Interpretation of groundwater flow patterns through a reconstruction of the tritium precipitation record in the Cochabamba Valley, Bolivia

Regional behaviour of the groundwater flow system in the Cochabamba Valley, Bolivia, is evaluated through the interpretation of tritium (³H) distributions in groundwater samples from wells and springs. In order to interpret groundwater ³H concentrations in Cochabamba Valley, where no historical record of ³H concentrations in rainfall exists, a reconstructed ³H precipitation record is developed. The record of ³H concentrations in precipitation is fairly extensive in the Amazon Basin and this record was extrapolated to the neighbouring Cochabamba Valley. Tritium concentrations in rainfall have been observed to increase under natural conditions with increasing latitude and with increasing distance from the ocean. By considering these trends, a linear relationship for increasing ³H concentration in precipitation is developed, based on data from the Amazon Basin, that realistically predicts regional ³H distributions from the northeast Brazilian coast to Cuzco, Peru. This ³H precipitation record is then extrapolated to the Cochabamba Valley and, after correction for radiogenic decay, is used to interpret trends in groundwater ³H concentrations within the valley.

The groundwater flow system in one of the principal alluvial fans, which serves as an important groundwater resource for the city, is studied in detail. Tritium concentrations drop from approximately 8–10 tritium units (TU) in the recharge area to concentrations below the detection limit of 0.8 TU further out in the valley. Groundwater velocities of approximately 0.3 to 0.9 m d⁻¹ are estimated from distributions of groundwater ³H concentrations along the alluvial fan with the use of the reconstructed precipitation ³H record. Regional characteristics of the groundwater flow system are discussed with respect to future development and protection of the groundwater resources.     

Origin and age of thermal waters in Cieplice Spa, Sudeten, Poland, inferred from isotope, chemical and noble gas data

Isotope and hydrochemical data of the thermal water system in Cieplice laskie Zdrój (Spa) indicate the existence of two subsystems that greatly differ in volume and which meet at the fault zones of a granitic horst, where they discharge at an altitude of about 340m. One of the subsystems is very small (about 4 × 10³ m³) as indicated by the tritium age of the order of 10 years and a low outflow rate. Its recharge area found from the δ¹⁸O and δD values, is about 200m above the springs, most probably on the slopes of the foothills of the Karkonosze Mountains south-southwest of the spa. The large subsystem contains water which is free of tritium and whose ¹⁴C content is from 1 to 8 pmc with δ¹³C = −8.0 to −9.2‰. The isotopic composition of this water reflects either the climatic effect (low-altitude recharge during a cooler pre-Holocene climate) or the altitude effect (recharge in the early Holocene period at about 1000m at the heights of the Karkonosze assuming that the ¹⁴C concentration is strongly reduced by exchange with calcite in veins). For the former hypothesis, the recharge area of this water is probably either at the foot of the southeastern slopes of the Kaczawa Mountains or/and at the foot of the Rudawy Janowickie Mountains, to the east of Cieplice. The noble gas temperatures are more consistent with the pre-Holocene recharge. Similarly, the ⁴He excess and ⁴⁰Ar/³⁶Ar ratio support the hypothesis of a pre-Holecene age. The constant ³He/⁴He ratio of 26 × 10⁻⁸ for highly different helium contents indicates crustal origin of helium. For the pre-Holocene age of water its volume is calculated at >- 10⁹m³ (stagnant water in micropores and mobile water in fractures) and the hydraulic conductivity of the host granite massif is estimated at about 7 × 10⁻⁸ ms⁻¹. Two outflows from this subsystem have different and variable fractions of a modern water component (bomb age), most probably originating from the bank infiltration of a nearby stream.     

Noble gas systematics of deep rift zone glasses from Loihi Seamount, Hawaii

We report new noble gas fusion and crushing data for six pillow rim glasses, recovered between 3 and 5 km water depth on the south rift zone of Loihi Seamount, Hawaii. Helium abundances of the glasses vary from 0.3 to 2.3 μcc/g, with ⁴He/³He ratios between 30000 and 27000 (24–27 R_A), similar to previously reported values. The neon data form a correlation line which is similar to the Loihi-Kilauea line reported by Honda et al. [1], but extends to much higher ratios, up to 12.9 and 0.0382 for the ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios, respectively. This provides conclusive evidence for the suggestion that the Hawaiian plume, thought to originate in the lower mantle, has a solar-like ²⁰Ne/²²Ne composition [1], but a slightly higher ²¹Ne/²²Ne ratio. ⁴⁰Ar/³⁶Ar ratios of the deep rift-zone glasses are as high as 2600, and show a positive correlation with neon isotopic ratios. In contrast to neon and argon, all xenon isotopic compositions are isotopically indistinguishable from air, which either suggests preferential atmospheric contamination of xenon, or could indicate an atmospheric xenon isotopic composition for the lower mantle.     

Helium and neon isotopes in the Imnaha Basalt, Columbia River Basalt Group: Evidence for a Yellowstone plume source

Helium, neon, and argon isotopic compositions were measured in two flows of the Columbia River flood basalt. The Imnaha Basalt has a ³He/⁴He ratio of 11.4 times atmospheric and ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios characteristic of a plume component. The measured ³He/⁴He is a lower limit, due to possible preferential ³He loss and/or addition of radiogenic ⁴He. A Wanapum Basalt flow, erupted approximately 2 Ma later in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that the Columbia River basalts were derived from the Yellowstone plume head which contained both ‘high-helium’ plume material and entrained depleted mantle. As the eruptions progressed the plume component in the melting region was gradually diluted or replaced.     

Tracing anthropogenically driven groundwater discharge into a coastal lagoon from southern Brazil

We investigated the distribution of naturally occurring geochemical tracers (²²²Rn, ²²³Ra, ²²⁴Ra, ²²⁶Ra, CH₄, δ¹⁸O, and δ²H) in the water column and adjacent groundwater of Mangueira Lagoon as proxies of groundwater discharge. Mangueira Lagoon is a large (90 km long), shallow (4–5 m deep), fresh, and non-tidal coastal lagoon in southern Brazil surrounded by extensively irrigated rice plantations and numerous irrigation canals. We hypothesized that the annual, intense irrigation for rice agriculture creates extreme conditions that seasonally change groundwater discharge patterns in the adjacent lagoon. We further supposed that dredging of irrigation canals alters groundwater fluxes.

While the activities of ²²²Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH₄ and radium isotopes were only 1 order of magnitude higher. Therefore, ²²²Rn appears to be the preferred groundwater tracer in this system. Radon concentrations and conductivities were dramatically higher near the pump house of rice irrigation canals, consistent with a groundwater source. Modeling of radon inventories accounting for total inputs (groundwater advection, diffusion from sediments, and decay of ²²⁶Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicated that groundwater advection rates in the irrigation canals (25 cm/d) are over 2 orders of magnitude higher than along the shoreline (0.1 cm/d). Nearly 75% of the total area of the canals is found in the southern half of the lagoon, where groundwater inputs seem to be higher as also indicated by methane and stable isotope trends. In spite of the relatively small area of the canals, we estimate that they contribute nearly 70% of the total (57,000 m³/d) groundwater input into the entire Mangueira Lagoon. We suggest that the dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into the lagoon.     

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 H₂O 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.     

Groundwater use by vegetation in a tropical savanna riparian zone (Daly River, Australia)

Soil water matric potentials (Ψ_m) and the deuterium (δ²H) composition at natural abundance levels of xylem water, soil water, river water and groundwater were used to evaluate whether trees use groundwater during the dry season in the riparian zone of the Daly River (Northern Territory, Australia). Groundwater was a significant source of water for plant transpiration, probably accounting for more than 50% of the water transpired during the dry season. Groundwater use occurred either when trees used water from the capillary fringe or when low Ψ_m induced by soil water uptake lifted groundwater in the vadose zone. Several water use strategies were inferred within the riparian plant community. Melaleuca argentea W. Fitzg and Barringtonia acutangula (L.) Gaertn. appeared to be obligate phreatophytes as they used groundwater almost exclusively and were associated with riverbanks and lower terraces with shallow (<5 m) water tables. Several species appeared to be facultative phreatophytes (including Cathorium umbellatum (Vahl.) Kosterm. and Acacia auriculiformis A. Cunn. ex Benth.) and tended to rely more heavily on soil water with increased elevation in the riparian zone. The levee-bound Corymbia bella K.D. Hill and L.A.S. Johnson mostly used soil water and is either a facultative phreatophyte or a non-phreatophyte. The temporal variability in groundwater utilisation by the trees is unclear because the study focused on the end of the dry season only. A decline in the regional water table as a result of groundwater pumping may affect the health of riparian zone vegetation in the Daly River because groundwater use is significant during the dry season.     

Influence of fracture anisotropy on ground water ages and chemistry,Valley and Ridge province,Pennsylvania

Model ground water ages based on chlorofluorocarbons (CFCs) and tritium/helium-3 (³H/³He) data were obtained from two arrays of nested piezometers located on the north limb of an anticline in fractured sedimentary rocks in the Valley and Ridge geologic province of Pennsylvania. The fracture geometry of the gently east plunging fold is very regular and consists predominately of south dipping to subhorizontal to north dipping bedding-plane parting and east striking, steeply dipping axial-plane spaced cleavage. In the area of the piezometer arrays, which trend north-south on the north limb of the fold, north dipping bedding-plane parting is a more dominant fracture set than is steeply south dipping axial-plane cleavage. The dating of ground water from the piezometer arrays reveals that ground water traveling along paths parallel to the dip direction of bedding-plane parting has younger ³H/³He and CFC model ages, or a greater component of young water, than does ground water traveling along paths opposite to the dip direction. In predominantly unmixed samples there is a strong positive correlation between age of the young fraction of water and dissolved sodium concentration. The travel times inferred from the model ages are significantly longer than those previously calculated by a ground water flow model, which assumed isotropically fractured layers parallel to topography. A revised model factors in the directional anisotropy to produce longer travel times. Ground water travel times in the watershed therefore appear to be more influenced by anisotropic fracture geometry than previously realized. This could have significant implications for ground water models in other areas underlain by similarly tilted or folded sedimentary rock, such as elsewhere in the Valley and Ridge or the early Mesozoic basins.     

Diffusive fractionation of noble gases and helium isotopes during mantle melting

The large differences in He and Ar diffusivities in silicate minerals could result in fractionation of the He/Ar ratio during melting of the mantle, producing He/Ar ratios in the primary mantle melts that are higher than those of the bulk mantle. Modeling noble gas diffusion out of the bulk mantle into fast diffusion pathways (such as fractures or melt channels) suggests that significant (order of magnitude) He/Ar fractionation will occur if the fast diffusion channels are spaced several meters apart and the noble gas residence in these diffusion channels is of the order days to weeks. In addition, the 15% difference in ³He and ⁴He diffusivities could also produce isotopic fractionation between the melt and its solid source. Modeling the behavior of He and Ar during melting shows that small increases (few %) in ³He/⁴He should be correlated with larger variations (factor of 5) in ⁴He/⁴⁰Ar. However, in order to test this hypothesis the effects of subsequent He–Ar fractionation that occur during degassing have to be corrected. I describe a scheme that can separate He/Ar variations in the primary melt from overprinted fractionation during magmatic degassing. Using the degassing-corrected data, there is a correlation between the primary melt’s ⁴He/⁴⁰Ar and ³He/⁴He in mid-ocean ridge basalts (MORBs). The slope of the correlation is consistent with the models of preferential diffusion of ³He relative to ⁴He and of ⁴He relative to ⁴⁰Ar from the solid mantle into the melt. Diffusive fractionation of noble gases during melting of the mantle can also account for low ⁴He/⁴⁰Ar ratios commonly found in residual mantle xenoliths: preferential diffusion of He relative to Ar will produce some regions of the mantle with low ⁴He/⁴⁰Ar, the complement of the high ⁴He/⁴⁰Ar ratios in basalts. Diffusive fractionation cannot, however, account for differences between the He and Ne isotopic compositions of MORBs compared with ocean island basalts (OIBs); not only are the extremely high ³He/⁴He ratios of OIBs (up to 50 Ra) difficult to produce at reasonable mantle time and lengthscales, but also the Ne isotopic compositions of MORBs and OIBs do not lie on a single mass fractionation line, therefore cannot result from diffusive fractionation of a single mantle Ne source. If preferential diffusion of He from the solid mantle into primary melts is a significant process during generation of MORBs, then it is difficult to constrain the He concentration of the mantle: He concentrations in basalts and the He flux to the ocean essentially result from extraction of He from a larger (and unknown) volume of mantle than that that produced the basalts themselves. The He concentration of the mantle cannot be constrained until more accurate estimates of the diffusion contribution are available.     

Isotope geochemistry of Pantelleria volcanic fluids, Sicily Channel rift: a mantle volatile end-member for volcanism in southern Europe

Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO₂–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured ³He/⁴He ratios and δ¹³C of both the free gases (4.5–7.3 R_a and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 R_a and −7.1 to −0.9‰), together with their covariation with the He/CO₂ ratio, constrain a ³He/⁴He ratio of 7.3±0.1 R_a and a δ¹³C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic ⁴He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in ³He and has a lower (closer to MORB) δ¹³C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 R_a, δ¹³C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, ³He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB ³He/⁴He ratio but higher than MORB CO₂/³He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.     

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven different localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like ³He/⁴He values of 9–12 × 10^–6 (6.4–8.6 times atmospheric) despite large variations in ⁴He. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger variations in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic ²¹Ne is observed in some samples. ⁴⁰Ar/³⁶Ar varies widely (atmospheric to 1880). Kr is atmospheric in composition for all samples. Some samples show a clear excess ¹²⁹Xe, which is a well-known MORB signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data reflect the interaction of a MORB-like magma with an atmospheric component such as seawater or of a depleted mantle source with a water-rich component that was probably derived from the subducting slab.     

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.     

Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as R_c/R_A (R_c=air corrected sample ³He/⁴He, R_A=air ³He/⁴He), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R_c/R_A<0.05, crustal helium domain). Further north, there is a resolvable ³He anomaly consistent with a mantle contribution (R/R_A>0.1, mantle helium domain). The highest values of 0.27–0.38 R_A occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50–100 km north of the Indus-Zangpo suture zone. There seems to be no association between the ³He anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the ³He anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the ³He anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet.     

甘肃东南地区温泉流体地球化学特征

根据氢、 氧、 氦同位素与水化学组分资料, 讨论了甘肃东南地区温泉水的来源、 地球化学变化及其与2008年汶川M_S8.0地震的关系。 测定结果表明: 样品的溶解性固体总量(TDS)范围为241.7~2 372.1 mg/L。 采集的7处温泉(通渭汤池河温泉、 清水地震台、 天水地震台、 武山地震台、 武山22号井、 成县地震台、 武都地震台)水样可归为四种化学类型: Na·Ca-SO₄、 Ca·Mg-SO₄、 Na-HCO₃·SO₄、 Ca·Mg-SO₄·HCO₃。 地下热水的化学类型与裂隙深度和围岩的岩性有关, 离子浓度和断裂深度基本成正相关。 通渭汤池河温泉和武都地震台的δ18O和δD值分别在-11.4‰ ~ -7.6‰和-85.7‰ ~ -57.1‰的范围内, 通渭汤池河温泉和武都地震台中³He/⁴He的值分别为0.4×10^-7和12.7×10^-7。 氢、 氧、 氦同位素组成特征表明温泉水源于大气降水, 在循环过程中经历了水岩反应, 且可能有地表水的混入。 2008年汶川M_S8.0地震发生后, 研究区域内温泉水中K⁺、 Ca²⁺含量总体上升, SO^2-₄、 Cl^-含量总体下降, Na⁺含量变化不明显; 热水循环深度受地震影响发生变化。 本文确定了甘肃东南地区温泉来源、 水化学类型成因及其与汶川M_S8.0地震的关系。     

Helium isotope characteristics of Andean geothermal fluids and lavas

The first comprehensive helium isotope survey of the Andes is reported here. We have sampled geothermal fluids and phyric lava flows from the Southern (svz) and Central (cvz) Volcanic Zones, the volcanically active Pun˜a region and the Precordillera, Salta Basin, Longitudinal Valley and the aseismic region between the two volcanic zones. Although the active areas are characterised by significant differences in crustal age and thickness, the svz, cvz and Pun˜a are characterised by a wide and overlapping range in ³He/⁴He ratios (for fluids and phenocrysts) from predominantly radiogenic values to close to the MORB ratio. The measured ranges in ³He/⁴He ratios (R) (reported normalised to the air ³He/⁴He—R_A) are: svz (0.18 < R/R_A< 6.9); cvz (0.82 < R/R_A< 6.0); and Pun˜a (1.8 < R/R_A< 5.4). Modification of magmatic ³He/⁴He ratios by water/rock interactions (fluids) or post-eruptive grow-in of radiogenic ⁴He or preferential diffusive loss of ³He (phenocrysts) is considered unlikely; this means that the wide range reflects the helium isotope characteristics of magma bodies in the Andean crust. The mechanism controlling the ³He/⁴He ratios appears to be a mixing between mantle (MORB-like) helium and a radiogenic helium component derived from radioactive decay within the magma (magma aging) and/or interaction with ⁴He-rich country rock: a process expected to be influenced by pre-eruptive degassing of the mantle component. Assimilation of lower crust is also capable of modifying ³He/⁴He ratios, albeit to a much lesser extent. However, it is possible that the highest measured values in each zone were established by the addition of lower crustal radiogenic helium to MORB helium. In this case, the higher ‘base level’ ratios of the svz would reflect the younger crustal structure of this region. In contrast to helium, there is no overlap in the Sr or Pb isotope characteristics of lavas from the active zones: in all areas, therefore, ³He/⁴He ratios appear to vary independently of Sr and Pb isotope variations. This decoupling between the lithophile tracers and helium reflects the different processes controlling their isotopic characteristics: crust-mantle interactions, alone, for Sr and Pb but for helium the effects of pre-eruptive degassing and possibly magma aging are possibly superimposed. The presence of mantle helium in the Pun˜a region, and to a lesser extent in the Salta Basin, gives an across-arc perspective to the helium isotope distribution and shows mantle melting to occur significantly to the east of the active arc: this is most probably a consequence of lithospheric delamination. The Precordillera hot spring water has the only pure radiogenic helium signal of the entire sample suite and thus marks the western limit of asthenospheric mantle under the Andes.     

Noble gases in submarine pillow basalt glasses from the Lau Basin: Detection of a solar component in backarc basin basalts

Noble gas elemental and isotopic abundances have been analysed in eight samples of youthful basaltic glass dredged from three different locations within the Lau Backarc Basin: (1) the King's Triple Junction, (2) the Central Lau Spreading Centre at 18°S and (3) the Eastern Lau Spreading Centre at 19°S. Samples from the Lau central and eastern spreading centres have MORB-like helium isotopic ratios of approximately 1.2 × 10⁻⁵ (8.5 R/R_A). In contrast, the samples from the King's Triple Junction yield helium isotopic ratios averaging 9.4 (±0.8) × 10⁻⁶ (6.7 ± 0.6 R/R_A), systematically lower than the MORB-like value, which may be reflecting the addition of radiogenic ⁴He released from the descending slab. Neon isotopic ratios are enriched in ²⁰Ne and ²¹Ne with respect to atmospheric ratios by as much as 23% and 62% respectively. These observations further confirm that non-atmospheric neon is a common characteristic of samples derived from the mantle. The helium and neon isotopic signatures in the samples can be explained by mixing of a primordial solar component, radiogenic and nucleogenic components produced by radioactive processes inside the Earth, and an atmospheric component. This reconnaissance survey of noble gases in a backarc basin indicates that current volcanism is dominated by magmas from the mantle wedge, a source similar to that from which MORBs are derived. The heavier noble gases (argon, krypton and xenon), however, show more atmosphere-like compositions, either indicating strong interaction of the magmas with the atmosphere or the presence of a recycled component derived from the underlying subducting slab.     

Helium isotope geochemistry of some volcanic rocks from Saint Helena

³He/⁴He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, includingin vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited³He/⁴He ratio at St. Helena is 4.3–5.9 R_A. Helium isotope disequilibrium is present within the phenocrysts, with lower³He/⁴He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding(U + Th)-rich lavas.

A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave³He/⁴He= 7.1 R_A.³He/⁴He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to³He/⁴He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low²⁰⁶Pb/²⁰⁴Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.³He/⁴He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 R_A). This suggests that the recycled materials were injected into the mantle within the last 10⁹ yrs.     

Helium isotope geochemistry of mid-ocean ridge basalts from the South Atlantic

We report new helium isotope results for 49 basalt glass samples from the Mid-Atlantic Ridge between 1°N and 47°S.³He/⁴He in South Atlantic mid-ocean ridge basalts (MORB) varies between 6.5 and 9.0 R_A (R_A is the atmospheric ratio of1.39 × 10⁻⁶), encompassing the range of previously reported values for MORB erupted away from high³He/⁴He hotspots such as Iceland. He, Sr and Pb isotopes show systematic relationships along the ridge axis. The ridge axis is segmented with respect to geochemical variations, and local spike-like anomalies in³He/⁴He, Pb and Sr isotopes, and trace element ratios such as(La/Sm)_N are prevalent at the latitudes of the islands of St. Helena, Tristan da Cunha and Gough to the east of the ridge. The isotope systematics are consistent with injection beneath the ridge of mantle “blobs” enriched in radiogenic He, Pb and Sr, derived from off-axis hotspot sources. The variability in³He/⁴He along the ridge can be used to refine the hotspot source-migrating-ridge sink model.

MORB from the 2–7°S segment are systematically the least radiogenic samples found along the mid-ocean ridge system to date. Here the depleted mantle source is characterized by⁸⁷Sr/⁸⁶Sr of 0.7022, Pb isotopes close to the geochron and with²⁰⁶Pb/²⁰⁴Pb of 17.7, and³He/⁴He of 8.6–8.9 R_A. The “background contamination” of the subridge mantle, by radiogenic helium derived from off-ridge hotspots, displays a maximum between 20 and 24°S. The HePb and HeSr isotope relations along the ridge indicate that the³He/⁴He ratios are lower for the hotspot sources of St. Helena, Tristan da Cunha and Gough than for the MORB source, consistent with direct measurements of³He/⁴He ratios in the island lavas. Details of the HeSrPb isotope systematics between 12 and 22°S are consistent with early, widespread dispersion of the St. Helena plume into the asthenosphere, probably during flattening of the plume head beneath the thick lithosphere prior to continental breakup. The geographical variation in theHe/Pbratio deduced from the isotope systematics suggests only minor degassing of the plume during this stage. Subsequently, it appears that the plume component reaching the mid-Atlantic ridge was partially outgassed of He during off-ridge hotspot volcanism and related melting activity.

Overall, the similar behavior of He and Pb isotopes along the ridge indicates that the respective mantle sources have evolved under conditions which produced related He and Pb isotope variations.     

Implanted radiogenic and other noble gases in crustal diamonds from Northern Kazakhstan

Noble gases were extracted in steps from grain size fractions of microdiamonds ( < 100 μm) from the Kokchetav Massif, Northern Kazakhstan, by pyrolysis and combustion. The concentration of ⁴He in the diamonds proper (liberated by combustion) shows a 1/r dependence on grain size. For grain diameters > 15 μm the concentration also decreases with the combustion step. Both results are clear evidence that ⁴He has been implanted into the diamonds from -decaying elements in the surrounding matrix. The saturation concentration of ⁴He(5.6 × 10⁻⁴ cm³ STP/g) is among the very highest observed in any terrestrial diamonds. Fission xenon from the spontaneous fission of ²³⁸U accompanies the radiogenic ⁴He; the ¹³⁶Xe_f/⁴He ratio of (2.5 ± 0.3) × 10⁻⁹ agrees well with the production ratio of 2.3 × 10⁻⁹ expected in a reservoir where Th/U 3.3. Radiogenic ⁴⁰Ar is predominantly ( > 90%) set free upon combustion; it also resides in the diamonds and appears to have been incorporated into the diamonds upon their formation.

³He, on the other hand is mainly released during pyrolysis and hence is apparently carried by ‘contaminants’. The concentration in the diamonds proper is of the order of 4 × 10⁻¹² cm³ STP/g, with a ³He/⁴He ratio of 1 × 10⁻⁸. Excess ²¹Ne, similarly, appears to be present in contaminants as well as in diamonds proper. These two nuclides in the contaminants must have a nucleogenic origin, but it is difficult to explain their high concentrations.     

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have ³He/⁴He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic ³He/⁴He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic ³He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high ³He/⁴He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high ³He/⁴He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.