High 3He Research Articles

Real-time drill mud gas logging at the USDP-4 drilling, Unzen volcano, Japan

During the Unzen conduit drilling project USDP-4, the gas phase dissolved in the drill mud was continuously analyzed. Starting from the volcano's north flank an almost complete gas profile was achieved to the final depth of 1995.75 m in July 2004. Limitations were given due to the extremely difficult drilling conditions. The highly fractured rock formation led to loss of drill mud circulation in the shallow parts of the drill hole. Significant fluid inflow horizons did not occur above 800 m (drill string length). Starting from that depth on, invading fluids were detected with the real-time gas monitoring system. Major variations in the mud gas composition occurred only below a depth of 1000 m. Of major importance are fluid inflow zones with high 3He/ 4He at depths of 1555 m (7.3 R A), 1755.5 m (7.48 R A) and 1977.4 m (6.21 R A). These values indicate a significant influence of fluids with mantle signature. Furthermore, enhanced methane, radon and helium concentrations were also detected at specific depth. These detected major fluid- and gas inflow horizons may be able to explain magmatic degassing processes, related to the Unzen's eruption mechanism. This is generally true for the main fluid inflow zones and especially for the detected inflows at 1555 m, 1755.5 m and 1977.4 m. Furthermore, a correlation between lithology and gas composition was observed. Higher H 2S concentrations were detected while drilling in pyrite-rich rocks. Cracks and fissures as well as lithological changes are often correlated with increasing amounts of gas. Trends with depth, from a minor to a more magmatic influenced regime were observed together with a change in hydrothermal alteration of the surrounding rock. This corresponds with the magmatic conduit zone which was penetrated at a depth of 1600 m, and supports the model of a high influence of hydrothermal fluid, accelerating cooling and mineral alteration.

The evolution of He Isotopes in the convecting mantle and the preservation of high 3He/ 4He ratios

A key requirement for any model of mantle evolution is accounting for the high 3He/ 4He ratios of many ocean island basalts compared to those of mid-ocean ridge basalts. The early, popular paradigm of primitive, undegassed mantle stored in a convectively isolated lower mantle is incompatible with geophysical constraints that imply whole mantle convection. Thus it has been suggested more recently that domains with high 3He/U ratios have been created continuously from the bulk mantle throughout Earth history. Such models require that the 3He/ 4He ratio of the convecting mantle was at least as high as the highest values seen in OIB at the time the OIB source was generated. These domains must also be created with sufficient He to impart distinctive He isotopic signatures to ocean island basalts. However, the He isotope evolution of the mantle has not been consistently quantified to determine if such scenarios are plausible. Here a simple model of the He evolution of the whole mantle is examined. Using a wide range of possible histories of continental extraction and He degassing, the bulk convecting mantle was found to have had 3He/ 4He ratios as high as those seen in the Iceland hotspot only prior to 3 Ga. Such high 3He/ 4He ratios can only be preserved if located in domains that are not modified by convective mixing or diffusive homogenisation since that time. Further, there are difficulties in producing, with commonly invoked magmatic processes, domains with sufficiently high 3He/U ratios and enough 3He to be able to impart this signature to ocean island basalts. The results are consistent with models that store such He signatures in the core or a deep layer in the mantle, but are hard to reconcile with models that continuously generate high 3He/ 4He domains within the mantle.

New Samoan lavas from Ofu Island reveal a hemispherically heterogeneous high 3He/ 4He mantle

New measurements of high 3He/ 4He ratios in Samoan lavas from Ofu Island (19.5–33.8 times atmospheric) extend the known range for 3He/ 4He in the southern hemisphere mantle. The Ofu data suggest that the high 3He/ 4He mantle component thought to be common to all oceanic hotspots, called FOZO ( Focus Zone), is not homogeneous. Sr, Nd and Pb isotopes in Ofu lavas indicate that the Samoan high 3He/ 4He component is isotopically distinct from the high 3He/ 4He lavas from Hawaii, Iceland and Galapagos. Along with Samoa, the highest 3He/ 4He sample from each southern hemisphere high 3He/ 4He hotspot exhibits lower 143Nd/ 144Nd ratios than their counterparts in the northern hemisphere (excluding lavas erupted in continental, back-arc, and submarine ridge environments). The observation of a large-scale isotopic enrichment (generally higher 87Sr/ 86Sr and lower 143Nd/ 144Nd) in the FOZO-A (austral) high 3He/ 4He mantle compared to the FOZO-B (boreal) high 3He/ 4He mantle is similar to the DUPAL anomaly, a globe-encircling feature of isotopic enrichment observed primarily in southern hemisphere ocean island basalts. Additionally, the recent discovery that terrestrial samples have 142Nd/ 144Nd ratios higher than chrondrites has potentially important implications for the origin of the FOZO reservoirs, and suggest that the high 3He/ 4He mantle has been re-enriched.

Thermochronology of non-volcanic hydrothermal activity in the Kii Peninsula, Southwest Japan: Evidence from fission track dating and helium isotopes in paleo-hydrothermal fluids

Although there is no indication of volcanism during the Pliocene and Quaternary in the Kii Peninsula, Southwest Japan, the peninsula has long been recognized to be unusual and atypical of non-volcanic regions, with anomalous high-temperature hot springs and high 3He content in hot spring gases. Recently, it has been suggested that aqueous fluids generated by dehydration of the subducting Philippine Sea plate carry mantle helium from the subcrustal lithosphere to terrestrial water, resulting in the emanation of high 3He gases at high-temperature hot springs [e.g., Matsumoto T., Kawabata T., Matsuda J., Yamamoto K. and Mimura K. 2003. 3He/ 4He ratios in well gases in the Kinki district, SW Japan surface appearance of slab-derived fluids in a non-volcanic area in Kii Peninsula. Earth Planet. Sci. Lett. 216, 221–230]. Fission track dating of zircon and apatite samples from the altered host rocks around three hot springs (Yunomine Hot Spring, Kamiyu Hot Spring and Tosenji Hot spring) was carried out in order to elucidate the thermochronology of the hydrothermal system in this non-volcanic region. Additionally, new helium isotope data from paleo-fluids trapped in fluid inclusions in hydrothermal minerals were obtained for comparison with the present-day hydrothermal fluids from the above hot springs. The apatite FT ages from the altered host rocks around the hot springs ranged from 2.5 Ma to about 5 Ma, which are significantly younger than the 12 Ma age of the non-altered host rocks far from the hot springs. In the southern Kii Peninsula, therefore, it is obvious that the non-volcanic hydrothermal activity already has been ongoing since Pliocene or early Pleistocene time. The 3He/ 4He ratios of quartz veins and pyrite disseminated in the altered host rock around the hot springs ranged from 1.1 to 3.6 R A , generally in agreement with those of the present-day hydrothermal fluids. Almost constant values of 3He/ 4He ratios over time indicate that no important changes have occurred in deep source gases, at least during the last several million years.

Formation processes of thermal waters in Green Tuff: A geochemical study in the Hokuriku district, central Japan

Thermal waters from Green Tuff region in the Hokuriku district, central Japan were investigated to reveal formation process of water, based on the chemical and isotopic compositions of water and dissolved gases in water. The thermal waters of local meteoric origin are typical Green Tuff-type water, Na–Ca–Cl–SO 4-type or Na–Cl–SO 4-type, which well reacted by water–rock interaction and is found to be saturated with calcite and undersaturated with gypsum. The Green Tuff-type waters have relatively high 3He/ 4He ratio indicating high contribution of mantle-derived helium, and have extremely low total carbon/ 3He (Ct/ 3He) ratio compared with the ratio of MORB due to removal of carbon species from waters caused by calcite deposition. In contrast, a mineral spring water close to an active fault is found to be an immature water with less water–rock interaction containing a large amount of gaseous CO 2 and is undersaturated with both calcite and gypsum. This water has high 3He/ 4He ratio and Ct/ 3He ratio indicating the high contribution of mantle-derived helium and deep-seated CO 2, and likely provided through the fault. We proposed a chemical evolution model considering existence of both chemically immature CO 2 gas-rich water and mature carbon-poor water which well reacted with rocks. Deep-seated CO 2 and mantle-derived helium are upwelling via faults and are supplied to the groundwater of local meteoric origin, and an immature water containing high content of CO 2 and mantle-derived helium is formed. During the circulation of this immature water in the Green Tuff layers, water–rock interaction such as ion exchanges with minerals, dissolution of gypsum and deposition of calcite occurred. At the interaction with rocks, deposition of calcite occurs and results in a loss of Ca 2+, and decrease of Ca 2+ brings dissolution of gypsum and more calcite deposition. Consequently, almost all the supplied CO 2 is eliminated from the water. Furthermore, we made a quantitative evaluation of the chemical evolution of Green Tuff-type water using geochemical modeling code PHREEQC taking account of addition of gas components, and found the good agreement with observational results. This study reveals that the supply of large amounts of CO 2 and subsequent water–rock interaction are necessary for the formation of “Green Tuff-type” water with high 3He/ 4He ratio and extremely low Ct/ 3He ratio.

186Os and 187Os enrichments and high- 3He/ 4He sources in the Earth’s mantle: Evidence from Icelandic picrites

Picrites from the neovolcanic zones in Iceland display a range in 187Os/ 188Os from 0.1297 to 0.1381 (γ Os = + 2.1 to +8.7) and uniform 186Os/ 188Os of 0.1198375 ± 32 (2 σ). The value for 186Os/ 188Os is within uncertainty of the present-day value for the primitive upper mantle of 0.1198398 ± 16. These Os isotope systematics are best explained by ancient recycled crust or melt enrichment in the mantle source region. If so, then the coupled enrichments displayed in 186Os/ 188Os and 187Os/ 188Os from lavas of other plume systems must result from an independent process, the most viable candidate at present remains core–mantle interaction. While some plumes with high 3He/ 4He, such as Hawaii, appear to have been subjected to detectable addition of Os (and possibly He) from the outer core, others such as Iceland do not. A positive correlation between 187Os/ 188Os and 3He/ 4He from 9.6 to 19 Ra in Iceland picrites is best modeled as mixtures of 1 Ga or older ancient recycled crust mixed with primitive mantle or incompletely degassed depleted mantle isolated since 1–1.5 Ga, which preserves the high 3He/ 4He of the depleted mantle at the time. These mixtures create a hybrid source region that subsequently mixes with the present-day convecting MORB mantle during ascent and melting. This multistage mixing scenario requires convective isolation in the deep mantle for hundreds of million years or more to maintain these compositionally distinct hybrid sources. The 3He/ 4He of lavas derived from the Iceland plume changed over time, from a maximum of 50 Ra at 60 Ma, to approximately 25–27 Ra at present. The changes are coupled with distinct compositional gaps between the different aged lavas when 3He/ 4He is plotted versus various geochemical parameters such as 143Nd/ 144Nd and La/Sm. These relationships can be interpreted as an increase in the proportion of ancient recycled crust in the upwelling plume over this time period. The positive correlation between 187Os/ 188Os and 3He/ 4He demonstrates that the Iceland lava He isotopic compositions do not result from simple melt depletion histories and consequent removal of U and Th in their mantle sources. Instead their He isotopic compositions reflect mixtures of heterogeneous materials formed at different times with different U and Th concentrations. This hybridization is likely prevalent in all ocean island lavas derived from deep mantle sources.

Noble gas compositions of the lithospheric mantle below the Chyulu Hills volcanic field, Kenya

We performed a noble gas isotope study on mantle xenoliths from the Quaternary volcanic field of Chyulu Hills, S Kenya, c. 100 km east of the Kenya rift graben. 4He/ 3He ratios were in the range of 100 000–120 000 and indistinguishable from other subcontinental lithospheric mantle compositions worldwide and similar to most results of the East African rift system. 21Ne/ 22Ne ratios corrected for atmospheric contributions displayed less nucleogenic values compared to mid ocean ridge basalts. This requires addition of Ne from a more primitive mantle source, that could be assigned to the postulated Kenyan mantle ‘plume’ or alternatively might be attributed to the ‘high 3He/ 4He’ Afar hotspot c. 2000 km distant. Whereas He and Ne isotopes reflect a rather homogeneous source composition, we observe a variable air corrected Ar isotopic composition. This might mirror additional contributions of atmospheric fluids strongly enriched in Ar. Nevertheless, in the least contaminated xenolith we were able to calculate a maximum 40Ar/ 36Ar ratio of 34,800 ± 500 assuming a mantle 20Ne/ 22Ne = 12.5. This is regarded as best approximation of the real 40Ar/ 36Ar ratio of the investigated mantle source. The same sample showed the highest observed 129Xe/ 130Xe and 136Xe/ 130Xe ratios of 7.01 ± 0.12 and 2.37 ± 0.05, respectively. The correlation of 129Xe/ 130Xe– 136Xe/ 130Xe ratios is in broad agreement with mantle Xe in OIB and MORB sources. Within uncertainties a minor contribution of crustally derived xenon is possible, though not compelling.

He, Ne and Ar isotopic composition of Fe-Mn crusts from the western and central Pacific Ocean and implications for their genesis

The noble gas nuclide abundances and isotopic ratios of the upmost layer of Fe-Mn crusts from the western and central Pacific Ocean have been determined. The results indicate that the He and Ar nuclide abundances and isotopic ratios can be classified into two types: low 3He/4He type and high 3He/4He type. The low 3He/4He type is characterized by high 4He abundances of 191×10−9 cm3·STP·g−1 on average, with variable 4He, 20Ne and 40Ar abundances in the range (42.8−421)×10−9 cm3·STP·g−1, (5.40−141)×10−9 cm3·STP·g−1, and (773−10976)×10−9 cm3·STP·g−1, respectively. The high 3He/4He samples are characterized by low 4He abundances of 11.7×10−9 cm3·STP·g−1 on average, with 4He, 20Ne and 40Ar abundances in the range of (7.57−17.4)×10−9 cm3·STP·g−1, (10.4−25.5)×10−1 cm3·STP·g−1 and (5354−9050)×10−9 cm3·STP·g−1, respectively. The low 3He/4He samples have 3He/4He ratios (with R/RA ratios of 2.04−2.92) which are lower than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (447−543) which are higher than those of air (295.5). The high 3He/4He samples have 3He/4He ratios (with R/R A ratios of 10.4−12.0) slightly higher than those of MORB (R/R A=8±1) and 40Ar/36Ar ratios (293−299) very similar to those of air (295.5). The Ne isotopic ratios (20Ne/22Ne and 21Ne/22Ne ratios of 10.3−10.9 and 0.02774−0.03039, respectively) and the 38Ar/36Ar ratios (0.1886−0.1963) have narrow ranges which are very similar to those of air (the 20Ne/22Ne, 21Ne/22Ne, 38Ar/36Ar ratios of 9.80, 0.029 and 0.187, respectively), and cannot be differentiated into different groups. The noble gas nuclide abundances and isotopic ratios, together with their regional variability, suggest that the noble gases in the Fe-Mn crusts originate primarily from the lower mantle. The low 3He/4He type and high 3He/4He type samples have noble gas characteristics similar to those of HIMU (High U/Pb Mantle)-and EM (Enriched Mantle)-type mantle material, respectively. The low 3He/4He type samples with HIMU-type noble gas isotopic ratios occur in the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain and the Mid-Pacific Seamounts whereas the high 3He/4He type samples with EM-type noble gas isotopic ratios occur in the Line Island Chain. This difference in noble gas characteristics of these crust types implies that the Magellan Seamounts, Marcus-Wake Seamounts, Marshall Island Chain, and the Mid-Pacific Seamounts originated from HIMU-type lower mantle material whereas the Line Island Chain originated from EM-type lower mantle material. This finding is consistent with variations in the Pb-isotope and trace element signatures in the seamount lavas. Differences in the mantle source may therefore be responsible for variations in the noble gas abundances and isotopic ratios in the Fe-Mn crusts. Mantle degassing appears to be the principal factor controlling noble gas isotopic abundances in Fe-Mn crusts. Decay of radioactive isotopes has a negligible influence on the nuclide abundances and isotopic ratios of noble gases in these crusts on the timescale of their formation.

Measurements of the Superfluid Joule–Thomson Refrigerator Using High Concentration 3He–4He Mixtures

The superfluid Joule–Thomson refrigerator (SJTR) uses a liquid superfluid 3He–4He mixture to provide cooling below 1 K. Performance measurements of the SJTR using 5% and 11% 3He concentration mixtures are reported. High concentration operation shows higher cooling powers at high temperature. Ultimate temperatures are seen to increase with increasing concentration due to a pinching of the temperature defect in the recuperative heat exchanger. This pinching effect is due to the variation of the heat capacity of the 3He–4He mixture with temperature and concentration and is discussed in detail and design changes are suggested to mitigate it.

Neon and helium isotopes as tracers of mantle reservoirs and mantle dynamics

He isotopes are usually used as tracers for the identification of mantle reservoirs and mantle dynamics. High 3He/ 4He ratios are thought to represent deep mantle material, whereas ratios around 8 R A (R A stands for the atmospheric 3He/ 4He ratio of 1.39 × 10 − 6 ) are thought to be representative of the upper mantle. Based on He, Ne and Ar fusion data from fresh, submarine volcanic glasses of several Mid-Atlantic Ridge off-axis seamounts we show that He isotopes can be susceptible to interferences during melt formation and evolution resulting in the decoupling of He from other elements, such as e.g. Ne or Pb. All obtained He data are indistinguishable from that of the MORB range. In contrast, Ne isotopic compositions are much more primitive than MORB. Combined He, Ne and Ar systematics show that the source region of these seamounts has experienced a preferential loss of He compared to Ne and Ar. This He loss, combined with subsequent 4He production, resulted in the decoupling of the He isotope systematics from Ne and Pb. Thus, when considering He and Ne, only Ne has preserved the evidence that a primitive mantle component contributed to the formation of the investigated seamounts. As these seamounts are not fed from a mantle plume being derived from the deep mantle, the primitive Ne component resides within the upper mantle, implying that primitive noble gases are not necessarily indicative of deep mantle material.

He, Sr, Nd, and Pb isotopic constraints on the origin of the Marquesas and other linear volcanic chains

The classic hotspot hypothesis [Morgan, W. J., 1971. Convection plumes in the lower mantle. Nature 230, 42–43], which posits that linear volcanic chains are traces of fixed plumes in the mantle on moving lithospheric plates, was instrumental in elevating the plate tectonics paradigm in the 1960s into a modern Earth Science theory. The hypothesis itself, however, remains conjectural because many of its predictions, particularly the simple age-progressive type of volcanism, are not observed in many linear volcanic chains. As an alternative explanation, it is proposed that linear volcanic chains are formed through magmatism along pre-existing lines of weakness such as transform zones and old sutures, or along cracks created by stresses on lithospheric plates. The Marquesas linear volcanic chain in south-central Pacific has geologic features that are consistent with some of the predictions of both hypotheses. To better constrain the origin of this volcanic chain, we collected major and trace element and Sr, Nd, Pb, and He isotopic data from several Marquesan lavas. Our new analyses combined with literature data classify the samples into the well established tholeiitic to mildly alkalic, low 87Sr/ 86Sr, high 143Nd/ 144Nd, shield-building volcanic phase lava group and highly alkalic, high 87Sr/ 86Sr, low 143Nd/ 144Nd, post-shield phase group. Lead isotopes show generally higher 206Pb/ 204Pb ratios and suggest evidence of crustal assimilation for the shield-building phase lavas, consistent with the argument that the shield-building phase volcanism has a lithospheric source component. On the other hand, post-shield phase lavas that are predicted to represent the true composition of the mantle source by the hotspot hypothesis have higher 3He/ 4He ratios and these are coupled to other geochemical tracers. Thus our results show that the Marquesas volcanic chain, similar to many other linear volcanic chains, has a high 3He/ 4He component in its mantle source. The presence of such a distinct source component cannot be easily explained by dispersed upper mantle heterogeneities, but provides a powerful constraint for the hotspot origin of many linear volcanic chains.

The role of lithospheric gabbros on the composition of Galapagos lavas

New geochemical data combined with a compilation of all published data for the Galapagos basalts indicate that the Galapagos Archipelago can be divided into two very different volcanic areas, east and west of the 91° W fracture zone. In both regions, the interaction between plume melts and depleted upper mantle lavas most likely controls the composition of the erupted basalts. However, we find important differences in the trace element compositions between lavas from the two regions. First, Ti/Gd and Nb/La ratios correlate with 3He/ 4He, and all three ratios decrease with increasing distance from Fernandina. These correlations suggest that the high Ti/Gd and Nb/La ratios are typical of the plume mantle source that carries the high 3He/ 4He signature. Second, there is an increase in the Ba/Th, K/La, Sr/Nd, Eu/Eu ⁎, Pb/Nd ratios, and a higher 87Sr/ 86Sr at similar 143Nd/ 144Nd (high ΔSr) from west to east across the fracture zone, and from north to south within the eastern region. These geochemical characteristics resemble those of plagioclase-rich cumulates from the oceanic crust and ophiolite complexes. The compositions of the lavas from the eastern region can be explained by interaction of basalts with plagioclase-rich cumulate during melt percolation through the oceanic lithosphere. We argue that kinetic interaction of magmas with plagioclase-rich cumulates, previously formed either in the Galapagos Spreading Center or beneath the leading edge of the plume on the western region, are responsible for the observed composition of basalts erupted in the eastern region.

Geochemistry of East African Rift basalts: An overview

Mafic lavas erupted along the East African Rift System from the Afar triangle in northern Ethiopia to the Rungwe province in southern Tanzania display a wide range of geochemical and isotopic compositions that reflect heterogeneity in both source and process. In areas with the lowest degree of crustal extension (the Western and Southern Kenya Rifts) primitive lavas record the greatest extent of lithospheric melting, manifest in elevated abundances of incompatible elements and highly radiogenic Sr–Nd–Pb isotopic compositions. Where prolonged extension has removed most or all of the mantle lithosphere (the Turkana and Northern Kenya Rifts), a larger role for sub-lithospheric processes is indicated. At intermediate degrees of extension (the Main Ethiopian Rift) both lithospheric and sub-lithospheric contributions are observed, and crustal assimilation occurs in some cases. Despite the wide compositional range of African Rift basalts, a restricted number of source domains contribute to magmatism throughout the area. These individual domains are: (1) the subcontinental mantle lithosphere; (2) a plume source with high-μ Sr–Nd–Pb–He isotopic affinities, present in all areas within and south of the Turkana Depression; and (3) a plume source with isotopic signatures analogous to those observed in some ocean islands, including high 3He/ 4He values, present throughout the Ethiopian Rift and the Afar region. The two plume sources may both be derived from the South African Superplume, which is likely to be a compositionally heterogeneous feature of the lower mantle.

Noble gas and geochronology study of the Hana Ridge, Haleakala volcano, Hawaii; implications to the temporal change of magma source and the structural evolution of the submarine ridge

In order to constrain the magma sources and the structural evolution of Hana Ridge, noble gas isotope ratios and 40Ar/ 39Ar ages were determined for this submarine extension of the east rift zone of Haleakala volcano. 40Ar/ 39Ar ages of ten lavas from Hana Ridge are bimodally distributed. Three samples have ages of 1.8–1.9 Ma and seven samples have ages of 1.4–1.5 Ma. The observation of older lavas overlying younger ones in two locations suggests that growth of Hawaiian rift zones occurs endogenously by continuous intrusion of magmas and patchy resurfacing at outbreak points. The 0.5 Ma age range for submarine Hana Ridge lavas, coupled with previously published K/Ar ages of 0.97–1.1 Ma for subaerial Honomanu lavas on the Haleakala Volcano, indicates that Haleakala shield volcanism persisted for nearly 1 My. Furthermore, the new 40Ar/ 39Ar ages, when considered with isotopic compositions of the lavas, suggests that the source for Hana Ridge magmas gradually shifted over a period of 0.5–1.0 My. The majority of samples from the ridge have relatively uniform 3He/ 4He ratios between 18 and 22 Ra, which is higher than 3He/ 4He of subaerial Honomanu tholeiites. 20Ne/ 22Ne and 21Ne/ 22Ne ratios define linearly correlated trend that overlaps with the Loihi–Kilauea trend. He–Ne systematics of the Hana Ridge indicate that the magmas comprising most parts of the ridge were derived from a source with a primordial less-degassed mantle component. Since Hana Ridge lavas predate the subaerial Honomanu lavas, temporal decrease of 3He/ 4He ratios suggests that contribution of this primordial component had decreased in the magma source during establishment of Hana Ridge and Haleakala volcano. Pb–Sr isotopes demonstrate that Hana Ridge magmas are representative of the Kea component. However, such isotopic signatures associated with elevated high 3He/ 4He ratios precludes that the Kea component is a distinct endmember, such as recycled oceanic crust or lithospheric mantle. Alternatively, we propose that it is a common sub-component that is a mixture of Loihi endmember and recycled oceanic crust. Ne isotope ratios of the Hawaiian samples, including the Hana Ridge, show primordial signature irrespective of 3He/ 4He ratios. Such apparent decoupling of He and Ne isotopes may be also attributed to mixing of the Loihi component and recycled component in the mantle plume, either of which components needs to be elementally fractionated prior to mixing.

Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.

Thermal springs, fumaroles and gas vents of continental Yemen: Their relation with active tectonics, regional hydrology and the country’s geothermal potential

Most thermal springs of continental Yemen (about 65 emergences at 48 sampling sites) and a couple of fumaroles and boiling water pools have been sampled and analyzed for chemical and isotopic composition in the liquid phase and the associated free-gas phase. Whatever the emergence, all the water discharges have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na–HCO 3-composition, clearly affected by an anomalous flux of deep CO 2 deriving from active hydrothermal systems located in the Jurassic Amran Group limestone sequence and/or the Cretaceous Tawilah Group, likely underlying the 2000–3000 m thick volcanic suite. At lower elevations, CO 2 also affects the composition of some springs emerging at the borders of the central volcanic plateau. Although mixing to a limited extent with organic CO 2 infiltrating together with the meteoric recharge waters cannot be ruled out, all the CO 2-rich gas samples have a δ 13C–CO 2 signature that falls in the range of mantle CO 2 (−3 < δ 13C < −7‰ V-PDB). The relatively high 3He/ 4He (1 < R/ R a < 3.2) ratios measured in all the CO 2-rich springs and also some mixed N 2–CO 2 gas vents in the far east Hadramaut region support the presence of mantle magmas and related hydrothermal systems residing at the crust level in several areas of Yemen. This well agrees with the presence of Quaternary basaltic magmatic activity along the Gulf of Aden, as well as inside the central Yemen volcanic plateau. Presently, the thermal springs of Yemen are prevalently used for spas and/or bathing. Nevertheless, liquid- and gas-geothermometry and geological considerations suggest that there are at least three areas (Al Lisi, Al Makhaya and Damt) inside the Yemen volcanic plateau (around Dhamar) that may be promising prospects for the future development of geothermal energy in Yemen. Alternatively, they could be used as a source of energy for small-to-medium scale agriculture and/or industrial purposes. Moreover, most of the thermal water discharges have the chemical characteristics of potable waters and, in an arid country like Yemen, they have to be considered as long-term water supply resources if the country should face water shortages.

Helium isotopes provide no evidence for deep mantle involvement in widespread Cenozoic volcanism across Central Asia

Small-volume alkali basaltic volcanism has occurred intermittently for the past + 30 My across a vast area of thick continental crust from southern Siberia, through Mongolia to northeast China. With a lack of evidence for Basin-and-Range-type crustal extension or rifting, models to explain the widely dispersed, yet long-lived, volcanism tend to favour involvement of one or more mantle plume(s). We examine the range of 3He/ 4He isotope values in olivine phenocrysts from basalts, and their entrained mantle xenoliths, from Hamar Daban in southern Siberia, and Hangai in central Mongolia, in order to examine whether upwelling lower mantle appears to be present beneath central Asia and thus test the validity of the plume model for this region. Our results show that the maximum 3He/ 4He value for the Siberian basalts is 8.12 ± 0.2 R a, and the maximum value for Mongolian basalts is 9.5 ± 0.5 R a. These values suggest that there is no significant contribution from a high 3He/ 4He primordial component that would strongly argue a lower mantle source. Overlap with commonly reported values for MORB leads us to propose that the source of the magmatism derives from the shallow asthenosphere. Alternative models to a deeply sourced mantle plume that may be able to explain the magmatism include: a shallow thermal anomaly confined to the upper mantle but either fed laterally or caused by thermal blanketing of the large Asian landmass; replacement or delamination of the lowermost lithosphere in response to tectonic stresses; or large-scale mantle disturbance or overturn caused by a protracted history of subduction beneath central Asia that ended regionally with the Jurassic closure of the Mongol-Okhotsk Ocean, but continues further afield with the present Indo-Asia collision.

A new geochemical model for the Earth's mantle inferred from 146Sm– 142Nd systematics

New measurements of 142Nd/ 144Nd in kimberlites, carbonatites, komatiites, ocean island basalts from Pitcairn, and mid-ocean ridge basalts from the Pacific and Indian Oceans show no deviation from the terrestrial Nd standard within an external reproducibility of 8 ppm. Measurements of mafic rocks from Isua Greenland confirm previous discoveries of an excess of between 0 and 17 ppm in the 142Nd/ 144Nd of these rocks. Since the terrestrial Nd standard has a 142Nd/ 144Nd approximately 20 ppm higher than measured in chondrites and eucrites, all terrestrial rocks so far analyzed have superchondritic 142Nd/ 144Nd. This requires either that the bulk silicate earth has a superchondritic Sm/Nd ratio or that the Earth experienced a global differentiation event within a couple of hundred million years of planet formation that created complementary incompatible element enriched and depleted mantle reservoirs. If that portion of the mantle involved in the production of continental crust was originally characterized by incompatible element depletion (i.e. high Sm/Nd) rather than chondritic relative abundances of refractory lithophile elements, as is commonly assumed, the mass of mantle affected by continent formation may constitute as much as 80% or more of the mantle. The abundances of K, U and Th in this early-formed depleted mantle are 60–70% of those calculated for a “chondritic” mantle, which removes the inference that half of the 40Ar produced over Earth history must still reside in the mantle and explains why the mantle component characterized by high 3He/ 4He has superchondritic Nd and Hf isotopic composition. If these concentrations characterize the whole mantle, this implies a bulk-earth Urey ratio of ∼ 0.3 and hence that much of the heat escaping the Earth's interior is primordial and not the result of radioactive decay over Earth history. If the bulk silicate earth is presumed to have chondritic abundances of refractory lithophile elements, there must be an incompatible element enriched reservoir buried deep enough in to mantle to preclude its involvement in mantle convection and its contribution to surface volcanism because no terrestrial rock with 142Nd/ 144Nd less than chondritic has been identified.

Helium isotopes in ferromanganese crusts from the central Pacific Ocean

Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/ 4He of VA13/2 samples varies from 18.5 to 1852 R a due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/ 4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

Cosmogenic 3He production rates revisited from evidences of grain size dependent release of matrix-sited helium

Measurements of the cosmogenic 3He ( 3He c) content of various size aliquots of exposed olivines show that the fine fraction (< 140 μm) has 3He c concentrations between 14 and 100% lower than that of the coarse fractions (0.14–1 mm). Such differences attest to a grain size dependent partial release of 3He c from the phenocrysts matrix during the preliminary in vacuo crushing. This result might have important implications since most 3He c measurements have used for ∼ 20 yr a standard routine based on the fusion of bulk powdered phenocrysts, whatever their grain size. A suite of new data obtained from coarse olivine grains yielded a mean Sea Level High Latitude 3He c production rate (SLHL P 3) of 128 ± 5 and 136 ± 6 at. g − 1 yr − 1 , depending on the scaling factors used. This new value, which is ∼ 15% higher than previously published rates, is obtained from 5 ropy flow surfaces of Mt Etna (38°N) and Hawaiian (19°N) volcanoes, at elevations between sea level and 870 m and ranging in age from 1.47 ± 0.05 to 149 ± 23 ka according to independent 14C or K/Ar dating. 3He loss during the crushing step might account for the discrepancy between the standard reference value of 110–115 at. g − 1 yr − 1 and the higher SLHL P 3 proposed here. More generally, removal of the powdered fraction before fusion is an important point to consider in further studies in order to avoid any 3He c systematic underestimates. An altitudinal section has also been sampled on the ropy surface of a ∼ 1500 yr single flow of Mauna Loa (19°N) which allowed a new empirical atmospheric attenuation length of 149 ± 22 g cm − 2 to be documented for 3He c in olivines between 2400 and 4000 m elevations.