Atmospheric Noble Gases Research Articles

Martian soil component in impact glasses in a Martian meteorite.

Chemical compositions of impact melt glass veins, called Lithology C (Lith C) in Martian meteorite EET79001 were determined by electron microprobe analysis. A large enrichment of S, and significant enrichments of Al, Ca, and Na were observed in Lith C glass compared to Lithology A (Lith A). The S enrichment is due to mixing of plagioclase- enriched Lith A material with Martian soil, either prior to or during impact on Mars. A mixture of 87% Lith A, 7% plagioclase, and 6% Martian soil reproduces the average elemental abundances observed in Lith C. Shock melting of such a mixture of plagioclase-enriched, fine-grained Lith A host rock and Martian soil could yield large excesses of S (observed in this study) and Martian atmospheric noble gases (found by Bogard et al., 1983) in Lith C. These mixing proportions can be used to constrain the elemental abundance of phosphorus in Martian soil.

Interpretation of dissolved atmospheric noble gases in natural waters

Several studies have used the temperature dependence of gas solubilities in water to derive paleotemperatures from noble gases in groundwaters. We present a general method to infer environmental parameters from concentrations of dissolved atmospheric noble gases in water. Our approach incorporates statistical methods to quantify uncertainties of the deduced parameter values. The equilibration temperatures of water equilibrated with the atmosphere under controlled conditions could be inferred with a precision and accuracy of ±0.2°C. The equilibration temperatures of lake and river samples were determined with a similar precision. Most of these samples were in agreement with atmospheric equilibrium at the water temperature. In groundwaters either recharge temperature or altitude could be determined with high precision (±0.3°C and ±60 m, respectively) despite the presence of excess air. However, typical errors increase to ±3°C and ±700 m if both temperature and altitude are determined at the same time, because the two parameters are correlated. In some groundwater samples the composition of the excess air deviated significantly from atmospheric air, which was modeled by partial reequilibration. In this case the achievable precision of noble gas temperatures was significantly diminished (typical errors of ±1°C).

Infiltration of river water to a shallow aquifer investigated with 3H/ 3He, noble gases and CFCs

Noble gas isotopes ( 3He, 4He, Ne, Ar, Kr, Xe), tritium ( 3H), chlorofluorocarbons (CFCs) and dissolved oxygen (O 2) 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 (CFCl 3) and CFC-12 (CF 2Cl 2) 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 O 2 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.

Mantle, crustal and atmospheric noble gases in ailaoshan gold deposits, Yunnan Province, China

Helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the Ailaoshan Gold Province, China. The province is a series of gold deposits hosted on a 200-km segment of a major normal fault formed during Eocene extension. The deposits have been well characterised using conventional geochemical and microthermometric techniques, the results of which are consistent with a predominantly high temperature “magmatic” fluid present in the ore minerals. In contrast, analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle-derived, magmatic fluid and two different low temperature fluids similar to modern groundwaters. The specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle-derived component and a surface-derived component to these fluids. The magmatic fluid dominates deposits from the northern end of the fault. The second groundwater is present only in deposits in the south of the province. By assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature, 3He/heat ratios can be estimated. The 3He/heat ratios vary systematically along the fault, from ≈ 30 × 10 −12 cm 3 STP J −1 in the northern deposits to <0.6 × 10 −12 cm 3 STP J −1 in the south, reflecting a major change in heat and fluid flow. Noble gas and heat compositions are consistent with direct injection of magmatic volatiles into hydrothermal fluids at the northern end of the fault. However, the flow regime clearly changed along the fault. There appears to be little or no direct input of magmatic volatiles in the south. It is likely that heat was introduced into the hydrothermal system by diffusion into low enthalpy, high He-concentration groundwater.

Assimilation of seawater-derived components in an oceanic volcano: evidence from matrix glasses and glass inclusions from Loihi seamount, Hawaii

We report major element, H 2O, Cl, B, and Be analyses of matrix glass and olivine-hosted glass inclusions from two pillow lava samples dredged from 4200 m on the southern rift zone of Loihi seamount, Hawaii. Matrix glasses (MgO∼9 wt.%) have H 2O, Cl, and B contents considerably in excess of the values expected from mantle melting or fractional crystallization of parental Loihi magmas. Glass inclusions have H 2O, Cl, and B contents ranging from the high values of the matrix glasses to lower concentrations that are more typical of Hawaiian magmas. Concentrations of other incompatible elements (e.g., K 2O, P 2O 5, and Be) in matrix glasses and glass inclusions are uncorrelated with their H 2O, Cl, and B contents. Glass inclusions show considerable scatter in major element compositions compared to matrix glasses, but except for H 2O, Cl, and B, the average glass inclusion composition corresponds well to the matrix glass compositions. We propose that the glass inclusions represent compositionally diverse liquids present within the magmatic plumbing system at Loihi that were mixed and homogenized to produce the liquid that quenched to the matrix glass on eruption. This range of liquid compositions present at depth was trapped by crystallizing olivine prior to blending and homogenizing and therefore preserves a compositional diversity not present in erupted whole rocks. The high H 2O, Cl, and B contents of matrix glasses and some glass inclusions, and the range of H 2O, Cl, and B concentrations in glass inclusions, are best explained by variable extents of assimilation by Loihi magmas of H 2O–Cl–B-rich, seawater-derived components prior to eruption. The required assimilants range from material similar in composition to seawater to materials with Cl/H 2O and B/H 2O ratios much higher than seawater. Our preferred explanation (similar to that suggested for MORB by Michael and Schilling, 1989) [Michael, P.J., Schilling, J.-G., 1989. Chlorine in mid-ocean ridge magmas: Evidence for assimilation of seawater-influenced components. Geochim. Cosmochim. Acta, 53, pp. 3131–3143.] is that most of the assimilated materials were brines (or rocks containing brines in inclusions or along grain boundaries) enriched in Cl by high temperature phase separation of seawater in sub-sea-floor hydrothermal circulation systems. Addition of ∼1.0 wt.% of a 15 wt.% NaCl brine can explain the H 2O and Cl contents of the matrix glasses. Addition of altered basalt cannot readily account for the Cl and H 2O contents of matrix glasses and glass inclusions, but may be required to account for their elevated B contents. The enrichment in Cl and contamination with atmospheric noble gases observed in other samples from Loihi could also result from assimilation of Cl-enriched, seawater-derived components.

Atmospheric noble gases in volcanic glasses from the southern Lau Basin: origin from the subducting slab?

Noble gas concentrations and isotopic compositions have been determined for four submarine volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin. The samples are the least differentiated ones from this area, and they display enrichments in fluid-mobile elements similar to the nearby island arc. 3He/ 4He ratios are slightly below average MORB (6.8–7.8 times atmospheric), whereas Ne, Ar, Kr, and Xe have isotopic compositions very similar to air. Together with previously published data from the Valu Fa Ridge and other spreading segments in the Lau Basin, our data show a systematic latitudinal variation of increasing Ne, Ar, Kr, and Xe abundances from north to south as well as Ne and Ar isotopic compositions changing from MORB-like to atmosphere-like in the same direction. Moreover, isotopic compositions and noble gas abundances of the lavas correlate strongly with Ba/Nb ratios and H 2O concentrations. Based on these observations and mass balance arguments, we propose that the atmospheric noble gases come from the subducting oceanic crust and are not due to shallow contamination with air dissolved in seawater or assimilation of old crust. Our data suggest that the noble gases released from the subducting slab are atmospheric and thus contain little or no solar He and Ne. In addition to the fact that ratios of He to heavy noble gases are small in aged ocean crust, He has possibly fractionated from the other noble gases due to its higher diffusivity, and thus He transport from the subducting slab into the mantle wedge is probably insignificant. We propose that the 3He/ 4He ratios lower than MORB observed in the VFR lavas result from radiogenic ingrowth of He in a highly depleted, and hence degassed, mantle wedge after the enrichment of U and Th released from the downgoing slab.

Noble gas and carbon isotopes in Mariana Trough basalt glasses

Noble gas elemental and isotopic compositions have been measured as well as the abundance of C and its isotopic ratios in 11 glasses from submarine pillow basalts collected from the Mariana Trough. The 3He/ 4He ratios of 8.22 and 8.51 R atm of samples dredged from the central Mariana Trough (∼18°N) agree well with that of the Mid-Ocean Ridge Basalt (MORB) glasses (8.4±0.3 R atm), whereas a mean ratio of 8.06±0.35 R atm in samples from the northern Mariana Trough (∼20°N) is slightly lower than those of MORB. One sample shows apparent excess of 20Ne and 21Ne relative to atmospheric Ne, suggesting incorporation of solar-type Ne in the magma source. There is a positive correlation between 3He/ 4He and 40Ar/ 36Ar ratios, which may be explained by mixing between MORB-type and atmospheric noble gases. Excess 129Xe is observed in the sample which also shows 20Ne and 21Ne excesses. Observed δ 13C values of ∼20°N samples vary from −3.76‰ to −2.80‰, and appear higher than those of MORB, and the corresponding CO 2/ 3He ratios are higher than those of MARA samples at ∼18°N, suggesting C contribution from the subducted slab.

Multiple fluid pulses in a Samoan harzburgite

Noble gases trapped in fluid inclusions in residual mantle xenoliths from Samoa have a wide range in 4He/ 40Ar * which anti-correlates with enrichments in non-atmospheric Ar, Ne and Xe isotopes and with the concentration of 40Ar * trapped in the mineral. These correlations cannot be due to post-eruption loss of He from the inclusions, but are likely due to mixing of a He-depleted mantle fluid and an unfractionated, plume-derived, fluid that has entrained significant atmospheric noble gases. Mixing of the two fluids occurred within the mantle or at the base of the Samoan lithosphere. Helium loss at depth is implied.

Detection of reprocessing activities through stable isotope measurements of atmospheric noble gases

Noble gas stable isotope abundance measurements may provide a tool for detecting reprocessing activities of nuclear fuels. An approach has been made by carrying out blending calculations of released fission xenon and krypton in air using the Isotope Mixture Programs which have been developed at the IRMM. After having obtained a reliable approximation to the expected range of the isotope ratios in the blends and the respective detection limit thereof through these calculations, the potential application of ultra-accurate measurements of the isotopic composition of anthropogenic and atmospheric noble gases is taken into consideration. Also the important role of radiometric measurements of 85Kr and 133Xe for the detection of nuclear fuel reprocessing is taken into account. The information provided by such activity measurements is limited, therefore a method to calculate the initial isotopic composition of released fission noble gases, through measuring of their atmospheric mixing ratio, is presented and discussed. Highly accurate stable isotopic measurements of atmospheric noble gases might provide more detailed information on the “history” of the reprocessed nuclear fuel. Therefore they could serve, in combination with radiometric detection techniques, as an excellent tool for the identification of reprocessing activities.

The Northwest Geysers high-temperature reservoir: evidence for active magmatic degassing and implications for the origin of the Geysers geothermal field

Noble gas isotope abundances in steam from the Coldwater Creek field of the Northwest Geysers, California, show mixing between a nearly pure mid-ocean ridge (MOR) type magmatic gas with high 3He/ 4He and low radiogenic 40∗Ar ( R/ Ra > 8.3 and 40∗Ar/ 4He < 0.07), and a magmatic gas diluted with crustal gas ( R/ Ra < 6.6 and 40∗Ar/ 4He > 0.25). The 3He-enriched component is positively correlated with 4 He/ 40∗ Ar ratios, total helium to non-condensable gas ratios, and the ratios of total helium to atmospheric noble gases, and is accompanied by mantle-like 3He/CO 2 and 4 He/ 36 Ar ratios. The steam samples most enriched in this high 3Hecomponent are produced from a high-temperature reservoir (HTR) and are also the most enriched in total gas and HCl. These results support the hypothesis of active magma degassing beneath the Northwest Geysers, suggest that a significant fraction of the non-condensable gases produced with steam from the HTR is magmatic, and add new constraints to genetic models of the system and its evolution. The intensity of the magmatic signal is inconsistent with deep boiling of connate or metamorphic waters and suggests active magma degassing. A correlation between magmatic helium and non-condensable gases implies that the HTR high-gas component is also magmatic and that the formation of the HTR was possibly related to magmatic intrusion. Magmatic input is unlikely to decline on the time scale of production, but injection of water into the HTR would have multiple benefits: (1) pressure and steam flow would increase; (2) gas concentrations would decrease by dilution with low-gas steam from vaporized liquid; and (3) HCl in steam would be removed once a liquid phase is established. High 3He/ 4He ratios in steam produced from the reservoirs in the southern parts of The Geysers suggest the presence of a similar HTR underlying these portions of the field. Monitoring of 3He content may provide early warning of potential pressure drawdown and entry of corrosive, high-gas HTR steam.

Release of anthropogenic xenon to the atmosphere: a large-scale isotope dilution

As a result of the reprocessing of nuclear fuel, the fission products xenon and krypton are released to the atmosphere. Due to their different isotopic composition as compared to natural atmospheric xenon (krypton) the dilution process in air results in isotopic alterations in the blend. The isotopic compositions of fission noble gases were calculated for two scenarios. Subsequently blending calculations were carried out in order to simulate the mixing of the fission off-gases with atmospheric noble gases. In order to estimate the detection limit of the alternations occurring, the calculated isotope ratios in the blend were compared to the ratios of natural xenon (krypton) as measured using the highly developed ultra-accurate measurement facilities for isotopic mass spectrometry at the Institute for Reference Materials and Measurements. The calculated sensitivity of this method lies within a range of some 10 9 m 3 of air g −1 fission gas. Finally the potential application of stable isotope measurements of xenon (krypton) as a supplementary method for detecting reprocessing activities is discussed.

Cometary origin of the biosphere: A progress report

Empirical evidence of the accretion temperature for undifferentiated meteorites coming from the asteroid belt, combined with any reasonable temperature gradient extending from the asteroid belt to the Earth's zone, suggests that the Earth accreted from very hot dust grains that were degassed from all volatile elements and depleted in labile compounds. Isotopic evidence from the atmospheric noble gases also shows that no primary atmosphere has survived on the Earth. The only possible source for the atmosphere and the oceans is therefore the cometary bombardment that is predicted as the inescapable consequence of the formation of the giant planets. This implies that comets are the only source of organic carbon, nitrogen and water, hence of the total biosphere of the Earth.

Noble gas isotopic composition of deep underground water in Osaka plain, central Japan: Evidence for mantle He and model for new volcanism

Abstract Elemental and isotopic compositions of noble gases extracted from the bore hole water in Osaka plain, central Japan were examined. The water samples were collected from four shallow bore holes (180‐450 m) and seven deep bore holes (600‐1370 m) which have been used for an urban resort hot spring zone. The water temperatures of the deep bore holes were 22‐50°C and that of the shallow bore holes, 13‐23°C. The elemental abundance patterns show the progressive enrichment of the heavier noble gases compared with the atmospheric noble gas composition except for He, which is heavily enriched in deep bore hole water samples. 3He/4He ratios from the bore holes reaching the Ryoke granitic basement were higher than the atmospheric value (1.4 × 10−6), indicating a release of mantle He through the basement. The highest value of 8.2 × 10−6 is in the range of arc volcanism. On the other hand, the bore holes in sedimentary rocks overlying the basement release He enriched in radiogenic 4He, resulted in a low 3He/4He ratio of 0.5 × 10−6. 4He/20Ne and 40Ar/36Ar ratios indicate that the air contamination is generally larger in shallow bore holes than in deep ones from each site. The helium enriched in mantle He is compatible with the previous work which suggested up‐rising magma in ‘Kinki Spot’, the area of Osaka and western Wakayama, in spite of no volcanic activity in the area. A model to explain an initiation of magma generation beneath this area is presented.

Carbon and noble gases in Archean chert

In order to study the origin and evolution of volatiles trapped in Archean and Early Proterozoic cherts, we have measured contents and isotopic compositions of carbon and noble gases using step-heating and mass spectrometry. The observed negative correlation between the δ 13C-value and nitrogen/carbon ratio in the samples suggests that trapped carbon in Archean cherts is a mixture of carbonate, sedimentary organic matter and an atmospheric component. Elemental abundance patterns of noble gases show a slight depletion of 20Ne and excess of 132Xe relative to air, which is explained by atmospheric noble gases dissolved in water at relatively low temperature. The 3 He 4 He ratios indicate an apparent crustal value of (1–3)·10 −3, consistent with the in situ radiogenic production ratio. Significant excess 21Ne, 131Xe and 132–136Xe as well as slight enrichment of 38Ar and 86Kr relative to atmosphere are found in the Onverwacht chert at the 800° and 1200°C steps. Similar but small isotope anomalies are detected in the Frere Formation chert at the 1800°C step. Excess 21Ne shows positive correlations with 38Ar, 86Kr and 132Xe- 136Xe enrichments while excess 131Xe has no relation with the other isotope anomalies. The 21Ne anomaly can be attributable to the nuclear reaction 18O(α,n) 21Ne. The enrichments of 86Kr and 132Xe- 136Xe are derived from spontaneous fission of 238U in the cherts. The slight excess 38Ar may be due to the 35Cl(α,p) 38Ar reaction based on the positive correlation between 21 Ne 22 Ne and 38 Ar 36 Ar ratios, but should be verified in future work.

Noble gases in the mantle wedge and lower crust: an inference from the isotopic analyses of xenoliths from Oki-Dogo and Ichinomegata, Japan.

The noble gas isotopic compositions of xenoliths sampled from Oki-Dogo island and Ichinomegata crater, Japan, have been measured to understand better the origin of noble gases in the mantle wedge and lower crust beneath the Japanese Island Arc which represents a typical convergent plate boundary. The 3He/4He ratios of lherzolites and a websterite derived from the upper mantle are the same as MORB-type He isotopic ratios, i.e., 8 ± 1 times the atmospheric value. The 40Ar/36Ar ratios (307–1870) of these samples are generally lower than that assumed for MORB-type Ar (>20000). These results indicate that the He and Ar initially present in the mantle wedge were similar to the MORB-type gases but were later contaminated by atmospheric noble gases in the subducting slab. This resulted in the decreased 40Ar/36Ar ratios and a negligible change in 3He/4He ratios. A high He concentration (1.2 × 10–6 cm3 STP/g) and 3He/4He ratio (10.5 × 10–6), similar to arc volcanic gases, was observed in an amphibolite sample which represents lower crust beneath Ichinomegata. This He isotopic ratio implies that magmatic noble gases associated with arc volcanism do not ascend directly from the mantle wedge but from the lower crust where they accumulate.

Mantle degassing and atmospheric evolution: Noble gas view.

On the basis of recent high quality noble gas experimental data, we propose a scenario for the origin of terrestrial noble gases. The isotopic compositions of non-radiogenic, terrestrial noble gases (except for atmospheric neon) were established, prior to the Earth's accretion, in planetesimals by gravitational capture of solar-type noble gases from the solar nebula. Gravitational capture by planetesimals produces a pattern in which the strongest fractionations occur in the heaviest elements. After the formation of the Earth, neon was further fractionated during hydrodynamic escape of hydrogen from the primitive atmosphere to give rise to the present atmospheric neon isotopic composition. Therefore, except for neon, isotopic compositions of nonradiogenic noble gases both in the atmosphere and the solid earth are the same. Noble gas degassing from the solid earth occurred during the magma ocean stage through bubble (gas)/melt partition. Subsequent magma production and eruption throughout geological time has contributed little to atmospheric noble gases except for radiogenic components. From the comparision of 3He/4He ratios between the upper and lower mantles, we estimate that the 40Ar/36Ar ratio in the lower mantle is more than 6, 000. Hence, we argue that the lower mantle has also undergone some degree of degassing.

Paleotemperatures in the Southwestern United States Derived from Noble Gases in Ground Water

A paleotemperature record based on measurements of atmospheric noble gases dissolved in ground water of the Carrizo aquifer (Texas) shows that the annual mean temperature in the southwestern United States during the last glacial maximum was about 5 degrees C lower than the present-day value. In combination with evidence for fluctuations in mountain snow lines, this cooling indicates that the glacial lapse rate was approximately the same as it is today. In contrast, measurements on deep-sea sediments indicate that surface temperatures in the ocean basins adjacent to our study area decreased by only about 2 degrees C. This difference between continental and oceanic records poses questions concerning our current understanding of paleoclimate and climate-controlling processes.

Constraints from noble-gas contents on the origin of carbonado diamonds

CARBONADOS are porous aggregates of micrometre-size diamond crystals1,2. Although they are not rare, they have not been found in kimberlites3, and they contain inclusion minerals typical of the Earth's crust, rather than the upper-mantle assemblages commonly found in kimberlite diamonds. Carbonados also have lower 13C/12C ratios than kimberlite diamonds1,2,4. Although these observations strongly suggest that carbonados were formed in the crust, crustal conditions are generally unlikely to provide pressures and temperatures in the diamond stability field (but see ref. 5 for a possible exception). Other suggestions for the origin of carbonado include the transformation of subducted carbon in the mantle6, impact metamorphism of crustal rocks containing organic carbon7, and (for very fine-grained carbonado) the irradiation of organic matter by decaying uranium and thorium in uranium-rich phases8–10. Here we present further evidence for a crustal connection, in the form of noble-gas data for four carbonados from Brazil and Africa. We find that all four contain large amounts of implanted xenon and krypton from 238U fission, nucleogenic neon and 4He, and tightly trapped atmospheric noble gases, from which we conclude that these carbonados formed in a uranium-rich crustal environment.

Comment on “Atmospheric contamination: A possible source for heavy noble gases in basalts from Loihi Seamount, Hawaii” by D. B. Patterson, M. Honda, and I. McDougall

Allègre et al. (1983), Staudacher et al. (1986) and Sarda et al. (1988) reported noble gas concentrations and isotopic ratios of basalt glasses from Loihi seamount and Hualalai, Hawaii. The 4He/3He and 40Ar/36Ar ratios of ∼25,000 and 390 ± 50 respectively are different than the air ratio and typical ratios found in MORB glasses. The Ne, Kr and Xe isotopic ratios are the same as those found in air or deep seawater. Based on the different isotopic ratios and the different noble gas concentration patterns of Ne, Ar, Kr and Xe compared to air, deep seawater and MORB, these authors concluded that the noble gases enclosed in vesicles from Loihi and Hualalai basalts are not significantly contaminated by atmospheric noble gases, and that the Loihi and Hualalai basalts came from a separate source, namely the lower mantle, with a distinct noble gas signature. In contrast to the upper mantle, which is highly depleted in noble gases, this lower mantle should be not or only slightly outgassed (Allègre et al., 1986).In a recent publication, Patterson et al. (1990), based on our noble gas data (Staudacher et al., 1986; Sarda et al., 1988), propose that Ne, Ar, Kr and Xe noble gas concentrations in basalt glasses could be plausibly explained by a) contamination of the Loihi plume magma prior to eruption with atmospheric noble gases disolved in deep seawater (which they model by elemental noble gas fractionation of a tholeiitic melt that was equilibrated with seawater derived noble gases), and b) elemental fractionation of noble gases during vesicle formation and vesicle loss. We completely disagree with such an interpretation and show here that their argumentation is inconsistent and incorrect.

The geothermal system of Wairakei, New Zealand: physical processes and age estimates inferred from noble gases

Atmospheric and non-atmospheric noble gases were identified by their isotopic composition in 31 samples from geothermal features from the Wairakei, Tauhara and Mokai geothermal fields, North Island, New Zealand. In the fumaroles, atmospheric noble gases (ANG) were found in concentrations that exceed the values in air-saturated recharge water (ASW) by a factor of up to 14. Significant concentrations of non-atmospheric (NA) 4He (He NA) and 40Ar (Ar NA) were observed, up to 4.3 × 10 −4and up to2.9 × 10 −4 cm 3 STP/g fluid, respectively. In contrast, the hot-spring waters revealed significant depletions in ANG (as low as 0.02 of the ASW concentrations) and low, but recognizable, concentrations of He NA and Ar NA. In geothermal wells, a wide spectrum of ANG concentrations was observed, from 0.02 to 30 times the concentrations in ASW. 4He NA and 40Ar NA varied accordingly. These observations lead to the simplified boiling model: meteoric water penetrated into the geothermal compartment, where it was heated and enriched by 4He NA and 40Ar NA of mantle and crustal origins. Boiling of ascending fluid, due to pressure decrease, results in formation of a noble-gas-enriched steam phase, issuing as fumaroles and drowned fumaroles, and a noble-gas-depleted residual liquid. The latter may cause secondary boiling of shallow groundwater, emerging in hot springs. Geothermal wells tap fluids at various degrees of steam separation. The depth of recharge of meteoric water into the geothermal system exceeds the depth of boiling, which is 600 m at the studied area. The ANG patterns, along with He NA/ANG ratios are applied to identify the following physical processes: (a) mixing of early separated steam with residual liquid; (b) mixing of a few per cent groundwater with residual liquid; (c) addition of air; (d) addition of excess Ne; and (e) encroachment of recent groundwater due to pressure decrease. Retention-enrichment values of the ANG were applied to reconstruct non-atmospheric initial (NAI) concentrations of He NAI, Ar NAI and the initial CO 2 in the geothermal water. Literature data on 3He/ 4He were used to separate the 4He NAI concentrations into crustal- and mantle-derived components. The crustal-radiogenic 4He concentration, calculated for the initial water, was applied to derive an age estimate of the geothermal water, assuming storage in rhyolitic rocks. Effective ages of 2 × 10 4, 1 × 10 5and2 × 10 5 a were obtained for the initial liquids feeding the Wairakei wells, fumaroles, and Mokai wells, respectively. Crustal Ar NA concentrations were estimated from those of crustal He and from the rhyolitic ( 4He/ 40Ar) production ratio. The calculated crustal 40Ar is≤ 0.1 of 40Ar NA, and ( 4He/ 40Ar) contribution is found at Wairakei to be ∼0.5, within the range of values measured by other workers in mantle-derived rocks.