Mantle 3He Research Articles

Testing mixing models of old and young groundwater in a tropical lowland rain forest with environmental tracers

We tested three models of mixing between old interbasin groundwater flow (IGF) and young, locally derived groundwater in a lowland rain forest in Costa Rica using a large suite of environmental tracers. We focus on the young fraction of water using the transient tracers CFC‐11, CFC‐12, CFC‐113, SF6, 3H, and bomb 14C. We measured 3He, but 3H/3He dating is generally problematic due to the presence of mantle 3He. Because of their unique concentration histories in the atmosphere, combinations of transient tracers are sensitive not only to subsurface travel times but also to mixing between waters having different travel times. Samples fall into three distinct categories: (1) young waters that plot along a piston flow line, (2) old samples that have near‐zero concentrations of the transient tracers, and (3) mixtures of 1 and 2. We have modeled the concentrations of the transient tracers using (1) a binary mixing model (BMM) of old and young water with the young fraction transported via piston flow, (2) an exponential mixing model (EMM) with a distribution of groundwater travel times characterized by a mean value, and (3) an exponential mixing model for the young fraction followed by binary mixing with an old fraction (EMM/BMM). In spite of the mathematical differences in the mixing models, they all lead to a similar conceptual model of young (0 to 10 year) groundwater that is locally derived mixing with old (>1000 years) groundwater that is recharged beyond the surface water boundary of the system.

Helium isotopic constraints on simulated ocean circulations: implications for abyssal theories

There is ongoing controversy as to the dynamical significance of geothermal heat flow in shaping the abyssal circulation. In this paper, we gauge the impact of geothermal heating and vertical mixing parameterizations in the general circulation model OPA. The experiments are evaluated by comparing simulated mantle 3He with observations collected during the GEOSECS and WOCE programs. This tracer is particularly adapted to the validation of our numerical simulations because its injection into the ocean interior is tightly linked to geothermal processes. In agreement with previous studies, the model circulation is found very sensitive to the parameterization of the vertical mixing. The meridional overturning circulation (MOC) is globally intensified when moving from a constant mixing to a version with enhanced mixing near the ocean bottom, with the most drastic variation observed for AABW (+ 50%). Adding the geothermal heat flux mainly affects AABW circulation in the model, enhancing it all the more as the meridional circulation is slow (low vertical mixing), but proportionally less so when it is more vigorous (enhanced vertical mixing). This can be understood from the requirement of the abyssal ocean to maintain heat balance. The evaluation with mantle 3He simulations reveals that the version with low vertical mixing, with its sluggish circulation, produces unrealistically high a 3He isotopic composition. However, with a vertical mixing that is enhanced at depth, the 3He distribution falls within an acceptable range of values in the deep ocean. Finally, adding the geothermal heating to this enhanced mixing case provides a substantial improvement of the simulation of AABW in all basins but the Indian Ocean. 3He isotopic composition is then in good agreement with the observations. Taken jointly with observational estimates of the MOC intensity, these independent isotopic constraints suggest that both geothermal heating and enhanced diapycnal mixing at depth are key ingredients in the realistic simulation of abyssal circulation.

Tritium Released from Mantle Source: Implications for Natural Nuclear Fusion in the Earth’s Interior

This paper summarizes the observation results of mantle tritium (3H) in two volcanic lakes, Lakes Nemrut (Turkey) and Laacher (Germany). The presence of excess 3H in the lakes can be explained as material released from mantle source because of the correlation of excess 3H with mantle 3He and 4He. We conclude that excess 3H in these two lakes, after the origin of the excess 3H from atmosphere and conventional nuclear reactions are excluded and the correlation of the excess 3H and mantle 3He is considered, might be from a mantle source and produced by nuclear fusion (d–d reaction) in the deep Earth. We have also investigated helium isotopes in the hydrothermal vent fluids at Mid-Ocean Ridge (MOR). The results show nearly constant 3He/4He ratio (3He/4He = 1.12 ± 0.13 × 10−5) and approximately constant 3He/heat ratios ((5–10) × 10−18 mol/J). The correlation of 3He with 4He and heat suggests that it is reasonable to suppose 3He is produced by nuclear fusion (d–d reaction) and 4He from α-decay of U and Th in the deep Earth. Based on that, 3He/4He ratios for 10 hydrothermal vent fluids are calculated. The results agree with the measurement at hydrothermal vent fluids and Mid-Ocean Ridge Basalts (MORB) on the average. We concludes that the narrow distribution of 3He/4He ratio peaked at ∼8 RA in MORB can be explained by the hypothesis that 3He is produced in nuclear fusion.

Conditions of formation of ore-geochemical specialization of modern hydrotherms in the Baikal Rift Zone

Modern hydrotherms of the Baikal Rift Zone (BRZ) are divided according to contents of ore and rare elements and gas composition into: (1) carbonaceous, forming in carbonate rocks, (2) hydrosulfuric and H 2S-free nitric and methane, discharged in Cenozoic deposits, and (3) nitric, produced in granitoids. Carbonaceous hydrotherms are enriched in Sr, Ba, Y, Co, and Cr; nitric ones, in Li, Rb, Cs, Mo, W, and other granitophile elements; and hydrosulfuric hydrotherms are saturated with complex metals. High contents of U and associated elements are specific for oxidized nitric alkaline thermal waters and are atypical of carbonaceous ones, and high contents of Au are observed in methane hydrotherms. Separation of ore elements between organic and clayey matter takes place as early as the initial stage of sediment formation. During sediment accumulation, these elements pass into hydrothermal solution in the same separated associations. In oxidizing (Eh ranges from +50 to +260 mV) alkaline medium, U and, obviously, other elements of uranium association, present not only in granitoids but also in the organic matter of sediments, pass into the solution most actively. Alkaline medium (7 < pH < 10) favors the transition of Mo and W from igneous rocks and sediments into the solution. Methane hydrotherms (8 < pH < 9, Eh < –70 mV) can accumulate gold. In strongly reducing (Eh ranges from –200 to –360 mV) alkaline (pH > 8) hydrosulfuric anoxic medium, chalcophile complex metals of the clayey matter of sediments are the most active to pass into the solution. Thus, as Eh decreases and H 2S concentration increases, hydrothermal solution extracts various element associations from igneous and sedimentary deposits in the sequence U–Mo, W–Au–Sn, Cu, Pb. In the solution the differentiation of elements continues. Hence, the separation of ore-element associations at the initial stage of sediment formation, long before the hydrothermal activity, is the first track in a chain of processes leading to the differentiation of ore deposits according to formational types. Hydrotherms confined to deep-fault zones contain an endogenous fluid with mantle 3He/ 4He values close to those in methane and carbonaceous hydrotherms of the Tunka rift depression in the BRZ. According to results of thermodynamic modeling, the fluid consists of CH 4, H 2O, and CO 2 and obviously lacks mantle ore elements, as it was initially of hydrocarbon composition.

Helium and argon isotopes in lower crustal xenoliths from the belomorian mobile belt

The isotopic systematics of noble gases (He and Ar) were studied in Neoarchean and Paleoproterozoic lower crustal xenoliths from the Belomorian mobile belt. The xenolith suite is dominated by garnet granulites (Grt + Cpx + Pl ± Opx ± Qtz ± Kfs ± Phl ± Hbl) and two-pyroxene or garnet pyroxenites (Cpx + Pl ± Grt ± Opx ± Hbl ± Qtz). The xenoliths and the host Devonian ultramafic lamprophyres forming diatremes and explosion dikes contain fluid with similar He and Ar isotopic compositions. It was found that the fluid was trapped by the rocks and xenolith minerals approximately simultaneously with the formation of the lamprophyres. This conclusion is based on the identical K-Ar ages of the majority of xenoliths and ultramafic lamprophyres. When the xenoliths were transported toward the surface by high-temperature ultrabasic melts, the noble gases occurring in them were partly (Ar) or completely (He) lost. The melts were contaminated by meteoric waters during their emplacement in the upper crust, which resulted in that the Ar isotopic composition of lamprophyres approached the composition of atmospheric Ar. The fluid phase that was liberated during melt crystallization severely affected the xenoliths, diminishing the difference between the isotope compositions of He and Ar in the xenoliths and ultramafic lamprophyres. The He isotope composition includes an admixture of mantle 3He, which is suggested by the high measured 3He concentrations, exceeding the calculated values, and high 3He/36Ar ratios in the xenoliths and their host lamprophyres. The fraction of the mantle component in the fluid trapped by the xenoliths was estimated as ∼20%.

Reply to a comment by Werner Aeschbach-Hertig on “Helium sources in passive margin aquifers new evidence for a significant mantle 3He source in aquifers with unexpectedly low in situ 3He/4He production” by M. C. Castro [Earth Planet. Sci. Lett. 222 (2004) 897–913

Reply to a comment by Werner Aeschbach-Hertig on “Helium sources in passive margin aquifers new evidence for a significant mantle 3He source in aquifers with unexpectedly low in situ 3He/4He production” by M. C. Castro [Earth Planet. Sci. Lett. 222 (2004) 897–913

A comment on “Helium sources in passive margin aquifers—new evidence for a significant mantle 3He source in aquifers with unexpectedly low in situ 3He/4He production” by M. C. Castro [Earth Planet. Sci. Lett. 222 (2004) 897–913

A comment on “Helium sources in passive margin aquifers—new evidence for a significant mantle 3He source in aquifers with unexpectedly low in situ 3He/4He production” by M. C. Castro [Earth Planet. Sci. Lett. 222 (2004) 897–913

A helium isotope perspective on the Dixie Valley, Nevada, hydrothermal system

Fluids from springs, fumaroles, and wells throughout Dixie Valley, NV were analyzed for noble gas abundances and isotopic compositions. The helium isotopic compositions of fluids produced from the Dixie Valley geothermal field range from 0.70 to 0.76 Ra, are among the highest values in the valley, and indicate that ∼7.5% of the total helium is derived from the mantle. A lack of recent volcanics or other potential sources requires flow of mantle-derived helium up along the valley bounding Stillwater Range Front Fault, from which the geothermal fluids are produced. Using a one-dimensional flow model, a lower limit fluid flow rate up through the fault of 7 mm/yr is estimated, corresponding to a mantle 3He flux of ∼10 4 atoms m −2 s −1. A comparison between the fluids from Dixie Valley springs, fumaroles, and wells and the fluids produced from the geothermal field reveals a mixing trend between the geothermal fluid and younger, cooler groundwaters. The exceptions are those features that either emanate directly from the Stillwater fault or wells that penetrate and extract fluids from the fault zone, all of which have helium isotopic compositions that are indistinguishable from the geothermal production fluids. The results of our study indicate that the Stillwater Range Front Fault system must act as a permeable conduit that can sustain high vertical fluid flow rates from deep within the crust and crust-mantle boundary and that high permeability may exist along most of its length. This suggests that the geothermal potential of the Stillwater fault may be significantly greater than the 6–8 km long system presently under production. Since all the numerous springs, wells, and fumaroles in the valley also contain a fluid component that is indistinguishable from the geothermal/Stillwater fault fluid, the potential for an additional deeper and more pervasive geothermal system also exists and should be further evaluated. Furthermore, we suggest that elevated helium isotope compositions in regions with little or no recent magmatism are an indicator of the deep crustal permeability that is required to drive and sustain extensional geothermal systems.

Helium sources in passive margin aquifers—new evidence for a significant mantle 3He source in aquifers with unexpectedly low in situ 3He/ 4He production

Using knowledge of groundwater dynamics in the Carrizo aquifer and surrounding formations in southwest Texas, United States, 3He and 4He transport was modeled and calibration on concentrations achieved for both isotopes. Three components were isolated: atmospheric, in situ production and external deeper crustal and mantle components. Among the three major sources for these isotopes, 3He concentrations are primarily a binary mixture from atmospheric and deeper external sources. Unlike 4He, 3He resulting from in situ production is negligible over the entire domain. Tests of the distribution of R/ R a values ( R is the groundwater 3He/ 4He ratio; R a is the atmospheric 3He/ 4He ratio), show that relatively high R/ R a values (∼0.14) entering the aquifer are quickly lowered by extremely low (0.0025) in situ production ratios almost totally devoid of 3He. Such lessening gives rise to “typical crustal” R/ R a values of about 0.055. Such effect on R/ R a ratios increases as 3He and 4He move upward through the sedimentary succession. Further analysis on the R/ R a behavior suggests that low crustal R/ R a values have the potential to mask high R/ R a ratios carrying a substantial 3He mantle component, particularly, in multilayered aquifer systems. This suggests that a high mantle 3He component is present in sedimentary groundwater systems. Groundwater R/ R a values between 0.01 and 0.05 (considered as typical crustal values) may be the result of a significant (perhaps dominant) mantle component modified by low 3He/ 4He in situ production ratios. It further suggests that the continental crust may constitute a much more permeable environment to mantle 3He than previously thought. The occurrence of deeper magma bodies is not needed to explain the presence of mantle 3He in the crust, and implies that mantle 3He fluxes might be higher in crustal rocks than previously thought.

Fluid mixing in carbonate aquifers near Rapolano (central Italy): chemical and isotopic constraints

Chemical (major and trace elements) and isotopic compositions (δD and δ 18O in waters and δ 13C in CO 2 and 3He/ 4He in gases) of natural thermal (11) and cold (39) fluids (spring waters and gases) discharging from a tectonic window of Mesozoic limestones in central Italy, have proved to be the result of mixing processes inside the limestone formations. The limestones provide a preferential route for subsurface fluid migration and they gather both descending cold, Ca-HCO 3, B-depleted groundwaters and rising convective Ca-SO 4(HCO 3), CO 2-saturated, B-rich thermal waters. Atmospherically-derived descending gas components (N 2, Ne, He), dissolved in rainfall that infiltrates the limestone system mix with N 2, Ne, He-depleted hot rising waters. Boron in the liquid phase and N 2 and Ne in the gas phase are the most useful elements to trace the mixing process. The deeper gas samples recognised in the area are associated with the hotter waters emerging in the area. In spite of being depleted in Ne and He and light hydrocarbons they have the higher measured 3He/ 4He ratios, suggesting a contribution of mantle 3He to the gas phase. This contrasts with deep circulation in the crust which would lead to increased concentration of 4He in the deeper gases. Paradoxically, there is more relative concentration of 4He in the more air-contaminated gas samples than in the deeper gas samples. A similar paradox exists when the δ 13C of CO 2 in the deeper gas samples is considered. The shallower air-contaminated gas samples, although they should be affected by the addition of soil- 13C depleted organic C, have δ 13C in CO 2 more positive than the deeper gas samples recognized. Since any deep hydrothermal source of CO 2 should generate CO 2 with more positive values of δ 13C than those measured at surface, a multiple (single) calcite precipitation process from hydrothermal solutions, with C isotopic fractionation along the rising path inside the Mesozoic limestone formations, is proposed.

The subcontinental mantle beneath southern New Zealand, characterised by helium isotopes in intraplate basalts and gas-rich springs

New helium isotope data measured in Cenozoic intraplate basalts and their mantle xenoliths are compared with present-day mantle helium emission on a regional scale from thermal and nonthermal gas discharges on the South Island of New Zealand and the offshore Chatham Islands. Cenozoic intraplate basaltic volcanism in southern New Zealand has ocean island basalt affinities but is restricted to continental areas and absent from adjacent Pacific oceanic crust. Its distribution is diffuse and widespread, it is of intermittent timing and characterised by low magma volumes. Most of the 3He/ 4He ratios measured in fluid inclusions in mantle xenocrysts and basalt phenocrysts such as olivine, garnet, and amphibole fall within the narrow range of 8.5 ± 1.5 Ra (Ra is the atmospheric 3He/ 4He ratio) with a maximum value of 11.5 Ra. This range is characteristic of the relatively homogeneous and degassed upper MORB-mantle helium reservoir. No helium isotope ratios typical of the lower less degassed mantle (>12 Ra), such as exemplified by the modern hot-spot region of Hawaii (with up to 32 Ra) were measured. Helium isotope ratios of less than 8 Ra are interpreted in terms of dilution of upper mantle helium with a radiogenic component, due to either age of crystallisation or small-scale mantle heterogeneities caused by mixing of crustal material into the upper mantle. The crude correlation between age of samples and helium isotopes with generally lower R/Ra values in mantle xenoliths compared with host rock phenocrysts and the in general depleted Nd and Sr isotope ratios and the light rare earth element enrichment of the basalts supports derivation of melts as small melt fractions from a depleted upper mantle, with posteruptive ingrowth of radiogenic helium as a function of lithospheric age. In comparison, the regional helium isotope survey of thermal and nonthermal gas discharges of the South Island of New Zealand shows that mantle 3He anomalies in general do not show an obvious relationship with either age or proximity to the Cenozoic intraplate volcanic centres or with major faults. In general, areas characterised by mantle 3He emission are interpreted to define those regions beneath which mantle melting and basalt magma addition to the crust are recent. The strongest mantle 3He anomaly (equivalent to >80% mantle helium component) is centred over southern Dunedin, measured in magmatic CO 2-rich mineral water springs issuing from crystalline basement rocks which outcrop at the southern extent of Miocene intraplate basaltic volcanism which ceased 9 Ma ago. This mantle helium anomaly overlaps with an area characterised by elevated surface high heat flow, compatible with a long-lived mantle melt/heat input into the crust. In comparison Banks Peninsula, another Miocene intraplate basaltic centre, is characterised by relatively low surface heat flow and a small mantle helium contribution measured in a nitrogen-rich spring. Here the thermal transient induced by the magmatic event has either dissipated or has not reached the surface. In the former case one might be dealing with storage and mixing of magmatic and crustal gases at shallow crustal levels and in the latter with active to recent mantle-melt degassing at depth. Along the most actively deforming part of the plate boundary zone, the transpressional Alpine Fault and Marlborough fault systems, mantle helium is present in gas-rich springs in all those areas underlain by actively subducting oceanic crust (the Australian plate in the south and Pacific plate in the north), whereas the central part of the Alpine transpressional fault is characterised by pure crustal radiogenic helium. Areas where the mantle helium component is negligible are restricted to the centre part of the South Island, extending along its length from Southland to northern Canterbury and Murchison. These areas are interpreted to delineate the extent of thicker and colder lithosphere compared to all other areas where mantle helium release from partial mantle melts at depth is recent to active being added to the lower lithosphere and/or lower crust. Areas characterised by mantle helium anomalies are equated with areas of thermal mantle anomalies, i.e., localised mantle heterogeneities such as upwelling less dense silicate melts in the upper asthenospheric mantle.

Presence of a primary solar-type atmosphere around the earth: evidence of dissolved noble gas

Recent noble gas data of mantle-derived samples show that there are two end members: PLUME-type and MORB-type. The estimated high 3He and 22Ne abundances of the PLUME source, possibly representing the lower mantle, should reflect the remnant of dissolved solar-type atmosphere. Calculations of the structure of the primary atmosphere and the noble gas dissolution into the magma ocean of the accreting planet suggest that the high 3He and 22Ne abundances can be explained if the primary atmosphere persisted until M∼0.4–0.6 M E ( M E being the present Earth mass). The PLUME source has higher 3He/ 4He and lower 21Ne/ 22Ne than the MORB source. This is explained by assuming that the lower mantle was less degassed during magma ocean cooling. The carbon abundance in the mantle can be constrained from the estimated abundance of mantle 3He and C/ 3He data of the present mantle-derived samples. Dissolved solar-type noble gas might explain high noble gas abundance in the present Venus, if the primary atmosphere persisted until the final stage of accretion under lower dust opacity of the atmosphere.

Mantle 3He in hydrothermal vents and plume of the Lucky Strike site (MAR 37°17′N) and associated geothermal heat flux

The helium isotopes composition of hydrothermal fluids at the Lucky Strike vent field has been determined by mass spectrometry. [ 4He] end-member concentration is in the 1.2 × 10 −5–2.6 × 10 −5 cm 3 STP/g range and the 3He/ 4He ratio, R, is 8.13 ± 0.05 × R a. The helium-3 budget over the active vent field was estimated from the 3He mapping of the plume advected along the rift valley, using a 1-D advection box-model constrained with current velocity measurements. This leads to a 3He flux of 2.3 × 10 −8 mol/s. The scaling of the heat flux from the Lucky Strike site to that of 3He, using the 3He/heat ratio measured in the hydrothermal fluids (5.2 × 10 −18–7.7 × 10 −18 mol/J) allows an initial evaluation of the total geothermal heat output from the site, in the 3800 ± 1200 MW range. This flux, of the same order as that of the Endeavour segment (Juan de Fuca Ridge), indicates the present high level of the hydrothermal activity in this section of the Mid-Atlantic Ridge.

Mantle-origin CO2 gas pools in petroliferous basins, Eastern China

Faulting and magma activities frequently occurred in Eastern China since Jurassic Period. This create favourable conditions for discharge of mantle-origin gas. Springs and volcanic lakes after Quaternary volcanic period in Wudalianchi volcanic area, Heilongjiang Province and Tianchi, Changbai Mountain, Jilin province, contain a large amount of CO2. CO2 content amounts to 77.80%-99.45%, and ~13Cco 2 value is --3.83 to --7.50, and 3He/aHe of gas seeps is (1.67 _+ 0.07) x 10-6~8.24 x l0 -6, namely R/ Ra is 1.19~5.89. According to gas flux statistics of 3 m 2 water area of Keyan Spring in Wudalianchi volcanic area, 279.3 m 3 gas is discharged daily. Volcanic rock and deep faults are generally distributed in juncture of Jiangxi Province, Guangdong Province. There are many gas seeps in hot spring in Zhegulong, Pingyuan, Guangdong Province and Xunwu, Jiangxi Province. Their CO2 content varies from 96.7% to 99.96%, and ~13Cco ~ value is --3.39 to --5.62%o, 3He/4He of gas seeps is (1.90--+0.06) x 10 -6 ~(3.09--+0.09) • l0 -6, namely R/Ra is 1.36-3.21. The above 813Cco2 is -3.39 to -7.50%o, which shows that CO2 is of mantle-origin; the R/Ra of 1.19~5.89 shows that helium gas contains a large amount of mantle-origined 3He. This shows that the discharge of mantle-origin CO2 and helium is widespread in volcanic area and outcrop area of igneous rock in Eastern China. The discharge of mantle-origin gas is also widespread in rift petroliferous basin of Eastern China. 24 CO2 gas pools(fields) has been discovered in Songliao Basin, Bohai Bay Basin, Northern Jiangsu Basin and Sanshui Basin, and East China Sea Basin, Zhujiangkou Basin. CO2 content of these gas pools is 72.5 to 99.76%, and CO2 content of most CO2 gas pools is over 92%, with also hydrocarbon gases(Cl_4). ~13Cco 2 value of gas pools is -3.47~-5.90%0, which suggest that CO2 is derived from mantle. 3He/gHe is (2.80_+0.08)• -6 (6.94_+0.20)• -6, namely R/Ra is 2.00~4.96. This shows that helium gas contains a large amount of mantle 3He. 11-56% of helium is derived from mantle helium. But the light 813C1 of CO2 gas pools ( 2, also associated with traps close to deep faults and intrusive rocks. The magnitude of mantle-origin CO2 flux is quite large, for example, COz reserves of Huangqiao Gas Field in the North Jiangsu Basin is estimated up to 624 x 108m 3.

Resolving the mantle He/Ne and crustal 21Ne/ 22Ne in well gases

He and Ne isotope data from 48 mantle-rich well gases ( 3He/ 4He>0.1 R a) in 6 different regions of crustal extension around the world are used to derive both the average mantle He/Ne elemental ratio beneath the extensional systems, and the regional average crustal 21Ne/ 22Ne nucleogenic production ratio, ( 21Ne/ 22Ne) crust. Assuming a mantle with R/ R a=6.0–8.0 R a, on a plot of measured 3He/ 4He vs. air-corrected 21Ne/ 22Ne, 44 of the 48 data points fall within error of a well- defined mixing line between mantle and crust isotopic end-member values. The mixing line requires a mantle 3He/ 22Ne range of 0.6–1.9 and is less than half that estimated for the upper mantle, assuming steady state and a Solar-like lower mantle [1, 2]. The derived ( 21Ne/ 22Ne) crust is 0.47–0.76. This is significantly smaller than the theoretical value of 3.5 [3]. The low ( 21Ne/ 22Ne) crust confirms that mineral scale preferential siting of U and Th with respect to O and F controls the regional ( 21Ne/ 22Ne) crust [4]. The difference between the derived well-gas mantle He/Ne and estimated upper-mantle values can be reconciled by a gas/melt solubility fractionation process. This requires that the gas/melt volume ratio is small (<0.001) in all of the rifting environments in order to produce the observed magnitude of He/Ne fractionation. These small gas/melt volume ratios can only be produced in the lithospheric mantle. Furthermore, this requires that the mantle He/Ne composition is similar to or less than models invoking a Solar-like upper mantle, with ( 3He/ 22Ne)≤4.4. These results are consistent with a model in which small melt fraction asthenospheric melts are emplaced into the subcontinental lithospheric mantle. These melts partially degas, to a similar magnitude in all regions, lose a maximum of ∼15% of their He content by gas/melt phase partitioning, and form the dominant source of mantle-derived volatiles in regions of crustal extension. During migration of the magmatic volatiles through the crust the fractionated mantle-derived gases mix with crustal radiogenic noble gases on a regional scale, at or near their elemental production ratios.

Multiple source components in gas manifestations from north-central Italy

Gas manifestations in north-central Italy consist of CO2-rich gases with minor N2 rich emissions and discharge either along with thermal springs or into cold and stagnant waters. ‘Cold’ gases are prevalently C02 -dominated (> 90%) while gases related to the thermal waters have variable composition: from CO2 > 99.5% to N2 > 90%. The variable composition of ‘thermal’ gases is caused by differences in the thermal regime and lithology of the ascent paths, where there is mixing of gases from multiple sources, such as N2 rich atmospheric and deep C02-rich metamorphic end-members. Elevated concentrations of CH4 and H2 in these gases are generally related to the presence of active geothermal systems at shallow depth, such as the Larderello-Travale field in Tuscany. The δ13C values between coexisting CH4 and CO2 in all samples analyzed suggest that CH4 originates abiogenically in 200–400°C hydrothermal systems. Far from geothermal areas, where the thermal gradient is lower or the water/gas ratio is high because of large inflow of meteoric waters, H2 and CH4 are usually lower. In some cases, they can be scrubbed or oxidized (especially H2), while the residual rising gas becomes indirectly enriched in N2 and C02. Carbon dioxide is also enriched in some discharged gases because it is produced at shallow depth in lower temperature conditions (<150°C). Heavy δ15N values for N2 to near + 7.0%o suggest that, for some gas samples that contain excess nitrogen (e.g. where N2/Ar >> 83), this component probably derives from ammonia-rich feldspars and micas within the Palaeozoic schist basement rocks. However, other samples show evidence of a shallow component of CH4 and N2 from Neogene basin sediments. The areal distribution of the 3He/4He ratio points to a general prevalence of atmospheric and crustal 4He in the gas discharges in central Italy. A significant component of mantle 3He is only found in the geothermal areas of Larderello where the large regional thermal anomaly suggests the presence of a deep magmatic body.

³He in Indonesian Seas: Inferences on deep pathways

Helium isotope measurements from the Indonesian Seas, and from the Java‐Australia Dynamic Experiment cruises define mantle ³He sources and deep water spreading patterns. ³He sources in the Sulawesi Sea and in the Makassar Strait are found, but these are prevented from spreading into the Banda Sea and the Indian Ocean by blocking sills. A major mantle source is identified below 2000m in the Flores Sea. The ³He distribution suggests that the Banda Sea water migrates westward along the floor of the Flores Sea, upwells and then advects the high ³He Flores signal. A tongue of higher‐than‐ambient ³He on the Indian Ocean side of the Flores chain indicates a deep outflow of the Flores and the Banda Sea water into the Indian Ocean near 1300m.

Constraints on mantle 3He fluxes and deep‐sea circulation from an oceanic general circulation model

We have simulated the steady‐state distribution of helium in the deep sea to investigate the magnitude and spatial and temporal variability of mantle degassing and to characterize deep‐sea circulation and ventilation. The simulation was produced by linking a simple source function for helium injected at mid‐ocean ridges with an oceanic general circulation model (GCM). By assuming that the flux of mantle helium is linearly proportional to the seafloor spreading rate and by using previous estimates for the total flux of mantle helium into the oceans, the GCM yields an oceanic 3He distribution which is in qualitative agreement with observations both in overall magnitude and in general distribution. This provides new evidence that the flux of mantle 3He into the oceans is about 1000 mol/yr and that mid‐ocean ridges are the dominant source of mantle helium. Although the match with observations is good in the Pacific and Indian Oceans, the simulated 3He anomalies throughout the Atlantic Ocean are much higher than has been measured. Because the GCM is thought to reproduce Atlantic circulation reasonably well, this discrepancy suggests an error in the helium source function. Either helium injection is not a linear function of seafloor emplacement rate, or eruption and concomitant degassing are highly episodic at the slow spreading rates characteristic of the Mid‐Atlantic Ridge (MAR). The latter explanation would imply minimal volcanic activity along the entire length of the MAR over the last few centuries. In addition to constraints on the degassing flux, our work provides information on the transport and ventilation of deep ocean waters and constrains the degree to which current GCMs can reproduce deep‐water circulation patterns. While the results generally support the GCM's abyssal circulation, our simulation reveals regions of overly‐intense lateral diffusion and upwelling in the model, particularly in the equatorial Pacific. Similarly, there appears to be insufficient production of He‐ventilated bottom waters in the model Antarctic. These observations suggest that further refinement of the GCM abyssal circulation is required.

Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: Evidence of crust/mantle interactions in a back-arc basin

We report helium and argon isotope analyses of fresh Lau Basin volcanic glasses from six sites within the central basin (~18°S), six localities along the Valu Fa Ridge (21°–22.4°S), and a single site from the northeastern basin (~ 15°S). Central basin basalts have 3He 4He ratios ( R) between 8.2–8.5 R A (R A = air 3He 4He ), 40Ar 36Ar ratios significantly greater than atmosphere (up to 4900), and 4He contents from 3.5–9.4 × 10 −6 cm 3 STP/g, similar to N-MORBs worldwide. These results are consistent with trace element and other (radiogenic) isotope data on these samples which indicate derivation from a depleted mantle source region. In contrast, evolved lavas from the Valu Fa Ridge all have R R A < MORB and show evidence of mixing with a component rich in radiogenic helium. There is a clear relationship between the 3He 4He ratios of these samples and their chemistry: basaltic andesites have 6.02 < R c R a < 7.65 , andesites have lower ratios (2.37−2.65 R A ), and a dacite has the lowest value of the entire sample suite (1.19 R A ). All lavas have 40Ar 36Ar ratios similar to the atmospheric value and low helium concentrations, from 3–11 × 10 −8 cm 3 STP/g, or between 30 and 300 times less than the central basin basalts. Although the helium isotope results of the Valu Fa lavas mirror the shift to more radiogenic values of other isotope systems (e.g., 206Pb 204Pb , 87Sr 86Sr ) which indicate addition of subducted sediment to these magma sources, we find no evidence that the radiogenic helium has a mantle or slab derivation or is in any way coupled to these other tracers. Instead, the most plausible mechanism to explain its incorporation into the Valu Fa lavas is by assimilation of old Lau crust in the near-surface environment by previously degassed magma. We argue that this mechanism has general applicability and can explain a number of hitherto apparently paradoxical geochemical features of some back-arc and ocean ridge lavas such as their high volatile and LIL element contents with low rare gas concentrations, and their mantle 3He 4He ratios with (hydrated oceanic) crustal D/H values. The realisation that the helium and argon systematics of the Valu Fa lavas are controlled by crust/mantle interactions has important implications for distinguishing between a number of models proposed for the formation of such evolved lavas, and we show that fractional crystallisation processes can most readily account for the low concentrations of, and systematic trends in, the mantle-derived helium and argon component of these lavas. In addition, because pre-existing crust in the Lau Basin must be old and/or altered enough to supply the radiogenic helium and atmospheric-like 40Ar 36Ar component to the Valu Fa lavas, the occurrence of crust/mantle interactions implies that old (forearc) crust may have been trapped within the Lau Basin: such a scenario has a clear bearing on ideas of the tectonic development of the basin. Finally, because of the potential of crust/mantle interactions to modify 3He 4He and 40Ar 36Ar ratios of mantle-derived melts, we assess the implications for using He and Ar tracers to characterise mantle sources in arcs, back-arcs, and spreading ridges, and consider the consequences for the combined use of rare gases with other (radiogenic) isotopic tracers of magma provenance at such settings. The basaltic andesite from the northeastern basin may also be influenced by the same kind of crustal interaction as the Valu Fa lavas as it falls within the 3He 4He range (6.9 R A ) of the other basaltic andesites. Interestingly, other helium isotope studies indicate that this part of the basin is characterised by a wide range in 3He 4He ratios, from MORB values up to 22 R A . The low 3He 4He ratio of the basaltic andesite, therefore, serves to illustrate the possible effects of magma chamber processes on the rare gas and other volatile characteristics of hotspot lavas: an observation which is important not only for this part of the Lau Basin but for other localities worldwide.

Variable 3He/heat ratios in submarine hydrothermal systems: evidence from two plumes over the Juan de Fuca ridge

The first vent fluid samples recovered from submarine hydrother-mal systems on the Galapagos Rift1 and at 21° N on the East Pacific Rise (EPR)2 had a nearly identical ratio of 3He/heat of ~0.5 x l0−12cm3STPcar−1, even though the two hydrothermal systems were separated geographically and had widely differing fluid exit temperatures (~20 and ~350 °C, respectively)3–5. Jenkins et al.3 combined this ratio with independent estimates of the flux of mantle 3He through the oceans6, to calculate a global oceanic hydrothermal heat flux of 4.9 x 1019cal yr−1, which is in excellent agreement with geophysical estimates for this flux7,8. Other investigators then combined this 3He flux with measured ratios of various chemicals in vent fluids to 3He (such as Mn/3He and Si/3He) to estimate global hydrothermal fluxes for these species1,9,10. Here we show that 3He/heat ratios vary by over an order of magnitude between submarine hydrothermal systems, suggesting that early measurements of the 3He/heat relation are not representative of all hydrothermal systems, and that flux calculations based on the oceanic 3He flux must be undertaken with caution.