Ratio Of He Research Articles

Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone

We have measured δ 13C values and CO 2/ 3He ratios of vesicle-gas, and chemical compositions of North Fiji Basin basalt glasses, to estimate the contribution of subducted carbon in back-arc basin basalt quantitatively. It has made clear that the CO 2/ 3He ratio increases and the δ 13C value decreases with K 2O content. Since the variety of K 2O content of North Fiji Basin basalt is the result of two-component mixing, the variety of the CO 2/ 3He ratio and δ 13C value of the North Fiji Basin basalt should be mainly a result of mixing between the mantle component (low-CO 2/ 3He, high- δ 13C and low-K 2O) and the subducted component (high-CO 2/ 3He, low- δ 13C and high-K 2O). From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most (∼90%) carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc, since the subducting carbonate and organic matter throughout the North Fiji subduction zone are estimated in a proportion of 20:1. This may be one clue that carbonate can be transported into the mantle through the subduction zones.

Systematic variations in the 3He/ 4He ratio and carbon of fumarolic fluids from active volcanic areas in Italy: Evidence for radiogenic 4He and crustal carbon addition by the subducting African plate?

The isotopic composition of helium from active volcanic areas in Italy differs from those of other arc volcanic areas (circum-Pacific region), even though the tectonic settings are similar [1–4]. Helium isotopic values of fumarolic gases were obtained for the southern Italian volcanoes, Mount Etna and the Eolian Islands ([5,6]; this work), and from central Italian volcanoes in Neapolitan, Roman and south Tuscany volcanic provinces ([7,8]; this work). The 3He/ 4He ratios decrease from south to north, from values slightly lower than those of MORB to near crustal values, including ratios lower than atmospheric (R a = 1). Contamination by crustal radiogenic helium, derived from the natural decay of uranium and thorium, is the most likely candidate for lowering 3He/ 4He ratios. Increases from south to north in the U and Th contents of volcanic rocks are in agreement with this hypothesis. Local crustal and surficial contaminations may also explain the systematic changes in ratios. Subduction of the African plate beneath the Tyrrhenian Sea seems to be the most logical process to explain the 3He/ 4He ratio decrease. Similarly, carbon isotopic data show higher values from south to north, which suggests increasing contamination of subducted sedimentary carbon into magmatic gases. Correlations with strontium and neodymium isotope data support the above mentioned hypothesis and demonstrate the remarkable link between helium and carbon isotope variations and tectonic processes. Comparison with low helium data from the Banda arc (Indonesia), although with a different mechanism, agrees with the presence of a continental end-member below the Tyrrhenian Sea volcanoes.

Excess helium and argon of radiogenic origin in Mediterranean brine basins

Extraordinary helium and argon signatures were observed in a recently discovered brine lake district in the eastern Mediterranean Sea at water depths of ∼ 3500 m. In the Urania Basin 4He concentrations are enriched by a factor of up to 32000 compared to normal seawater. Low 3He/ 4He ratios of 1 × 10 −7 and high 40Ar/ 36Ar ratios of up to 470 indicate a radiogenic source. These data represent by far the highest radiogenic supersaturations ever observed in an oceanic environment. We propose a new conceptional model assuming that the Urania Basin is fed by advective transport from a deep fluid reservoir below the Messinian evaporite sequence. The evaporite seal provides the unique condition for accumulating the extraordinary high amounts of radiogenic helium from deep sedimentary strata. Injection of radiogenic helium from brine pools like the Urania Basin may contribute significantly to the inventory of terrigenic helium in the deep waters of the eastern Mediterranean.

Genesis of thermal groundwaters from Siping'an district, China

Thermal groundwaters (40–52°C, pH = 7.4–7.8, Eh = 210–245 mV) from Siping'an district, Shanxi Province, northwestern China, are hydrogeochemically unique. Their occurrence is controlled by faulted structures in Precambrian host rocks. Their hydrochemical type (5 springs and 2 wells) is mainly Cl SO 4 Na, with TDS values around 1.0 g/l. Some minor elements such as Si, Br, Sr, and Li, as well as neutral and acid bituminous substances are so enriched that the thermal waters can also be regarded as mineral waters. Their origin is meteoric, as indicated by 3 lines of geochemical evidence: (1) their δD and δ 18O compositions are very close to the Craig meteoric line; (2) their dissolved gas compositions are N 2-dominated, with less O 2 and CO 2; and (3) the 3He/ 4He ratios are low (0.028). Geochemical processes responsible for the genesis of the hydrochemical features of the waters include dissolution, mixing, and oxidation. The most important water-rock interaction is dissolution or hydrolysis of alumino-silicate minerals in the magmatic and metamorphic host rocks, since the waters are still undersaturated with respect to albite, anorthite, K-spar, and chlorite, as shown by saturation indices. The tritium contents of some thermal waters (46–53 TU), higher than the tritium concentration of local meteoric water, result from the mixing of thermal waters with cold, shallow-lying groundwaters that are from the 1960s. The predominant species of Fe in the thermal waters is Fe(OH) 3, as a result of oxidation processes under aerobic conditions of the aquifers.

Helium isotopic composition of the Tabar-Lihir-Tanga-Feni island arc, Papua New Guinea

Helium abundances and isotopic ratios have been measured in samples of geothermal gases, submarine ultramafic xenoliths, and subaerial mafic phenocrysts and xenoliths from the Tabar-LihirTanga-Feni (TLTF) arc in the Bismarck Archipelago of Papua New Guinea. These unusual volcanoes are produced by adiabatic decompression melting of subduction-modified upper mantle and apparently carry an exceptionally large slab-derived component. They, therefore, provide an ideal setting for studying the role of subducted helium in arc volcanoes. Helium isotopic ratios in the geothermal fluids, submarine xenoliths, and most helium-rich subaerial samples indicate the mantle source regions beneath the Tabar Island Group and Lihir Island have 3He/ 4He ratios of about 7.2 times the atmospheric ratio ( R A), whereas the mantle source for Ambitle Island (Feni Group) in the more southern TLTF arc has a lower ratio of about 6.6 RA. These ratios are only slightly lower than typical depleted upper mantle values of about 8.5 R A, indicating that even severely slab-modified mantle wedge carries a relatively minor slab-derived helium component. The systematically lower 3He/ 4He ratios of the Ambitle samples are interpreted to reflect a slightly greater slab-derived helium component. Helium isotopic ratios as low as 2 R A, which are correlated with low total helium abundances, were observed in subaerial augitic clinopyroxene phenocrysts. Coexisting olivine and sodium-rich acmitic clinopyroxene have higher helium abundances and isotopic ratios. This isotopic disequilibrium between coexisting mineral phases is interpreted as crustal-level addition of a low 3He/ 4He component to an actively outgassing magma from which different mineral phases trap helium sequentially.

Production of nucleogenic neon in the Earth from natural radioactive decay

Nucleogenic neon isotope production resulting from α decay of naturally occurring U and Th was calculated for typical compositions of the mantle and crust using a simple non‐Monte Carlo code for neutron transport. The (α,n), (α,p), and (n, α) reactions, leading to the production of neon isotopes in the outgoing reaction channel, are taken into account in the calculations. In the first stage of calculation, neon production from (α,n) and (α,p)reactions on 17O, 18O, and 19F is considered, and the neutron energy spectrum from (α,n) reactions on the major elements is derived. This spectrum is further used for calculation of the neon isotope yield from the reactions 24Mg (n,α)21Ne, 25Mg(n,α)22Ne, and 23Na(n,α)20Ne. The calculated production of nucleogenic neon is dominated by (α,n) reactions with oxygen in the Earth's mantle and crust. The present calculations give 21Ne/4He production ratios that are approximately half those of the previous estimates by Rison [1980] and Kyser and Rison [1982]. The present study also has shown that the nucleogenic 21Ne production from the 24Mg(n,α) reaction is only about one‐fifth the previous estimate. The contribution of nucleogenic 21Ne from the 24Mg(n,α) reaction is insignificant compared with nucleogenic 21Ne production from 18O(α,n). Our estimate of the nucleogenic 21Ne/radiogenic 4He production ratio both in the mantle and in the crust is 4.5×10−8. This production ratio is quite consistent with nucleogenic 21Ne/radiogenic 4He ratios actually observed in crustal fluid samples ((4.6±0.8) ×10−8). The error in calculation of the nucleogenic neon production, resulting from a random statistical variation of the entry neutron yields and cross sections, is estimated to be less than 5%.

Secular variations of helium and carbon isotopes at Galeras volcano, Colombia

We have measured 3He/ 4He, 4He/ 20Ne andCO 2/ 4He ratios, and δ 13C values of CO 2 in eighteen samples from crater fumaroles of Galeras volcano, Colombia, and seven samples of bubbling gases of the Pandiaco mineral spring located about 8 km east of the volcano from April 1988 to July 1995. The 3He/ 4He ratios of crater fumaroles, corrected for atmospheric contamination, increased significantly from 5.47 R atm in December 1989 to 7.93 R atm in August 1990 and reached a maximum of 8.84 R atm in May 1992. The ratio then decreased gradually until July 1995. Overall variation of helium isotopes appears to be related to the current activity of the volcano. The 3He/ 4He ratios of Pandiaco mineral spring were constant between April 1988 and November 1991 and increased in April 1993, although experimental error is large. The δ 13C values of CO 2 of crater fumaroles show negative correlation to the helium isotopes, while those of Pandiaco are relatively constant. Based on the δ 13C values andCO 2/ 3He ratios, it is possible to estimate fractions of upper mantle carbon in these samples. Calculated mantle fractions are 3–10% in crater fumaroles, while those in Pandiaco spring are negligibly small.

The chemical and isotopic composition of fumarolic gases and spring discharges from Galeras Volcano, Colombia

Galeras fumarole discharges have been collected since its reactivation, in 1988, through December 1995. The gases are dominated by H 2O, CO 2, S (as SO 2 and H 2S) and HCl. The relative proportions of these gases classify them as ‘magmatic’. Thermodynamic equilibrium temperatures of the gases range from 260 to > 600 °C. The relative abundance of inert gases, N 2, Ar and He, can be used as ‘tracers’ to identify the source of the fumarole discharges. At Galeras the majority of the samples have a composition characteristic of gases originating from arc-related magmas, with relatively high N 2 contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition changed to basaltic or ‘mantle-derived’ gases, with significantly higher He contents. This is interpreted to be the result of injection of volatiles from a basaltic magma body at depth prior to and during the increased eruptive activity of 1993. The δ 13C values for CO 2 in fumarole discharges are typical of andesitic volcanoes and may indicate addition of MORB-derived CO 2. The δ 15N values for N 2 may indicate significant contribution of N 2 from marine sediments and only minor contribution of MORB-derived N 2. The δ D and δ 18O values of the discharging steam lie on a mixing trend between the isotopic composition of ‘arc-related’ magmatic water and 18O-shifted meteoric water. The most magmatic discharges have δ D values of −30 to −35‰; while the most meteoric discharges have values of −70 to −75‰, similar to Galeras thermal spring waters. Galeras thermal water discharges consist of acid sulfate and bicarbonate waters.S/Cl ratios in the acid sulfate waters are similar to fumarole ratios, suggesting direct absorption of magmatic gases into shallow ground waters. This is supported by the essentially meteoric δD and δ 18O values of the discharges and by elevated 3He/ 4He ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yield acid waters which are capable of dissolving rocks. The thermal waters, however, are far from equilibrium with Galeras lavas and pyroclastic rocks, providing evidence of the immaturity of the Galeras hydrothermal system. The SO 4 and Cl content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the volcano. The 7-year sampling program at Galeras revealed intriguing results concerning the activity of Galeras, its magmatic-hydrothermal system and the origin of the volatiles. Despite decreasing outlet temperatures since 1992, deep temperatures remain high, implying continued unrest in the Galeras magmatic system.

Mantle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy

We report the first detailed investigation of both helium and carbon isotopes in groundwaters and gases of Mt. Etna, providing new insight into the distribution, origin and budget of magmatic gas release at this very active volcano [1]. A mantle-derived magmatic component, with ultimate 3He/ 4He ratio of 6.9 ± 0.2 R a and δ 13C of about −4‰, is identified in both types of fluids, depending on their location and the extent of their dilution by either air (gases) or a mixture of dissolved air and organic carbon (waters). Apart from the summit zone, this magmatic component is preferentially concentrated in CO 2-rich groundwaters that issue from two remote sectors of the south-southwest and eastern volcano flanks, where its proportion increases with the altitude of meteoric recharge (or the length of pathflow) of the waters. Such a pattern suggests that, in addition to possible local gas input, the groundwaters collect much of their dissolved magmatic He and C while they infiltrate and flow through one of the more elevated, gas-effusing parts of the volcanic pile, among which the south-southeast fracture zone of Etna is the best candidate. These observations provide a new framework for remote geochemical monitoring of the volcano. The 3He/ 4He ratio of the magmatic gas end-member coincides with that of helium trapped in the He-rich olivine crystals of Etna basalts (mean range: 6.7 ± 0.4 R a, [2,3]), pointing to its negligible dilution by radiogenic He from the crustal basement and further constraining the 3He/ 4He ratio of the present-day Etna magma. While being lower than the typical MORB value of 8 R a, this ratio is the highest for an active volcano in continental Europe and probably tracks a relatively radiogenic upper mantle zone that is upwelling beneath this region [4]. The estimated outputs of mantle-derived CO 2 and 3He from Etna account for 10% and 15%, respectively, of estimates for global subaerial volcanic emissions. This huge contribution results from continuous degassing of mostly unerupted He- and C-rich alkaline basaltic magma, which occurs principally through the central open conduits and secondarily through the flanks of the volcano. Groundwaters carry only a minor fraction (≈ 3%) of total emitted CO 2 and 3He.

Effects of hydrothermal processes on the chemical and isotopic composition of mantle-derived gases in SE China

Chemical and isotopic (He, Ar and C) compositions have been measured for bubbling gases discharging from two closely-spaced earthquake prediction wells in Guangdong, SE China. Observed 3He/ 4He ratios ( R/ R A ranging from 1.1 to 4.9) indicate the occurrence of a mantle component that has been diluted by a crustal radiogenic 4He component. δ 3C values of CO 2 suggest a different origin in the two wells: magmatic for ZK#1 (−6.8 to −8.4%‰:) and biogenic for ZK#7 (−14.5 to −18.2%). Observed 40 Ar/ 36Ar and air-corrected N 2/Ar ratios suggest that nitrogen and argon of both wells are mostly meteoric. While the CO 2/ 3He ratios (∼1 x 109) in ZK#1 are close to that of MORB (Middle Oceanic Ridge Basalts), the ZK#7 is characterized by much lower CO 2/ 3He ratios (∼1 x10 7) with lower CO 2 concentration. The latter cannot be ascribed in terms of a simple mixing of the two components but has to be explained by removal of CO 2, probably caused by calcite precipitation. Alternatively, CO 2 removal from and 4He addition to the magmatic component may be responsible for the coupled variations of 3He/ 4He and CO 2/He ratios in the two wells, which is most likely controlled by the degree of gas-water-rock interaction.

The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust system

We report analyses of the 176Hf/ 177Hf ratio for 25 chondrites from different classes of meteorites (C, O, and E) and the 176Lu/ 177Hf ratio for 23 of these as measured by plasma source mass spectrometry. We have obtained a new set of present-day mean values in chondrites of 176Hf/ 177Hf= 0.282772 ± 29 and 176Lu/ 177Hf= 0.0332 ± 2. The 176Hf/ 177Hf ratio of the Solar System material 4.56 Ga ago was 0.279742 ± 29. Because the mantle array lies above the Bulk Silicate Earth in a 143Nd/ 144Nd versus 176Hf/ 177Hf plot, no terrestrial basalt seems to have formed from a primitive undifferentiated mantle, thereby casting doubt on the significance of high 3He/ 4He ratios. Comparison of observedHf/Nd ratios with those inferred from isotopic plots indicates that, in addition to the two most prominent components at the surface of the Earth, the depleted mantle and the continental crust, at least one more reservoir, which is not a significant component in the source of oceanic basalts, is needed to account for the Bulk Silicate Earth Hf-Nd geochemistry. This unaccounted for component probably consists of subducted basalts, representing ancient oceanic crust and plateaus. The lower continental crust and subducted pelagic sediments are found to be unsuitable candidates. Although it would explain the Lu-Hf systematics of oceanic basalts, perovskite fractionation from an early magma ocean does not account for the associated Nd isotopic signature. Most basalts forming the mantle array tap a mantle source which corresponds to residues left by ancient melting events with garnet at the liquidus.

Hydrogen and carbon isotopic composition of methane as evidence for biogenic origin of natural gases from the Green Tuff Basin, Japan

Hydrogen and carbon isotopic analyses of methane were carried out to investigate the origin of natural gases commercially produced from volcanic rocks in the Green Tuff Basin of northeast Japan, whose high 3He/ 4He ratios have been interpreted as an indication of their abiogenic origin. Methane isotopic signatures (δ 13C ≈ −32 to −67‰, δD ≈ −163 to −202‰, respectively) suggest that these are thermogenic gases or mixtures of thermogenic gases with bacterial gases. The correlations of methane δD vs. δ 13C for the gases from specific fields are consistent with a mixing model between bacterial and thermogenic end-members, which was previously proposed on the basis of 13C contents of C 1 to C 4 hydrocarbons (Sakata, 1991; Berner and Faber, 1996). The Green Tuff Basin gases are isotopically distinct from well-known abiogenic methanes, e.g. geothermal methane from the East Pacific Rise. A significant contribution of abiogenic methane would, therefore, be unlikely for these gases.

Carbon isotopes of hydrocarbons and carbon dioxide in natural gases in China

130 natural gases in the continent of China were determined for the abundance and carbon isotopes of hydrocarbons and carbon dioxide. The δ 13C (PDB) values of methane range from −68 to − 24.4%, and the largest isotopic variability for hydrocarbons is of δ 13C 1 < δ 13C; < δ 13C; < δ 13C 4 < δ 13C 5. These characteristics suggest that the gaseous hydrocarbons originated principally from thermal and or microbial decomposition of organic matter. The wide variations of δ 13C 0 distribution patterns (n = 1, 2, 3, 4 and 5) can be explained by (1) extensive mixture between thermogenic and microbial gases, or between thermogenic gases produced during different thermal stage, and or (2) preferential microbial oxidation. There is no direct evidence for the presence of abiogenic hydrocarbon in natural gases in China. One sample from Songliao basin shows a heavier δ 13C 1 ( − 24‰) and reverse isotopic distribution pattern ( δ 13C 1 > δ 13C 2 > δ 13C 3). Concerning the occurrence of mantle-derived He, Ne, Ar and Xe in the south Songliao basin, we propose that the methane mantle derived and heavy hydrocarbons (ethane, propane, butane, etc.) formed from polymerization of the mantle-derived methane. The δ 13C values for carbon dioxide range from − 13.9 to + 13.5‰ suggesting the multiple origins. Gases with CO 2 concentrations greater than 10% have a narrow range of δ 13C values, from − 8 to − 2‰, and show higher 1He 4He ratios ( > 0.1 times the atmospheric value), suggesting that a mantle-derived component is diluted by CO 2 derived from carbonate and or organic matter. Positive δ 13C values ( + 4 – + 14‰) indicate CO 2 reduction to CH 2, CO 2 with trace concentration in CH 4-rich gases can be attributed to microbial decomposition of organic matter in sedimentary rocks.

Helium isotope studies of geothermal fields in the Taupo Volcanic Zone, New Zealand

We report new measurements of 3He, 4He and Ne on 69 samples of gas from geothermal fields in the central section of the NE-SW trending Taupo Volcanic Zone (TVZ). We find 3He 4He ratios in the range 5.9-7.3 times the atmospheric ratio (5.9-7.3 R A), typical of subduction zone systems. In some areas near the boundaries of the volcanic zone the ratio is much lower (2–4 R A). The values in the Wairakei field are extremely uniform and vary by only a few per cent from 7.2 R A. In contrast, the wells in the Ohaaki-Broadlands field show a striking factor of two variation, with the ‘West bank’ section characterised by values near 6 R A while the ‘East bank’ section has values as low as 3.0 R A. This suggests that mixing of two types of geothermal source fluids may be occurring at Ohaaki-Broadlands. An inverse relationship has been found between the 3He 4He ratio of the gases and the B Cl ratios in the associated fluids. Thus the helium isotope ratios appear to be affected both by the flux of mantle helium associated with mantle subduction and also by the presence of sedimentary rocks beneath the geothermal field. Leaching, by the hot ascending fluid, of radiogenic 4He together with boron from the underlying high-boron, low-chloride Mesozoic greywacke basement, is thought to be the cause of this inverse relationship. The CO 2 3He ratio in the TVZ exceeds the MORB ratio by a factor of up to 100, with higher values corresponding to higher B Cl fluid ratios, suggesting that the greywacke may also be the major source of the CO 2. We have also measured argon concentrations and isotopes on 19 samples in the Ohaaki-Broadlands area in order to further elucidate the nature of the striking helium isotope variations. The argon isotope ratios are relatively close to atmospheric indicating that the majority of the argon comes from recirculated groundwater. The simplified boiling model of Mazor et al. (1990) has been used to calculate the 3He heat ratio in the Ohaaki-Broadlands and Wairakei fields. The values obtained fall within the range previously reported for mid-ocean ridge systems. This model has been extended and has led us to the conclusion that, compared to the 3He 4He ratio, the ( 3He 36Ar ) ASW ratio is a much better indicator of the potential geothermal heat. The ( 4He 40Ar ) NA (where NA = non-atmospheric) ratio of both the high (mantle-derived) and low (crustal-derived) 3He 4He end-members of the analyses reported here is about 3.6, but for samples with low 3He 36Ar ratio, this ratio falls to values below 0.5. The value of 3.6 is in the range which is expected for the radiogenic component and although considerably lower than that often found in subduction type mantle gases it is still within the accepted range of values.

Isotopic tracing of volcanic source regions from Hawaii: decoupling of gaseous from lithophile magma components

The well established geology and geochemistry of Hawaiian volcanism allows a detailed evaluation of the relationships between geochemically distinct tracers, such as the noble gases and the lithophile elements Sr, Nd and Pb. Four volcanic evolutionary stages show characteristic Sr-Nd-Pb isotopic progressions: early and late stages include depleted melts, whereas the intermediate shields tend to be less depleted, including source components similar to bulk earth values. In contrast, 3He/ 4He ratios gradually decrease from high values during the early stage to MORB-like values for the late stage. This lack of correlation between the lithophile and noble gas records may be accounted for by: (1) a unique ‘Loihi-like’ mantle component for Hawaii, or (2) a systematic offset between the two records. Whereas many studies have preferred the first scenario, we opt for the latter, and suggest that both systems are coupled in the deep mantle, but become decoupled when a gas-rich melt phase separates from the plume. We suggest that incipient CO 2-dominated melts, carrying the noble gases, could separate in the plume flexure, and may buoyantly migrate ahead of the plume, carrying the bulk of its lithophile inventory. These melts permeate the lithosphere, which, upon sufficient heating by the plume, could generate the initial stage alkali basalts and tholeiites. The subsequent formation of the shield stage tholeiites occurs when the main plume, carrying the less depleted lithophile signature, has risen to shallower levels, and commences to melt on a large scale. The melts of this stage show lower 3He/ 4He ratios owing to asthenospheric entrainment by the plume, which has previously lost a major part of its volatile inventory, allowing for dilution of the isotopic signature by MORB-type noble gases. If this model is correct, the less depleted (Sr, Nd and Pb) and less degassed (noble gases) components are from one and the same source, and may provide the best estimate for the isotopic composition of the deep mantle.

Helium isotopic variations in Ethiopian plume lavas: nature of magmatic sources and limit on lower mantle contribution

Oligocene Continental Flood Basalts (CFB) from Ethiopia exhibit a wide range of 3He/ 4He ratios, from 0.035 Ra in crust-contaminated Low-Ti lava to 19.6 Ra in plume-derived high-Ti lava (where Ra is the atmospheric ratio of 1.38 × 10 −6). Quaternary basalts sampled in the Main Ethiopian Rift and in Afar also display dramatic 3He/ 4He variations from 0.009 Ra to 16.9 Ra. Low isotopic ratios partly reflect crustal assimilation whereas 3He/ 4He values higher than the mean isotopic ratio of the upper mantle (8 ± 1 Ra as measured in Normal Mid-Ocean Ridge Basalts [N-MORB]) indicate the contribution of a lower mantle component. The geographical extension of plume-type He is consistent with an approximate radius of ∼ 1000 km for the flattened plume head. Helium isotopic data show that strong lower mantle signals were already apparent during early emissions of CFB and pre-dated by more than 15 Ma the major phases of rifting in the African Horn region, contrary to the view of passive decompression melting contemporary with large-scale rifting. The He-Sr composition of the plume component cannot result from a simple binary mixing between lower mantle and upper mantle end-members and requires the contribution of other component(s) such as recycled crust. The proportion of the lower mantle contribution to the total mass of material involved in the building of the Ethiopian magmatic province, as estimated from He contents and isotopic ratios in the respective mantle sources, is found to be small (< 5%). Except for He (and Ne), such contribution has no impact on the trace element and isotopic compositions of plume basalts, which are dominated by mixing between upper mantle, continental crust and recycled sources. If the thermal anomaly necessary to produce CFB originates in the lower mantle, as generally proposed, then there exists a dramatic decoupling between mass and heat transfers across the lower mantle-upper mantle boundary layer.

Radiogenic helium thermodesorption from titanium tritide

The thermal desorption technique has been used to investigate the 3He release from titanium tritide samples with the initial composition TiH * 1.8–1.9 (H * — protium, deuterium, tritium) varied in tritium content. It is shown that, depending on the holding time or amount of 3He generated by tritium decay (to the atomic ratio of 3He gen/Ti = 0.81), thermal desorption spectra for 3He in the range from room temperature to ∼ 1500°C may have from one to five peaks. Data are included about thermodesorption peak temperature varying with titanium tritide ‘aging’. Estimates have been made for the quantities of 3He accounting for different peaks, with storage until 3He gen/Ti = 0.24. The few peaks as observed in thermal desorption spectrum are attributed to radiogenic helium of the titanium tritide lattice having different states.

Resolving lithospheric and sub-lithospheric contributions to helium isotope variations in basalts from the southwestern US

The 3He/ 4He ratios of continental basalts and their peridotitic xenoliths delimit the (U + Th)/ 3He evolution of continental lithospheric mantle, but their interpretation is complicated by the potential for differential mobility of He with respect to lithophile elements. In this study we report 3He/ 4He ratios of 5.5–7.7 R A for olivines separated from young basalts in the southwestern US. Helium isotope ratios are lowest in samples that exhibit Th and Pb isotope evidence for derivation from an enriched source in the lithospheric mantle that has remained unmodified since ∼ 1.7 Ga. There is a strong, positive correlation between 3He/ 4He and ε Nd in southwestern US lavas, suggesting a time-integrated affinity between light rare earth- and (U + Th)/ 3He-enrichment in the underlying Proterozoic lithospheric mantle. This relationship further suggests that the Proterozoic lithosphere mantle in the southwestern US is probably not a highly degassed reservoir which is contaminated by He derived from the underlying asthenosphere. Instead, 3He/ 4He ratios of 5.7 ± 0.3 R A in the most enriched source regions suggest that their (U + Th)/ 3He is only slightly elevated compared to the depleted upper mantle, and that processes which enrich the lithospheric mantle in large-ion lithophile elements also enrich it in He.

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.

Helium and argon isotopes in rocks, minerals, and related ground waters: A case study in northern Switzerland

To address the problems related to the transfer of helium and argon isotopes from rocks into related aquifers, the concentrations of U, Th, Li, and K and helium and argon isotopes were measured in a sedimentary sequence from Tertiary to Permo-Carboniferous and in crystalline rocks in northern Switzerland. In addition to whole-rock samples, mineral separates have also been investigated. The observed concentrations of 3He, 4He, and 40Ar in rocks, minerals and groundwaters are compared to calculated values which would result from in situ radiogenic production in a closed system. This comparison shows that the rocks and minerals have almost completely lost radiogenic in situ produced helium. Loss of 3He and 4He has been controlled by different retention capacities of the rock- forming and accessory minerals and fillings. Therefore, measured 3He/ 4He ratios in these rocks can differ from the calculated production ratios. The deviation is generally less than a factor of 3. However, in certain chemical sediments, e.g., anhydrites, elevated ratios of 3He/ 4He up to 15 × 10 −8, which is 20 times the production ratio, were observed. It is postulated that 3H, the precursor of 3He, might be chemically bound in such sediments during the 12.3 years half-life of 3H and that the beta decay energy of 3H is too small to liberate 3He. The calculated closed-system concentrations of helium in groundwater exceed the measured ones by up to three orders of magnitude, implying that the water/rock system has not been a closed system since the time of sedimentation and that movable waters have removed helium to a discharge area and to the atmosphere. Stagnant old waters, however, could contain high concentrations of helium and supply it to younger movable waters. Aquitards with low permeability (Permian shales) appear to be a more important source of helium isotopes than water-bearing rocks (Permian sandstones). High abundances of all parent elements in the shales, almost complete loss of radiogenic helium, and the enhanced production ratio of 3He/ 4He = 7.2 × 10 −8 in these rocks, similar to the ratio observed in the neighbour Permian aquifer, 9.4 × 10 −8, suggest the shale as a major source of helium isotopes for this aquifer and diffusion as a principal process for helium transfer from the stagnant porewaters in shales into the movable waters in sandstones. Some sedimentary rocks, e.g., Permian sandstones and shales, contain more helium than could have been produced since the time of sedimentation. While Permian sandstones mainly have retained this excess rare gas component, they have released radiogenic helium. The difference in the retention of these two components most probably demonstrates that migration of radiogenic helium from damage tracks along crystal imperfections to grain boundaries is an important process controlling its loss. The abundances of excess 3He and 4He in rocks cannot be predicted by in situ production calculations; studies of rocks and minerals are required. The average ratio of 40Ar/ 36Ar = 385 in groundwaters collected from Permian sediments from the Weiach borehole is much higher than that in the aquifers from the crystalline basement below the sediments. Radiogenic 40Ar* in Permian sediments is mainly liberated by diagenetic water-rock interactions. The low contribution of radiogenic 40Ar* in deep aquifers of the crystalline basement and the fact that chlorine concentrations in groundwaters from the basement are also much lower than in the overlaying Permian aquifer eliminates a hypothetical upwards fluid flux. From this study we envisage intrabasin sources for helium and argon isotopes in groundwaters with mainly lateral migration of waters through sedimentary layers at different rates and diffusion of helium from “slower-velocity” older waters in more stagnant zones (shales) into flow-passes with “highervelocity” younger waters in sections with higher permeability (sandstones). An external source for noble gases is not required in such a model.