High 3He Research Articles

Dynamic changes in hydrogeochemical conditions caused by tunnel excavation at the Aspo Hard Rock Laboratory (HRL), Sweden

Hydraulic changes caused by tunneling at the Aspo Hard Rock Laboratory (HRL) in Sweden have been investigated over a period of 2a using different hydrochemical approaches, i.e. noble gas content, isotopic measurements and major ion concentrations. The dissolved noble gases ( 4He and Ne contents, and the ratio of 3He/ 4He, 40Ar/ 36Ar), stable isotopes, chemical concentrations of major ions, and 36Cl/Cl ratios, were determined in groundwater samples collected in the tunnel from borehole sections isolated by inflated packers. Groundwater was categorized into 3 groups based on 4He and Cl − contents: undisturbed groundwater (i.e. prior to tunnel construction) with high 4He and Cl − contents, groundwater that has been gradually changed by mixing with Baltic seawater and whose 4He and Cl − contents have gradually increased with increasing depth, and groundwater that has been totally changed due to a rapid mixing of Baltic seawater and/or shallow groundwater and whose 4He and Cl − contents are extremely low compared with other samples collected at the same surrounding depth. The oldest groundwater with a high salinity of more than 14,000 mg l −1 of Cl − is estimated to be more than 1.8 Ma old. The groundwater residence time ranges from 0.9 to 900 Ka in the mixing-zone. Groundwater in the disturbed zone where rapid mixing has occurred is hard to date reliably and its primary hydrochemical character has already been lost.

3He dimers in two-dimensional 3He– 4He mixture films

Ground state properties of the two-dimensional 3He– 4He mixture are studied at zero temperature. We find that at low concentrations 3He atoms dimerize and form bosonic mixture with 4He. The binding energy of the dimer ranges from 10 −3 K near the saturation density to 10 −6 K at the solidification density. The radius of such a weakly bound dimer is tens of Ångströms. A stable atomic mixture is found at concentration larger than ≈5% varying with density. At high 3He concentrations the mixture phase separates and critical concentrations are obtained by calculating the second derivative of the enthalpy.

Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): implications for HIMU mantle sources

We report a comprehensive study of He isotope ( 3He/ 4He) variations on well-characterized lavas from La Palma, the current locus of activity of the Canary hotspot. Cogenetic olivine (OL) and clinopyroxene (CPX) phenocrysts from 11 basaltic lava flows representing all stages of the island’s evolution were analyzed by crushing in vacuo. Additionally, the sample set is supplemented by a CO 2-rich bubbling cold spring in Taburiente caldera. Phenocryst 3He/ 4He ratios vary between 6.3 to 8.9R A; both extremes occur in the 2 to 0.6 Ma shield-building Taburiente lavas. Historic flows vary between 7.0 to 7.8R A, whereas phenocrysts from a single submarine basement sill (4.0–2.9 Ma) have 3He/ 4He ratios of 8.3R A (OL) and 8.4R A (CPX). He isotopic equilibrium characterizes all the phenocryst pairs except one Taburiente lava. The absence of a correlation between 3He/ 4He and [He] contrasts with other ocean island basalts, e.g., Heard Island (Hilton et al., 1995) and, together with equilibrium in phenocrystic 3He/ 4He, suggests that the La Palma sample suite has 3He/ 4He ratios that have suffered minimal crustal contamination. The cold spring (geothermal) sample gives a 3He/ 4He ratio of 9.5R A, which is higher than any of the phenocrysts and in excellent agreement with a previously reported value for Taburiente caldera (Perez et al., 1996). The geothermal 3He/ 4He values indicate that La Palma is the first HIMU-like ocean island (where HIMU implies a distinctive mantle source with high time-integrated 238U/ 204Pb (μ) ratio) with reported 3He/ 4He ratios higher than the canonical value (8 ± 1R A) characteristic of depleted mid-ocean ridge basalt (MORB) mantle. We present two simple mixing scenarios that are compatible with the He–Pb isotope systematics of La Palma: a) a high- 3He mantle plume, extensively but variably depleted in its original He content, mixes with a HIMU source characterized by both radiogenic He and lead isotope compositions; and b) mixing between a composite plume, with high 3He/ 4He and radiogenic lead, and depleted mid-ocean ridge basalt mantle. The latter model is favored as it appears to account more readily for the mid-ocean ridge MORB-like He isotope characteristics of the lavas. Addition of radiogenic He—most likely produced in mantle melts frozen into the sub-Canarian mantle lithosphere—to resultant mixtures may subsequently occur in either model. Such a process can explain the absence of correlations between He and Pb isotope ratios in the lavas, and the lower 3He/ 4He values of the phenocrysts with respect to the geothermal fluids. We find no need to invoke a radiogenic growth/diffusion model (Hanyu and Kaneoka, 1998) to explain the He–Pb isotope relationships for La Palma. These mixing models may also be applicable to other HIMU islands: however, any high- 3He plume signature of St. Helena and Mangaia may be obscured due to the high proportion of the HIMU component at these localities and/or by addition of radiogenic He.

High ³He/4He ratio in xenoliths from Takashima: Evidence for plume type volcanism in southwestern Japan

Cenozoic basalts distributed along the southwestern Japan arc system have geochemical features different from typical island‐arc basalts, suggesting that the volcanism might have been triggered by a mantle plume. To identify the source materials in these volcanics, noble gas isotopic compositions of olivines separated from dunite xenoliths in alkaline basalts from Takashima, northwestern Kyushu, were measured. Most samples contain MORB‐type helium. However, some samples showed ³He/4He ratios significantly higher than the MORB value, from 9.4 up to 16.6 times the atmospheric ratio of 1.4×10−6. This finding strongly supports the contribution of a mantle plume to the volcanism of this region.

Tritium and stable isotopes of magmatic waters

To investigate the isotopic composition and age of water in volcanic gases and magmas, we analyzed samples from 11 active volcanoes ranging in composition from tholeiitic basalt to rhyolite: Mount St. Helens (USA), Kilauea (USA), Pacaya (Guatemala), Galeras (Colombia), Satsuma Iwo-Jima (Japan), Sierra Negra and Alcedo (Ecuador), Vulcano (Italy), Parı́cutin (Mexico), Kudryavy (Russia), and White Island (New Zealand). Tritium at relatively low levels (0.1–5T.U.) is found in most emissions from high-temperature volcanic fumaroles sampled, even at discharge temperatures >700°C. Although magmatic fluids sampled from these emissions usually contain high CO2, Stotal, HCl, HF, B, Br, 3He R/RA, and low contents of air components, stable isotope and tritium relations of nearly all such fluids show mixing of magmatic volatiles with relatively young meteoric water (model ages≤75y). Linear δD/δ18O and 3H/δ18O mixing trends of these two end-members are invariably detected at arc volcanoes. Tritium is also detected in fumarole condensates at hot spot basalt volcanoes, but collecting samples approaching the composition of end-member magmatic fluid is exceedingly difficult. In situ production of 3H, mostly from spontaneous fission of 238U in magmas is calculated to be <0.001T.U., except for the most evolved compositions (high U, Th, and Li and low H2O contents). These values are below the detection limit of 3H by conventional analytical techniques (about 0.01T.U. at best). We found no conclusive evidence that natural fusion in the Earth produces anomalous amounts of detectable 3H (>0.05T.U.).

Helium desorption from cavities induced by high energy 3He and 4He implantation in silicon

A detailed study has been made of helium release from silicon wafers implanted with MeV helium ions at fluences of 5×10 16 cm −2 and 10 17 cm −2. Thermal desorption spectrometry (TDS), neutron depth profiling (NDP), non-Rutherford elastic backscattering (NREBS) and nuclear reaction analysis (NRA) have been employed to measure the helium content and release rate during isothermal annealing at annealing temperatures of 800 and 1000°C. TDS has also been used for isochronal annealing. Transmission electron microscopy (TEM) is used to monitor changes in morphology in the formed bubble layer. The helium release results can be modeled rather well when it is assumed that the helium initially is present in overpressurized bubbles. The present study reveals a single activation energy for helium release of 1.83 (0.05) eV.

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.

Large volume recycling of oceanic lithosphere over short time scales: geochemical constraints from the Caribbean Large Igneous Province

Oceanic flood basalts are poorly understood, short-term expressions of highly increased heat flux and mass flow within the convecting mantle. The uniqueness of the Caribbean Large Igneous Province (CLIP, 92–74 Ma) with respect to other Cretaceous oceanic plateaus is its extensive sub-aerial exposures, providing an excellent basis to investigate the temporal and compositional relationships within a starting plume head. We present major element, trace element and initial Sr–Nd–Pb isotope composition of 40 extrusive rocks from the Caribbean Plateau, including onland sections in Costa Rica, Colombia and Curaçao as well as DSDP Sites in the Central Caribbean. Even though the lavas were erupted over an area of ∼3×10 6 km 2, the majority have strikingly uniform incompatible element patterns (La/Yb=0.96±0.16, n=64 out of 79 samples, 2σ) and initial Nd–Pb isotopic compositions (e.g. 143Nd/ 144Nd in=0.51291±3, ϵ Ndi=7.3±0.6, 206Pb/ 204Pb in=18.86±0.12, n=54 out of 66, 2σ). Lavas with endmember compositions have only been sampled at the DSDP Sites, Gorgona Island (Colombia) and the 65–60 Ma accreted Quepos and Osa igneous complexes (Costa Rica) of the subsequent hotspot track. Despite the relatively uniform composition of most lavas, linear correlations exist between isotope ratios and between isotope and highly incompatible trace element ratios. The Sr–Nd–Pb isotope and trace element signatures of the chemically enriched lavas are compatible with derivation from recycled oceanic crust, while the depleted lavas are derived from a highly residual source. This source could represent either oceanic lithospheric mantle left after ocean crust formation or gabbros with interlayered ultramafic cumulates of the lower oceanic crust. High 3He/ 4He in olivines of enriched picrites at Quepos are ∼12 times higher than the atmospheric ratio suggesting that the enriched component may have once resided in the lower mantle. Evaluation of the Sm–Nd and U–Pb isotope systematics on isochron diagrams suggests that the age of separation of enriched and depleted components from the depleted MORB source mantle could have been ≤500 Ma before CLIP formation and interpreted to reflect the recycling time of the CLIP source. Mantle plume heads may provide a mechanism for transporting large volumes of possibly young recycled oceanic lithosphere residing in the lower mantle back into the shallow MORB source mantle.

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.

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.

Anomalous helium and heat signatures associated with the 1998 Axial Volcano Event, Juan de Fuca Ridge

Water‐column plumes sampled before and after the 1998 eruptive event on Axial Volcano, Juan de Fuca Ridge, showed a complex pattern in ³He and heat very different from that observed during previous magma injection events. Plumes detected in 1997 before the event had relatively high ³He/heat consistent with the volatile‐rich character of the known hydrothermal vents within the summit caldera. A response cruise in February 1998 immediately after the event failed to detect any event plumes, but instead found an intense steady‐state type plume with high‐temperature signals, moderate ³He enrichments, and low ³He/heat ratios. This plume was present over the caldera and extended a considerable distance off axis. In contrast, all of the plumes detected during follow‐up cruises in July–September 1998 had moderate temperature signals, high ³He concentrations, and elevated ³He/heat ratios. Comparison with previous events suggests that the low ³He/heat plume sampled in February was either the remnant of an event plume which had already migrated off axis, or an event plume‐like discharge which was smeared out by strong currents. It is likely that the high ³He/heat plumes observed in July–September were derived from the injected dike or from the new lava flow.

Geochemical observations and one layer mantle convection

Rare gas systematics are at the heart of the discrepancy between geophysical and geochemical models. Since more and more robust evidence of whole mantle convection comes from seismic tomography and geoid modeling, the interpretation of high 3He/ 4He in some oceanic island basalts as being primitive has to be revisited. A time dependent model with five reservoirs (bulk mantle, continental crust, atmosphere, residual deep mantle and D″) is studied for Rb/Sr and U/Pb/He systems. The dynamics of this model correspond to whole mantle convection in which subducted oceanic crust, transformed into dense assemblages, partially segregates to form a D″ layer growing with time as in Christensen and Hofmann [J. Geophys. Res. 99 (1994) 19867–19884]. A complementary cold and depleted harzburgitic lithosphere remains above D″. We assume that hotspots arise from the deep thermal boundary layer and tap, in variable proportions, material from both the residual deep mantle and D″. The difference between HIMU and Hawaiian basalts is attributed to HIMU being mostly from strongly degassed oceanic crust, though enriched in incompatibles (D″), while Hawaii is mostly from MORB source residuals that are variably degassed and depleted. We suggest that a significant part of the Earth’s radioactive elements (∼1/3) is trapped in the D″ layer.

A Tale of Two Planets

Most of Earth's volcanism occurs at plate boundaries—mid-ocean ridges and island arcs. Most of the 3He and other noble gases also are expelled from the interior at these locations. A small amount, compared to the global budget, of magma (<10%) and 3He («1%) escapes at mid-plate volcanoes, the socalled “hotspots.” Locations of hotspots are controlled by stresses and cracks, not by deep, narrow mantle upwellings. Most, or all, of Earth's volatile inventory appears to have been brought in by a late veneer. The high 3He content and high He/Ne and He/Ar ratios of mid-ocean ridge basalts, plus the excess 129Xe and 136Xe, suggest that the ridge basalt reservoir is relatively undegassed and that there is no primordial undegassed reservoir. The two prevailing views of the origin of Earth are summarized in the context of this information.

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.

Depth profiling of light elements using a nuclear microprobe

In this paper, we present some examples of depth profiling of light elements with a nuclear microprobe performed at LARN during the last decade. Some new possibilities of ion beam microanalysis of light elements with our 2 MV Tandetron accelerator are also discussed.The first example of application consists of depth profiling of nitrogen and aluminium on a SiAl alloy implanted with nitrogen. The nuclear microprobe was used to determine three-dimensional distribution of aluminium, silicon and nitrogen in a specific grain of the implanted alloy. The nitrogen depth profile was measured using the well known 15N(p,αγ)12C nuclear resonant reaction at 429 keV. The aluminium depth profile was measured with the resonant nuclear reaction 27Al(p,γ)28Si at 991.8 keV.Depth profiling of carbon and oxygen is also possible using nuclear reactions induced by 3He particles. Nuclear reactions like 12C(3He,pi)14N (i=0,1,2) or 16O(3He,α0)15O were used to measure local wear tracks on a diamond coating after a fretting test against a Cr steel ball.PIXE microprobe and nuclear reactions induced by deuterons were also used to characterise the gold–silicon alloy formed by the diffusion of silicon into gold foils. The nuclear reaction 28Si(d,p)29Si in a transmission geometry was used in order to depth profile silicon especially in the grain boundaries of the gold–silicon alloy.Some new perspectives of depth profiling light elements are also presented using our new 2 MV Tandetron accelerator, such as high energy 4He microbeams for depth profiling of carbon or nitrogen.

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.

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.

4He as a tracer of continental dust: a 1.9 million year record of aeolian flux to the west equatorial Pacific Ocean

The mass accumulation rates (MARs) of aeolian dust in the ocean basins provide an important record of climate in the continental source regions of atmospheric dust and of the prevailing wind patterns responsible for dust transport in the geologic past. The incorporation of other terrigenous components such as volcanic ashes in seafloor sediments, however, often obscures the aeolian dust record. We describe a new approach which uses the delivery rate of crustal 4He to seafloor sediments as a proxy for the mass accumulation rate of old continental dust which is unaffected by the addition of other terrigenous components. We have determined the flux of crustal 4He delivered to the seafloor of the Ontong Java Plateau (OJP) in the western equatorial Pacific over the last 1.9 Myrs. Crustal 4He fluxes vary between 7.7 and 30 ncc cm −2 kyr −1 and show excellent correlation with global climate as recorded by oxygen isotopes, with high crustal 4He fluxes associated with glacial periods over the entire interval studied. Furthermore, the onset of strong 100 kyr glacial–interglacial climate cycling is clearly seen in the 4He flux record about 700 kyrs ago. These data record variations in the supply of Asian dust in response to climate driven changes in the aridity of the Asian dust sources, consistent with earlier work on Asian dust flux to the northern Pacific Ocean. However, in contrast to previous studies of sites in the central and eastern equatorial Pacific Ocean, there is no evidence that the Inter Tropical Convergence Zone (an effective rainfall barrier to the southward transport of northern hemisphere dust across the equator in the central and eastern Pacific) has influenced the delivery of Asian dust to the OJP. The most likely carrier phase for crustal helium in these sediments is zircon, which can reasonably account for all the 4He observed in the samples. As a first order estimate, these results suggest that the mass accumulation rate of Asian dust on the OJP over the last 1.9 Myrs varied from about 4 to 15 mg cm −2 kyr −1. In contrast, previous studies show that over the same interval the total MAR of terrigenous dust (i.e. Asian dust plus local volcanics) on OJP varied between about 34 and 90 mg cm −2 kyr −1.

Extraterrestrial 3He in seafloor sediments: evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust, orbital parameters, and Quaternary climate

We have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the Ontong Java Plateau, western equatorial Pacific Ocean (ODP Site 806). These results provide a two million year record of the burial flux of extraterrestrial 3He, which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles. The 3He burial flux prior to ∼700 ka was relatively low, ∼0.5 pcc cm −2 kyr −1, but from 700 ka to the present, the burial flux gradually increased to a value of ∼1.0 pcc cm −2 kyr −1. 100 kyr periodicity in the 3He burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate, with high 3He burial fluxes associated with interglacial periods. This periodicity and phase are consistent with previous 3He measurements in North Atlantic sediments. Although 100 kyr periodicity in 3He burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris, the measurements and predictions differ by one half cycle in phase. Nevertheless, our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and/or inclination relative to the solar-system invariable plane. Such control is a necessary but not sufficient condition for the hypothesis of Muller and MacDonald (1995) that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period. We also identify several brief (<25 kyr) intervals of strongly enhanced 3He burial, possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles.

Diffuse emission of carbon dioxide, methane, and helium‐3 from Teide Volcano, Tenerife, Canary Islands

Diffuse emission of CO2, CH4 and ³He was investigated in the summit crater of Teide volcano, Tenerife, Canary Islands. The results indicate that Teide volcano releases abundant CO2 not only from its active crater, but also from its flanks as diffuse soil emanations. The spatial distribution of these emanations correlates quite closely with that of geothermal anomalies and manifestations. Our flux measurements indicate a total output of 380 ton·day−1 of CO2 and 1.6 on·day−1 of CH4 by diffuse degassing. From the ³He/CO2 ratios and the CO2 flux we estimate a diffuse ³He flux of 0.052 mol·year−1. Isotopic analysis of carbon in soil gas samples from sites of high soil CO2 flux indicates a magmatic origin for the CO2, also reported by high ³He/ 4He ratios of 5.50 and 5.87 Ra. The origin of methane is more uncertain with low δ13C values suggesting a biogenic contribution.