molal) and⁸⁴Kr1.8 × 10⁻⁹ molal) in the fluid inclusions are equal to those of fresh surface waters in equilibrium with air at approximately20 ± 5°. The mole ratios ofBr/Cl (1.2 × 10⁻⁴) andI/Cl (1–2 × 10⁻⁶) are among the lowest measured in any natural waters, similar to those of modern brines formed by dissolution of Permian NaCl-bearing evaporites in southeast New Mexico.⁴⁰Ar/³⁶Ar ratios (600) are twice that of air, and indicate that the fluid inclusions had excess radiogenic⁴⁰Ar (1.4 × 10⁻⁵ molal) when trapped. The amount of excess⁴⁰Ar appears to be too large to have been acquired with Cl by congruent dissolution of halite-bearing evaporites, and possibly too small to have been acquired with Pb by congruent dissolution of granitic basement rocks with Proterozoic KAr ages.

From thelmngms data, combined with published Pb and S isotope data, we infer the following sequence of events in the history of the Hansonburg MVT hydrothermal brine: (1) the brine originated as relatively dilute meteoric water, and it did not gain or lose atmospheric Ar or Kr after recharge; (2) the originally dilute fluid acquired the bulk of its Cl and sulfate in the subsurface after recharge by dissolving halite-bearing Permian? marine evaporites; (3) the high salinity brine then acquired most of its Pb and excess radiogenic⁴⁰Ar from interactions with aquifer rocks other than evaporites, possibly clastic sedimentary rocks or basement rocks with Phanerozoic KAr “ages”; and (4) the brine deposited fluorite without having boiled or degassed.