Ratio Of He Research Articles

Evidence for an 18O-depleted mantle plume from contrasting 18O/ 16O ratios of back-arc lavas from the Manus Basin and Mariana Trough

Back-arc basin glasses from the Mariana Trough and Manus Basin display contrasting oxygen isotope characteristics that require differences in their mantle sources. In both basins, the lavas that are most depleted in high field strength elements possess δ 18O values of around 6.0‰, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the more fertile mantle component differs between the two basins. The lowest δ 18O values in the Mariana Trough are similar to MORB and suggest that the ambient upper mantle interacts with a subduction-modified mantle to produce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basaltic glasses from the Manus Basin display a negative correlation with helium isotope ratios. The subduction-modified component is associated with 3He/ 4He ratios typical of the upper mantle. Glasses with 3He/ 4He ratios greater than average MORB, characteristic of a deep mantle plume, have δ 18O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is 18O-depleted relative to the source of MORB. Two mechanisms are identified that might generate this reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large 18O-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate and iron alloy during the segregation of the Earth’s core may have the potential to generate such reservoirs. Resolution between these mechanisms requires further experimental investigation of oxygen partitioning between silicates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume.

Mapping out the conduit of the Iceland mantle plume with helium isotopes

In order to investigate the lateral dimensions and the shallow dispersion of the Iceland mantle plume we have measured the 3He/ 4He ratios in closely spaced primitive (Mg#: 59–70) basaltic subglacial pillow lavas erupted along the Northern Rift Zone (NRZ) of Iceland. This detailed sampling has revealed for the first time a clear gradient in 3He/ 4He ratios along the NRZ, increasing from 8.0 times atmospheric ( R/ R a) in the northernmost samples to 20.4 R/ R a in central Iceland. In combination with previously published 3He/ 4He data, the new results from subglacial basaltic glasses from the neovolcanic rift zones of Iceland reveal a ‘plateau’ of high values ( 3He/ 4He ∼20 R/ R a), which is approximately 100 km in diameter and located over the southeastern part of the island. The high 3He/ 4He plateau is well-defined and coincides with a regional gravity minimum and a seismic low velocity anomaly within the upper mantle beneath Iceland. We suggest that this zone of elevated 3He/ 4He ratios (>20 R/ R a) outlines the width of the Iceland plume conduit at depth. 3He/ 4He ratios are well correlated with Nb/Th and Nb/Rb, but not well correlated with other isotope ratios (as indicated by Pb isotopes). The correlations between helium isotopes, the geophysical variations, and some extremely incompatible elements, such as Rb and Th, and the lack of correlation between He and Pb isotopes, are most likely due to preferential concentration of He into volatile-rich incipient melts, and the subsequent lateral dispersion of the plume at shallower levels.

沈み込んだ炭素と背弧海盆におけるその挙動

Based on δ 13C values and CO2/ 3He ratios of North Fiji Back-Arc Basin basalt glasses, wediscuss the carbon geochemical cycle in the subduction zone. Among the North Fiji Back-ArcBasin basalt glasses, there is a close correlation among CO2/ 3He ratios, δ 13C value, and143Nd/ 144Nd ratios. The CO2/ 3He ratios and the δ 13C values of North Fiji Basin basalt maybe attributed to binary mixing between the mantle component (low-CO2/ 3He, high-δ 13C, andhigh-143Nd/ 144Nd) and the subducted (recycled) component (high-δO2/ 3He, low-δ13C, and low-143Nd/ 144Nd). From a simple mass balance calculation, it is derived that the subductedend-member (recycled carbon) has 70% carbonate and 30% organic matter in origin.Assuming that complete decomposition of the subducted organic matters has occurred, most (about 90%) carbonates are not decomposed, because the amounts of subducting carbonatesand organic matters throughout the North Fiji subduction zone are estimated in a ratio of20: 1. This suggests that carbonate can be transported into the mantle through the subduction zones.

Dynamics of the Galapagos hotspot from helium isotope geochemistry

We have measured the isotopes of He, Sr, Nd and Pb in a number of lava flows from the Galapagos Archipelago; the main goal is to use magmatic helium as a tracer of plume influence in the western volcanoes. Because the Galapagos lava flows are so well preserved, it is also possible to measure surface exposure ages using in situ cosmic-ray-produced 3He. The exposure ages range from <0.1 to 580 Ka, are consistent with other chronological constraints, and provide the first direct dating of these lava flows. The new age data demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compositions simultaneously for the last ∼10 Ka. The magmatic 3He/ 4He ratios range from 6.9 to 27 times atmospheric (R a), with the highest values found on the islands of Isabela (16.8 R a for Vulcan Sierra Negra) and Fernandina (23 to 27 R a). Values from Santa Cruz are close to typical mid-ocean ridge basalt values (MORB, of ∼9 R a) and Pinta has a 3He/ 4He ratio lower than MORB (6.9 R a). These data confirm that the plume is centered beneath Fernandina which is the most active volcano in the archipelago and is at the leading edge of plate motion. The data are consistent with previous isotopic studies, confirming extensive contributions from depleted asthenospheric or lithospheric mantle sources, especially to the central islands. The most striking aspect of the helium isotopic data is that the 3He/ 4He ratios decrease systematically in all directions from Fernandina. This spatial variability is assumed to reflect the contribution of the purest plume component to Fernandina magmatism, and shows that helium is a sensitive indicator of plume influence. The highest 3He/ 4He ratios are found in volcanoes with lowest Na 2O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between 3He/ 4He and Nb/La, suggesting that the Galapagos plume source is characterized by high concentrations of Nb (and Ta). The major and trace element correlations demonstrate that helium is controlled by silicate melting and source variations rather than degassing and/or metasomatic processes. Although lavas with radiogenic isotopic compositions tend to have higher 3He/ 4He, the island-wide isotopic variability cannot be explained by simple two components mixing alone. The preferred model to explain the isotopic data includes a heterogeneous plume, centered at Fernandina, which undergoes polybaric melting, and spatial divergence and mixing with asthenospheric material at shallower depths. The unique regional pattern of the helium isotopic data suggests that helium is extracted more efficiently than other elements during the early stages of melting in the ascending plume.

Helium isotopes in historical lavas from Mount Vesuvius: Comment on ‘Noble gas isotopic ratios from historical lavas and fumaroles at Mount Vesuvius’ by D. Tedesco et al.

Helium isotope results recently published by Tedesco et al. [1] appear to show a decrease in 3He/4He since 1631 AD at Mount Vesuvius. The authors proposed that this change is the result of a residual magma with a fractionated (low) 3He/ 4He ratio, produced by preferential degassing of 3He during historical time. If such a process generally occurred in nature, it would seriously hinder our ability to characterize the 3He/4He ratio of mantle source regions. In proposing their model, Tedesco et al. [1] disregarded the previously published data of Graham et al. [2] which showed no change in 3He/4He with time at Vesuvius and implied that diierences between the two studies result from diierent methods of gas extraction. In response, we have reanalyzed several of the critical samples by fusion. These new results are in complete agreement with our original analyses performed by crushing [2] and are completely at odds with any measurable change in the 3He/4He ratio of Vesuvius lavas since 1631 AD. 2. Results

Volatiles (He, C, N, Ar) in mid-ocean ridge basalts: assesment of shallow-level fractionation and characterization of source composition

To document the characteristics of volatiles in the terrestrial mantle, the abundances and the isotopic ratios of carbon, nitrogen, helium, and argon have been analyzed in 45 mid-ocean ridge basalts (MORB) glasses from the Mid-Atlantic Ridge between 24°N and 36°N, the East Pacific Rise at 21°N, 13°N, and 17–19°S, the Red Sea (18–20°N), the Indian Ocean near the Triple Junction, and the central North Fiji basin. Gases were extracted by crushing and subsequently were split for purification and analysis. Static mass spectrometry was used for N, He, and Ar, and conventional dynamic mass spectrometry was used for C. The data confirm the occurrence of near-constant He isotope ratios ( 3He/ 4He = 8.53 ± 0.79 Ra; n = 36) and C isotope ratios (δ 13C = −5.2 ± 0.7‰ vs. PDB; n = 21), and of a light nitrogen component in the convective mantle. Overall, the δ 15N signature of MORB (mean δ 15N = −3.3 ± 1.0‰ vs. ATM, for samples with 40Ar/ 36Ar > 1000; n = 19) does not drastically differ from that of diamonds, some of which being presumably ≥3 Ga, and is thought to represent the nitrogen isotopic ratio of the asthenospheric mantle. A common mantle source for diamond-bearing magmas and MORB magmas is unlikely, and this similarity may imply either active exchange of volatiles between the respective mantle sources or homogeneous distribution of nitrogen isotopes in these sources during most of Earth’s history. Variations of the 4He/ 40Ar∗ ratios as well as 40Ar/ 36Ar ratios are consistent with fractional crystallization–assimilation–degassing taking place in the depth range of 3 to 6 km. The corrected average C/N of the MORB mantle is 535 ± 224, significantly higher than potential cosmochemical and geochemical end-members. C/He and C/N ratios corrected for fractional crystallization–assimilation–degassing fractionation increase with the degree of MORB enrichment (e.g., increasing K 2O/TiO 2) and are thought to reflect carbon heterogeneities in the mantle source, as a result of fractional recycling of a (fluid?) component extremely enriched in carbon (C/N > 3000). The N/ 3He ratios are less variable than the C/ 3He ratios, suggesting limited recycling of nitrogen. Such limited recycling is required to preserve a N isotope ratio in the convective mantle distinct from that of the atmosphere–hydrosphere sediments. Overall, neon, argon, nitrogen, and carbon abundances in the mantle appear to be chondritic, rather than solar, although the neon isotopic signature of the mantle indicates contribution of a solar component. This apparent discrepancy may reflect mixing between a solar-type component mostly seen at present in light rare gases and a chondritic-type component and has strong implications for the origin of terrestrial matter and the processes of their accretion.

Extreme 3He/ 4He ratios in northwest Iceland: constraining the common component in mantle plumes

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. 3He/ 4He ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently ˜37 R A (R A = air 3He/ 4He) — significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189–218]). The Sr, Nd and Pb isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517–520]) — in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275–24300]). This observation implies that Selardalur may represent the best estimate available to date of the He–Sr–Nd–Pb isotopic composition of the 5th mantle component common to many mantle plumes.

An active subcontinental mantle volatile system in the western Eger rift, Central Europe: gas flux, isotopic (He, C, and N) and compositional fingerprints

The composition and flux of gas emanations, and the isotopic ratios of CO 2, He and N 2 of 74 mineral springs and dry gas vents (mofettes) in the western Eger rift (Czech Republic) have been analyzed. Four geochemically similar, but tectonically separate, gas escape centers are distinguishable, out of which 3 show a free gas flux >85000 dm 3 h −1. All gases from the centers are CO 2-rich (>99 vol.%) and have δ 13C values ranging from −1.8 to −4.0‰. 3He/ 4He ratios are as high as R/R a = 5, and are among the highest measured in Europe. The discharge of the gas mixture decreases with distance from the emanation centers with both decreasing fractions of CO 2 and δ 13C values, whereas the fractions of N 2 and trace gases increase. These changes in chemical and isotopic composition are associated by a decrease in R/R a ratios from about 5 in the centers to <2 in the peripheries. The changes of the contents and isotopic composition of CO 2 can be explained by physico-chemical fractionations of CO 2 between gaseous and aqueous phases. Towards the periphery, the contents of free CO 2 and its δ 13C are reduced by dissolution of CO 2 in groundwater, whereby the content of N 2 increases. 3He/ 4He ratios give evidence for mixing of He from both a deep-seated magmatic and a crustal source. The gas emanation centers, with their strongly magmatic δ 13C value of about −2.7‰, seem to outline the intersections of the Eger rift and the Mariánské Lázně fault, which are considered to represent a deep-reaching fracture system that enables the ascent of gases from a magmatic body in the European subcontinental mantle (SCM). Therefore, the European SCM is suspected to be the main source of CO 2. The most mantle-like He (and probably N 2) occurs in the centers of gas release. The total regional gas flux in the western Eger rift is determined to be 3.6 × 10 8 mol a −1. When related to the investigated area of 1500 km 2, flux densities greater than 0.24 × 10 6, 52, and 0.65 mol km −2 a −1 for CO 2, N 2 and He respectively are calculated.

Separation of groundwater-flow components in a karstified aquifer using environmental tracers

Groundwater discharges from the intensively karstified Taurus Mountains to the Mediterranean Sea, either along the contact zone between the mountains and the Travertine Plateau (the Kirkgozler Springs, 15 m 3/s), or through the travertine (e.g. the Dudenbasi Spring, 18 m 3/s) and underneath it (unnamed submarine springs, unknown discharges). In an attempt to identify the hydraulic connections between the various outlet points, groundwater was analyzed for stable and radioactive isotopes, CFCs and He. The upgradient springs, belonging to the Kirkgozler–Dudenbasi system, were proven to be a mixture of recent and older water on the basis of their low 14C values (12–22.4 pmc), their exceptionally high He content (429–991 μcc/kg) and 3He: 4He (R:Ra) ratios (1.471–2.602) and their measurable 3H and CFC contents (1.9–5.9 TU and 0.84 to 3.27 pmoles/kg, respectively). The older component probably contains an even lower amount of modern C. However, the undersaturation of the mixture with respect to calcite, its high CO 2 content (up to 83 mg/L) and its enriched 13C values (−2.2 to −4.1‰) suggest intensive water/rock interactions, which would contribute 14C-devoid bicarbonates to the solution. Downgradient springs discharging along the Mediterranean coast contain groundwater contributions from higher altitudes, as evidenced by their depleted δ 18O and δD composition with respect to the local precipitation; however, a larger portion of the recent water component could be contributed from direct precipitation on the travertine. This larger component is reflected in the increased 3H (3.4 to 8.4 TU) and 14C (32.7–63.6 pmc) contents, atmospheric He (43–82 pmoles/kg), R:Ra values (1.006–1.198) and CFC contamination of the water.

Noble gas behaviour and composition in the mantle: constraints from the Iceland Plume

Current geochemical models envisage that noble gases in the upper mantle source region of Mid-Ocean Ridge Basalts (MORB) arise from three sources; an elementally unfractionated component, introduced by bulk transfer from the deeper mantle via plumes; a radiogenic/nucleogenic component arising from in-situ radiogenic decay of U, Th and K, and accumulated within the upper mantle over a residence time of approximately 1.4 Ga; a subducted atmospheric component, possibly restricted to the heavy noble gases. Our new data from Icelandic sub-glacial basaltic glasses have the highest 20Ne/ 22Ne so far reported in terrestrial samples, indistinguishable from the solar wind value of 13.8. Inferred 4He/ 3He, 21Ne/ 22Ne and 40Ar/ 36Ar ratios in the source are much lower than MORB values, reflecting the presence of a deep mantle component. Measured 22Ne/ 3He ratios are higher than upper mantle values, but correlate with 21Ne/ 4He, implying a recent localised enrichment of Ne in the magma. The implied 22Ne/ 3He ratio prior to this enrichment is identical to MORB estimates. This enrichment and the parallel enrichment in Ne/He appears to require a multi stage process during partial melting, in the magma chamber, and/or during eruption which fortuitously left the ratio of mantle He to Ar and Xe unchanged. 36Ar/ 3He ratios, estimated for the source region, are significantly higher than MORB values implying the introduction of atmospheric 36Ar to the magma at some stage. The samples exhibit only a small 129Xe excess, however, the implied 129Xe */ 3He is indistinguishable from the MORB value. Overall the elemental and isotopic ratios in the Iceland samples support mantle models which predict similar primordial noble gas ratios in the MORB and OIB source regions.

Origin of trace gases in submarine hydrothermal vents of the Kolbeinsey Ridge, north Iceland

Two hydrothermal fields of the Kolbeinsey Ridge area, north of Iceland, show vent gas characteristics which can be related to the subsurface conditions. Helium isotopes ( R/R air = 9.8, 10.9) indicate a mantle-derived origin and can be considered as a mixture of MORB helium and a deep-mantle plume helium component. The carbon isotope composition of CO 2 ranges between −2.4 and −7.8‰. The less negative δ 13C-CO 2 values were found at Grimsey. The data from Grimsey are very similar to those previously published and regarded as being characteristic for the Icelandic magmatic source. However, small amounts of biogenic CO 2 and/or subsurface calcite precipitation are responsible for the lighter isotope values of CO 2 from Kolbeinsey. CH 4/ 3He ratios which are higher than in MORB indicate an additional (sedimentary) methane source for Kolbeinsey and Grimsey hydrothermal gases. The presence of higher hydrocarbons up to butane, together with the carbon isotope values of methane (δ 13C = −26.1 to −39.8‰) suggest a probably high-mature organic source within thick sediments of the Tjörnes Fracture Zone and smaller depressions on the west side of the Kolbeinsey Ridge crest. Geochemical characteristics of hydrocarbons present in KR hydrothermal fluids are, however, typical for a mixed (thermogenic and high-temperature hydrothermal, e.g. EPR-type) origin. Moreover, it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases.

Volatile element isotopic systematics of the Rodrigues Triple Junction Indian Ocean MORB: implications for mantle heterogeneity

Mid-ocean ridge basalts (MORBs) from the Indian Ocean have been well-known to differ in Sr–Nd–Pb isotopic composition from those of the North Atlantic and the East Pacific. To understand if such heterogeneity exists also in volatile elements, we have measured elemental and isotopic composition of carbon, nitrogen, helium, and argon in trapped-gas-vesicles of the Rodrigues Triple Junction (RTJ) Indian Ocean MORBs (IOMs). The observed C–N–He–Ar isotopic ratios of the IOMs generally agree with average values of North Atlantic MORBs (NAMs) and East Pacific MORBs (EPMs). In contrast, the N 2/ 40Ar, C/N, and CO 2/ 3He ratios in fluid inclusions of the IOMs are quite unusual: current estimates of the N 2/ 40Ar, C/N, and CO 2/ 3He trapped in vesicles from the NAMs and EPMs are 91±20, 215∼4600, and (1.5±0.5)×10 9, respectively, whereas those of the IOMs are 120±36, 26∼190, and (0.3±0.1)×10 9, respectively. The 40Ar/ 36Ar and the 4He/ 40Ar * ratios indicate that the unusual N 2/ 40Ar, C/N, and CO 2/ 3He ratios of IOMs in this study have not been produced by atmospheric contamination or progressive fractional degassing, respectively, but more likely by mantle heterogeneity. The IOMs analyzed in this study seem to be depleted in the recycled carbon than the previous reported NAMs and EPMs. Assuming that values of IOMs in this study as those of the primitive (juvenile) mantle carbon, it is derived that most (65∼95%) carbon in the previously reported NAMs and EPMs is the recycled carbon (carbonate and organic matter) in origin, from a simple mass balance calculation. This may be one clue of the carbon recycling in the mantle.

Radiogenic helium isotope fractionation: the role of tritium as 3He precursor in geochemical applications

Reduced 4He/ 3He ratios, e.g., down to ≈1/100 times those expected from radiogenic production, were observed in sedimentary rocks. Formation and history of these rocks eliminate a contribution of mantle 3He-bearing fluid. To explain the difference between the observed and the calculated production 4He/ 3He ratios Loosli et al. (1995) and Tolstikhin et al. (1996) suggested a different behaviour of helium and tritium in damage tracks produced by emission of these nuclides. Generally, the tracks cross grain boundaries or some imperfections within a rock or mineral allowing a fast loss of noble 4He and 3He atoms. However, radiogenic 3He has the precursor 3H, generated in the exothermic 6Li(n t, α) 3H + 4.5 MeV reaction. The energetic tritons produce damage tracks comparable with those from α-decay of U and Th series. If 3H is chemically bound within a track, and the track is able to recover via some diagenetic process before the 3H decay, then 3H and daughter 3He atoms are trapped within the recovered track. This mechanism would explain the shorter residence time of 4He in the rocks/minerals than of 3He; therefore, 4He/ 3He ratios could decrease through time. To check this mechanism 4He, 3H, and 3He (from 3H-decay) were produced by the above reaction in special targets, consisting of layered composites of thin sections of quartz, sample, Li-bearing cover, sample, and quartz. The samples were the same rocks in which reduced 4He/ 3He ratios have been previously observed. Each target was placed in a quartz ampoule, which was then pumped out, sealed off, and then exposed to the flux of thermal neutrons in a reactor. After irradiation and cooling down (total duration 145 days), the nuclides produced during ( 3H, 3He, 4He) and after ( 3He) irradiation were measured in the gas phase above the targets and compared with their total quantities expected from the Li abundance and the integrated neutron flux. The ratios obtained were 3H(gas)/ 3H(total) < 0.05 and 3He(gas)/ 3He(total) varying from 0.2 to 0.9. The average residence times τ of 3H and 3He, respectively, were estimated to be ≈16 and ≈0.25 yr for this first time interval, which included the irradiation of the targets. After these first measurements, the targets were kept in a vacuum system under room temperature for 210 days and the amounts of 3H and 3He, which accumulated above the targets during this second time interval under fully controlled conditions, were also measured. Much slower rates of gas loss from the same targets with average residence times of τ( 3H) ≈ 600 yr and τ(He) ≈ 1.6 yr resulted for this second time interval. Probably these longer residence times are closer to those in the relevant natural environments, the 3H residence time being much longer than the 3H half-life. In all cases the inequality τ( 3He) ≪ τ( 3H) is valid. This confirms the proposed scenario envisaging longer retention of 3H than He in damage tracks. Within the frame of this scenario the life-time of 3H gives a time constraint on diagenetic processes; at least one to several newly formed atomic layers should appear during ∼10 yr to recover the tracks.

Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/ 3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from São Miguel and Terceira. The eastern part of São Miguel island displays radiogenic helium ( 4He/ 3He > 140,000, R/ R a<5.1) and lead (20.00, 15.75 and 40.33 for 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb). The 207Pb/ 204Pb and 208Pb/ 204Pb ratios for São Miguel are unusually radiogenic for oceanic basalts. Terceira basalts contain relatively unradiogenic/primitive 4He/ 3He ratios, with a minimum value of 64,000 ( R/ R a=11.3), and relatively high lead isotopic ratios ( 206Pb/ 204Pb = 20.02, 207Pb/ 204Pb = 15.64 and 208Pb/ 204Pb = 39.35). We propose that the Terceira source has a composition produced by a mixing between recycled oceanic crust (high 206Pb/ 204Pb) and entrained lower mantle (high 3He) material. The São Miguel island isotopic signature may be due to sampling of local (km-size) heterogeneity located at relatively shallow depth. The preferred origin of this heterogeneity is the Jurassic delamination of subcontinental lithosphere, which occurred during rifting and opening of the North Atlantic. The primitive helium ratios were also observed on the Mid Atlantic ridge at 38.5°N, reflecting plume–ridge interaction, whereas radiogenic ratios (>100,000) were observed at latitude higher than 40°N and may reflect the influence of the São Miguel component at the ridge.

Noble gas study of HIMU and EM ocean island basalts in the Polynesian region

Noble gas isotopes of HIMU and EM ocean island basalts from the Cook-Austral and Society Islands were investigated to constrain their origins. Separated olivine and clinopyroxene (cpx) phenocrysts were used for noble gas analyses. Since samples are relatively old, obtained from the oceanic area and showing chemical zoning in cpx phenocrysts, several tests on sample preparation and gas extraction methods were performed. First, by comparing heating and crushing methods, it has been confirmed that the crushing method is suitable to obtain inherent magmatic noble gases without radiogenic and cosmogenic components which were yielded after eruption, especially for He and Ne analyses. Second, noble gas compositions in the core and the rim of cpx phenocrysts were measured to evaluate the zoning effect on noble gases. The result has been that noble gas concentrations and He and Ne isotope ratios are different between them. The enrichment of noble gases in the rim compared to the core is probably due to fractional crystallization. Difference of He and Ne isotope ratios is explained by cosmogenic effect, and isotope ratios of the trapped component seem to be similar between the rim and the core. Third, leaching test reveals no systematic differences in noble gas compositions between leached and unleached samples. 3He/ 4He ratios of HIMU samples in the Cook-Austral Islands are uniform irrespective of phenocryst type (olivine and cpx) and age of samples (10–18 Ma), and lower (average 6.8 R A) than those of the Pacific MORB. On the other hand, 3He/ 4He of EM samples in the Cook-Austral Islands are similar to MORB values. EM samples in the Society Islands show rather higher 3He/ 4He than MORB. Ne, Kr and Xe isotope ratios are almost atmospheric within analytical uncertainties. 40Ar/ 36Ar are not so high as those of MORB. Anomalous noble gas abundance pattern such as He and Ne depletion and Kr and Xe enrichment relative to atmospheric abundances was observed. Furthermore, Ne/Ar and Kr/Ar show correlation with some trace elemental ratios like La/Yb. Lower 3He/ 4He of HIMU than MORB values requires relatively high time-integrated (U + Th)/ 3He for the HIMU source, which suggests that the HIMU source was produced from recycled materials which had been once located near the Earth’s surface. Moreover, extreme noble gas abundance pattern and strong correlation of Ne/Ar and Kr/Ar with La/Yb indicate that the HIMU endmember is highly depleted in light noble gases and enriched in heavy noble gases. Such feature is not common to mantle materials and is rather similar to the noble gas abundance patterns of the old oceanic crust and sediment, which supports the model that the HIMU source originates from subducted oceanic crust and/or sediment. If the HIMU source corresponds to the oceanic crust which subducted at 1–2 Ga as suggested by Pb isotope studies, however, the characteristic 3He/ 4He of HIMU (6.8 R A) would be too high because radiogenic 4He produced by U and Th decay should dramatically decrease 3He/ 4He. To overcome this problem, the He open system model is introduced which includes the effects of 4He production and diffusion between the HIMU source material and the surrounding mantle. This model favors that the HIMU source resides in the upper mantle, rather than in the lower mantle. Furthermore, this model predicts the thickness of the HIMU source to be in the order of 1 km. In contrast to low and uniform 3He/ 4He character of HIMU, 3He/ 4He of EM are rather variable. Entrainment of upper mantle material and/or a less-degassed component are required to explain the observed 3He/ 4He of EM in the Polynesian area. Participation of the less-degassed component would be related to the “superplume” below the Polynesian region.

Hotspot–ridge interaction along the Southeast Indian Ridge near Amsterdam and St. Paul islands: helium isotope evidence

We report new helium isotope analyses for basaltic glasses recovered along the Southeast Indian Ridge (SEIR) between longitudes 77°E and 88°E. In this region, the SEIR shoals to a depth less than 1800 m as it crosses the Amsterdam–St. Paul (ASP) plateau. Atop the plateau, where the average sample spacing is ∼10 km, 3He/ 4He ratios range between 9.3 and 13.4 R A ( R A = atmospheric ratio) with significant variability over short distances. Away from the plateau, ridge segments show a more restricted 3He/ 4He range, between 7 and 9 R A. Elevated 3He/ 4He ratios on the plateau are evidence for a deep mantle plume component that has been injected into the sub-ridge mantle beneath the region. Ridge segments southeast of the plateau show lower 3He/ 4He (7.6–8.3 R A) than segments to the northwest (8.3–9.0 R A), suggesting either a systematically varying `background contamination' of the MORB asthenosphere in the region, or localized input of a `low 3He/ 4He' component along the southeastern ridge segments, possibly derived from the distant Kerguelen hotspot. A double peak structure exists in the regional 3He/ 4He spatial distribution, because there is also a strong plume-derived He isotope signal which is offset from the shallowest, thickest section of the ridge. High 3He/ 4He ratios (up to 14.1 R A) are present all along the ridge segment immediately to the northwest of the ASP plateau (segment H), where anomalies in K/Ti ratio are also observed. The high 3He/ 4He ratios persist to the northern end of segment H, beyond which they abruptly drop to MORB background levels, which apparently persist all the way to the Rodrigues Triple Junction. This pattern suggests that the high- 3He/ 4He ASP plume supplies a shallow north-northeastward mantle flow toward adjacent portions of the SEIR. There is a systematic difference in 3He/ 4He–K/Ti behavior between axial lavas from the ASP plateau and those from segment H, suggesting a higher He/Ti elemental abundance ratio in the plume source material beneath segment H compared to that beneath the ridge axis on the ASP plateau. These observations are consistent with a model in which plume material supplied to segment H is derived from the outer portions of the ASP plume, where removal of He (relative to Ti) has been less efficient than within the core of the plume located beneath the Amsterdam and St. Paul islands. These outer portions of the plume have lower temperatures and have undergone less partial melting, leading to a higher relative He/Ti ratio compared to the inner portions of the plume that feed portions of the sub-ridge mantle beneath the ASP plateau.

He, Ne, Ar, and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < 3He/ 4He < 8.6 Rc/Ra) and carbon (−20 < δ 13C PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ 13C PDB and 3He/CO 2 ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO 2 en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The 3He/ 4He signature of summit fumaroles, thought to reflect the 3He/ 4He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the 3He/ 4He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature ( 3He/ 4He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower 3He/ 4He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low 3He/ 4He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low 3He/ 4He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments. The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.

Helium isotopes in lithospheric mantle: evidence from tertiary basalts of the western USA

The isotopic compositions of He, Sr, and Nd were measured in Tertiary-age basalts from the Basin and Range province of the western USA to evaluate models for the He isotopic character of subcontinental mantle lithosphere (SCML) and assess the role of recycled SCML in models of mantle evolution. Previous isotopic and trace element measurements suggested that most of these basalts were formed by melting of SCML. 3He/ 4He ratios, measured by in-vacuo crushing of olivine phenocrysts, vary from 2.9 to 7.8 times the atmospheric value (2.9 to 7.8 Ra) consistently below the MORB value of 8.7 ± 0.5 Ra. The lowest R/Ra values, associated with low ε Nd, high 87Sr/ 86Sr, and high La/Nb, are attributable to lithospheric mantle, and indicate that SCML is not dominated by MORB-type He, nor by high R/Ra, plume-type He. Consideration of geographic variability indicates there are two, and possibly three, distinct regions of SCML with differing He isotopic characteristics. SCML beneath the eastern Sierra Nevada is inferred to have 3He/ 4He of ∼5.5 Ra and a He/Nd ratio slightly less than MORB-type mantle; SCML beneath the central Basin and Range has 3He/ 4He of ∼4 Ra and a higher He/Nd ratio than MORB-type mantle. The SCML under southwestern Utah shows less systematic correlation of He isotopes with other geochemical parameters, but also has a lower bound R/Ra value of about 4 Ra. The inferred SCML helium ratios are consistent with retention of radiogenic 4He over 800 Ma for the eastern Sierra Nevada and 1700 Ma for the other two regions. The results are not consistent with models of He infiltration from the underlying asthenosphere and suggest the lithosphere of the Basin and Range region was not delaminated during the early Tertiary. The He, Sr, Nd, and Pb isotopic compositions inferred for the SCML of the southwestern USA are a reasonably good match to the characteristics of the EMII component of mantle heterogeneity identified in oceanic island basalts. High R/Ra mantle reservoirs identified in these basalts are not likely to represent recycled SCML.

Noble gas isotopic ratios from historical lavas and fumaroles at Mount Vesuvius (southern Italy): constraints for current and future volcanic activity

Helium, neon and argon isotope ratios have been analysed from phenocrysts of eleven lava samples belonging to the last eruptive cycle of Mount Vesuvius (1631 until 1944). The phenocrysts separates include pyroxene ( N=10) and olivine ( N=1). All phenocryst samples show similarly low gas contents (He, Ne and Ar ∼10 −10 cm 3/g). 3He/ 4He ratios, 5.3–2.11 R a, are generally low if compared to those typical of the MORB and those of the European Subcontinental Mantle (ESCM), respectively R/ R a 8.5±1 and 6.0–6.5. A decreasing trend is found from 1631 to 1796, while a more homogeneous set of data is obtained for more recent eruptions, as evidenced by an average R/ R a value of 2.85. Neon ratios ( 21Ne/ 22Ne and 20Ne/ 22Ne) strongly differ from those typically found on volcanoes and suggest that a crustal component has been added in the source region to Mt. Vesuvius magmas. Argon ratios ( 40Ar/ 36Ar and 38Ar/ 36Ar) have values similar to the atmosphere and are well correlated. The low 40Ar/ 36Ar ratio (max. 302) is, however, in the range of the 40Ar/ 36Ar ratios obtained from several lava samples at other Italian volcanoes and might be considered to have a deep origin. Two hypothesis have been discussed: (1) a deep argon-like-air source, due to subduction of air-rich sediments and/or (2) a preferential loss of Ar, in comparison to lighter noble gases, from silicic melts. Helium isotopic analysis of gas samples recently collected from crater and submarine fumaroles are similar to those of lavas belonging to the final part of this eruptive cycle. This result supports the idea that no new juvenile fluids from the source region have been injected into the magmatic reservoir during the 1631–1944 eruptive cycle and, more importantly, until 1993. Both sets of data help to understand the genesis of these fluids and to constrain the current activity of the volcano.

Plume-derived rare gases in 380 Ma carbonatites from the Kola region (Russia) and the argon isotopic composition in the deep mantle

In an effort to document the source of the parental melts to carbonatites, we have measured rare gases in 380 Ma carbonatites and associated mineral assemblages from the Kola Peninsula, eastern part of the Baltic shield in Russia. These series were emplaced during widespread Devonian magmatism when several large ultrabasic–alkaline–carbonatite massifs were formed. 4He/ 3He ratios vary from 1×10 6 to 1×10 7 in the bulk He extracted by melting of samples from three localities, including the large Kovdor massif. A comparison of measured abundances of 3He and 4He with those expected from in-situ production revealed a large (up to 10 5 times) excess of 3He, implying a significant contribution from a mantle-derived 3He-bearing fluid. Crushing of these samples allowed extraction of fluids with 4He/ 3He ratios down to 38,000, lower than those of mid-ocean ridge basalts and in the range of 4He/ 3He observed in 3He-rich ocean island basalts (OIBs) related to mantle plumes. 20Ne/ 22Ne up to 12.1±0.2 are higher than the atmospheric value of 9.80, implying the occurrence of primordial (solar-type) neon in the carbonatite source. 20Ne/ 22Ne and 21Ne/ 22Ne ratios display a good correlation, with the regression line close to (slightly to the right of) the Loihi Seamount correlation. Extrapolation of the regression to solar 20Ne/ 22Ne of 13.8 gives a 21Ne/ 22Ne of 0.045 for the plume end-member, well below the mid-ocean ridge basalt (MORB) source (upper mantle) end-member of 0.07. The measured 40Ar/ 36Ar ratios up to 2790 correlate very well with the Ne isotopic ratios, and the best estimate of the 40Ar/ 36Ar ratio of the plume source is within 5000±1000. Although the 3He/ 22Ne ratio in the plume source appears to be comparable to the solar value within a factor of 2, the 22Ne/ 36Ar ratio, computed from Ne–Ar isotope correlation, is two orders of magnitude lower than the solar value. Such difference is unlikely to be due to magmatic fractionation since the observed 4He/ 40Ar * ratios are close to values expected for radiogenic production and accumulation in the mantle source. It may rather represent a characteristic of the plume source. The isotope composition of light noble gases in samples from ultrabasic–alkaline rocks of the Kola Peninsula, and associated carbonatites, indicate a contribution of material with lower time-integrated (U + Th)/( 3He, 22Ne) and ( 40K/ 36Ar) ratios than those in the asthenospheric upper mantle, the subcontinental lithosphere, and the continental crust. The location of such material is likely to be below the convective mantle supplying MORB magmas, and reflects the contribution of a plume source material to Kola carbonatitic magmatism. These data support models which advocate a structure of the Earth heterogeneous in its refractory/volatile content.