Isotopic Compositions Of He Research Articles

Mineralogy and noble gas isotopes of micrometeorites collected from Antarctic snow

We have investigated seven micrometeorites (MMs) from Antarctic snow collected in 2003 and 2010 by means of electron microscopy, X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy (TEM) observation, and noble-gas isotope analysis. Isotopic ratios of He and Ne indicate that the noble gases in these MMs are mostly of solar wind (SW). Based on the release patterns of SW 4He, which should reflect the degree of heating during atmospheric entry, the seven MMs were classified into three types including two least heated, three moderately heated, and two severely heated MMs. The heating degrees are well correlated to their mineralogical features determined by TEM observation. One of the least heated MMs is composed of phyllosilicates, whereas the other consists of anhydrous minerals within which solar flare tracks were observed. The two severely heated MMs show clear evidence of atmospheric heating such as partial melt of the uppermost surface layer in one and abundant patches of dendritic magnetite and Si-rich glass within an olivine grain in the other. It is noteworthy that a moderately heated MM composed of a single crystal of olivine has a 3He/4He ratio of 8.44 × 10−4, which is higher than the SW value of 4.64 × 10−4, but does not show a cosmogenic 21Ne signature such as 20Ne/21Ne/22Ne = 12.83/0.0284/1. The isotopic compositions of He and Ne in this sample cannot be explained by mixing of a galactic cosmic ray (GCR)-produced component and SW gases. The high 3He/4He ratio without cosmogenic 21Ne signature likely indicates the presence of a 3He-enriched component derived from solar energetic particles.

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

AbstractWe measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides 10Be, 26Al, and 36Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic 36Cl and 36Ar measured in the metal separates, we determined 36Cl‐36Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation 3He/21Ne versus 22Ne/21Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation 3He/21Ne versus 22Ne/21Ne breaks down at high shielding. Using 36Cl‐36Ar exposure ages and measured noble gas concentrations, we determine 21Ne and 38Ar production rates as a function of 22Ne/21Ne. The new data agree with recent model calculations for the relationship between 21Ne and 38Ar production rates and the 22Ne/21Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for 21Ne and 38Ar production rates as a function of the shielding indicator 22Ne/21Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the 10Be/21Ne and 26Al/21Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.

Light noble gases in 12 meteorites from the Omani desert, Australia, Mauritania, Canada, and Sweden

AbstractWe measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shişr 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for 3He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shişr 033 all have excess 4He, either of planetary or solar origin. However, very high 4He/20Ne ratios occur at relatively low 20Ne/22Ne ratios, which is unexpected and needs further study. The measured 40Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped 36Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).

ISOTOPIC MASS FRACTIONATION OF SOLAR WIND: EVIDENCE FROM FAST AND SLOW SOLAR WIND COLLECTED BY THEGENESISMISSION

NASA's Genesis space mission returned samples of solar wind collected over ~2.3 years. We present elemental and isotopic compositions of He, Ne, and Ar analyzed in diamond-like carbon targets from the slow and fast solar wind collectors to investigate isotopic fractionation processes during solar wind formation. The solar wind provides information on the isotopic composition for most volatile elements for the solar atmosphere, the bulk Sun and hence, on the solar nebula from which it formed 4.6 Ga ago. Our data reveal a heavy isotope depletion in the slow solar wind compared to the fast wind composition by 63.1 ± 2.1‰ for He, 4.2 ± 0.5‰ amu–1 for Ne and 2.6 ± 0.5‰ amu–1 for Ar. The three Ne isotopes suggest that isotopic fractionation processes between fast and slow solar wind are mass dependent. The He/H ratios of the collected slow and fast solar wind samples are 0.0344 and 0.0406, respectively. The inefficient Coulomb drag model reproduces the measured isotopic fractionation between fast and slow wind. Therefore, we apply this model to infer the photospheric isotopic composition of He, Ne, and Ar from our solar wind data. We also compare the isotopic composition of oxygen and nitrogen measured in the solar wind with values of early solar system condensates, probably representing solar nebula composition. We interpret the differences between these samples as being due to isotopic fractionation during solar wind formation. For both elements, the magnitude and sign of the observed differences are in good agreement with the values predicted by the inefficient Coulomb drag model.

Noble gas isotopic constraints on the origin and evolution of the Jinchuan Ni–Cu–(PGE) sulfide ore-bearing ultramafic intrusion, Western China

The Jinchuan Ni–Cu–PGE sulfide deposit in Western China is the largest single magmatic sulfide deposit hosted by a small ultramafic intrusion in the world. The abundances and isotopic compositions of He, Ne and Ar in 23 silicate and sulfide mineral separates from peridotites, pyroxenites and sulfide ores have been determined by stepwise heating to investigate the formation and evolution of the intrusion and associated sulfide ores. Intriguingly, the volatiles released from elevated temperatures (400–1300°C) are all characterized by low 3He/4He (<1Ra) that are close to crustal value, varying 3He/4He, 21Ne/22Ne, 20Ne/22Ne and 40Ar/36Ar ratios suggest a mixture of crust, mantle and air components. Numerical modeling reveals that the apparent “crustal signatures” of the noble gases released from the Jinchuan samples are due to significant radiogenic production from U and Th decay after the crystallization from magma derived from a subcontinental lithospheric mantle at ~830Ma. In contrast, the noble gases released at low temperatures (200–400°C) are characterized by abundant atmosphere-origin components, suggesting that they are mainly derived from secondary fluid inclusions. The noble gases in the 400–1300°C fractions collectively exhibit a negative correlation between 3He/4He and 4He, and a positive correlation between 40Ar/36Ar and 40Ar, which are consistent with significant radiogenic ingrowths. The sulfide separates have slightly higher 3He/4He ratios than coexisting silicates in the intrusion, indicating more abundant radiogenic components in the latter. Mass balance calculations of He and Ar isotopic mixing models indicate that about 20% crustal fluid and 63% air saturated fluid (ASF) were added into ore-forming magma. Our data are consistent with a recent suggestion by some researchers that significant amounts of crustal fluids were added to the parental magma of the Jinchuan intrusion.

Isotopic and elemental fractionation of solar wind implanted in the Genesis concentrator target characterized and quantified by noble gases

Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The 20Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass‐dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions.

He and Ar isotopic compositions and genetic implications for the giant Shizhuyuan W–Sn–Bi–Mo deposit, Hunan Province, South China

The Shizhuyuan ore deposit in southern Hunan Province is a world-class W–Sn–Bi–Mo occurrence hosted by Devonian limestone in the thermal aureole of the Qianlishan granite. Mineralization coincided with intrusion of the granite pluton during Mesozoic crustal extension in South China. We present new He and Ar isotope data for volatiles released from pyrite in the Shizhuyuan deposit. Concentrations of 40Ar range from 0.21 to 2.38 × 10−6 cm3 STP 40Ar/g, and 4He concentrations range from 0.8 to 65.1 × 10−6 cm3 STP 4He/g. 3He/4He ratios vary from 0.06 to 1.66 Ra (where Ra is the 3He/4He ratio of air = 1.39 × 10−6) and 40Ar/36Ar ratios range from 293 to 1072. The isotopic compositions of He and Ar indicate that the ore-forming fluids were mantle-derived, modified by air-saturated crustal fluids. Shallow-level boiling increased 3He concentrations, whereas crustal contamination decreased 3He/4He ratios in the magmatic fluids. The occurrence of mantle-derived components in the magmatic fluid indicates that the associated Qianlishan granite is not a typical S-type pluton that formed entirely by crustal melting. We propose that the mineralization was related to mantle upwelling and Mesozoic lithosphere extension of the South China Block.

Cosmogenic nuclides in the solar gas‐rich H3–6 chondrite breccia Frontier Mountain 90174

Abstract— We re‐evaluated the cosmic‐ray exposure history of the H3‐6 chondrite shower Frontier Mountain (FRO) 90174, which previously was reported to have a simple exposure history, an irradiation time of about 7 Ma, and a pre‐atmospheric radius of 80–100 cm (Welten et al. 2001). Here we measured the concentrations and isotopic compositions of He, Ne, and Ar in 8 aliquots of 6 additional fragments of this shower, and 10Be and 26Al in the stone fractions of seven fragments. The radionuclide concentrations in the stone fractions, combined with those in the metal fractions, confirm that all samples are fragments of the FRO 90174 shower. Four of the fragments contain solarwind‐implanted noble gases with a solar 20Ne/22Ne ratio of ˜12.0, indicating that FRO 90174 is a regolith breccia. The concentrations of solar gases and cosmogenic 21Ne in the samples analyzed by us and by Welten et al. (2001) overlap with those of the FRO H‐chondrites from the 1984 season, suggesting that many of these samples are also part of the large FRO 90174 chondrite shower. The cosmogenic 21Ne concentrations in FRO 90174 show no simple correlation with 10Be and 26Al activities. We found 21Ne excesses between 0.3‐1.1 × 10−8cm3STP/g in 6 of the 17 samples. Since excess 21Ne and trapped solar gases are not homogeneously distributed, i.e., we found in one fragment aliquots with and without excess 21Ne and solar 20Ne, we conclude that excess 21Ne is due to GCR irradiation of the regolith before compaction of the FRO 90174 object. Therefore, the chondrite shower FRO 90174 did not simply experience an exposure history, but some material was already irradiated at the surface of an asteroid leading to excess 21Ne. This excess 21Ne is correlated to implanted solar gases, clearly indicating that both processes occurred on the regolith.

The complex exposure history of the Jiddat al Harasis 073 L‐chondrite shower

Abstract— We measured the concentrations and isotopic compositions of He, Ne, and Ar in 29 bulk samples from 11 different strewn field fragments of the large Jiddat al Harasis (JaH) 073 L6 chondrite shower, including 7 samples from known locations within the main mass. In addition, we measured the concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in 10 samples. All fragments of this shower are characterized by low 10Be concentrations (7.6–12.8 dpm/kg), high 26Al/10Be ratios (3.5‐5), large contributions of neutron capture 41Ca (200–1800 dpm/kgCa), low 3He/21Ne ratios (1.5‐3.0), large variations in cosmogenic 21Ne (1.2–12) × 10−8cm3STP/g, and significant contributions of neutron‐capture 36Ar. Stepwise heating experiments show that neutron‐capture produced 36Ar is predominantly released between 1000–1200 °C. All these results are consistent with a first‐stage exposure of ˜65 Ma within ˜20 cm of the surface of the L‐chondrite parent body, followed by ejection of a 1.5‐2 m large object, which was then delivered to Earth within about 0.5 and 0.7 Ma. The cosmogenic nuclide data in JaH 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history. The observed 3He/21Ne ratios of 2.5‐3.0 in the most shielded samples (including those of the main mass) are lower than predicted by model calculations, but similar to the lowest values found in the large Gold Basin L‐chondrite shower. The Bern plot, which gives a linear correlation for 3He/21Ne versus 22Ne/21Ne, is evidently not valid for very high shielding. Some of our measured 22Ne/21Ne ratios in JaH 073 are lower than 1.06, which is not well understood, but might be explained by loss of cosmogenic neon from shocked sodium‐rich plagioclase during terrestrial weathering. The amount of trapped atmospheric argon in the JaH 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments, which range from &lt;100 g to &gt;50 kg. Finally, low concentrations of radiogenic 4He and 40Ar indicate incomplete degassing &lt; 1 Ga ago, probably at the main collision event on the L‐chondrite parent body ˜480 Ma ago.

He, Ne and Ar systematics in single vesicles: Mantle isotopic ratios and origin of the air component in basaltic glasses

An outstanding problem in understanding the origin of the gaseous phase, particularly the rare gas compositions in magmatic rocks, is the ubiquitous atmospheric component in bulk rock samples, and whether this atmospheric component is a late stage contamination of the sample, or a recycled component though sediments or altered oceanic crust. In the present study we address this problem by analyzing single vesicles from the “popping rock 2∏D43” sample from the Mid-Atlantic Ridge using a UV laser ablation system. We have determined both elemental and isotopic compositions of He, Ne and Ar in single vesicles as well as Kr and Xe abundances. All vesicles analyzed have an isotopic composition identical to the referred degassed mantle value estimated from this same sample, despite analyzing vesicles from a wide size distribution. The atmospheric component, which is always detected in bulk samples by crushing or heating, was not detected in the single vesicles. This implies that the recycling of atmospheric noble gases in the mantle cannot explain the air-like component of this sample. The addition of the atmospheric component must occur either during the eruption, or after sample recovery.

Genesis of Cenozoic basalts in Central Asia and northern Mongolia (from helium isotope and petrochemical data)

Isotopic compositions of He (27 samples) and Ar (7 samples) as well as major and trace elements have been determined in Cenozoic effusive rocks and hosted olivine and pyroxene phenocrysts from northern and central Mongolia. The R = 3He/ 4He values are within (0.1–13)⋅10 −6. Abnormally high R value, 13⋅10 −6, atypical of Cenozoic basaltic-volcanism areas in Mongolia, has been first revealed at one of the sites of the Hangayn upland. In composition the rocks under study correspond to tephrites, trachybasalts, and subalkalic andesite-basalts. Analysis of their REE patterns and spidergrams shows that the elements participating in the formation of basalt fields of the Hangayn upland were supplied from the enriched mantle (EM1). These patterns are similar to the OIB ones; (La/Yb) n = 9–53. The R values in the olivine phenocrysts are higher as compared with the pyroxene phenocrysts and the bulk rock compositions of the same samples. Based on the elemental composition of the rocks, their contents of radiogenic 4He and 3He were calculated. The rate of 3He formation is 5.65⋅10 −2 at/(g⋅year). The calculated and measured R values in the rock samples point to the presence of trapped mantle helium.

Mantle-derived CO2 in Hot Springs of the Rehai Geothermal Field, Tengchong, China

Gas concentrations and isotopic compositions of He and CO2 were determined on free gas samples from ten hot springs of the Rehai geothermal field, Tengchong, China. The results showed that hot-spring CO2 gas, together with He, was derived mainly from the mantle, indicating the accumulation of mantle-derived volatiles beneath the survey area. The δ 13C values of CO2, higher than those of the typical mantle-derived carbon and the isotopic composition of hot-spring-free CO2 in unequilibrium with dissolved CO2, are recognized only in the Rehai geothermal field, suggesting that there seems to be a still-degassing magmatic intrusion at depths, which provides mantle-derived volatiles to the hydrothermal system above. The accumulation of those volatiles has probably played an important role in triggering earthquakes in this region. In addition, the isotopic characteristics of He and C also indicate that the magmatic intrusion seems to have been derived from the MORB source, and could be contaminated by crustal materials during its upwelling through the continental crust.

Evaluation of argon ages and integrity of fluid-inclusion compositions: stepwise noble gas heating experiments on 1.87 Ga alunite from Tapajós Province, Brazil

Quantitative analyses are reported for active (N 2, CH 4, CO, CO 2, H 2, O 2, HF, HCl, H 2S, SO 2) and noble (He, Ar, Ne) gases released by crushing and step heating of magmatic-hydrothermal alunite from the Tapajós gold province in Brazil. This is the oldest known alunite ( 40Ar/ 39Ar age of 1.87 Ga), and because it has undergone minimal postdepositional thermal or tectonic strain, it is excellent material to test the retention of gas species in fluid inclusions and within the crystal structure over geological time. The gas compositions of a single sample, in combination with Ar age-spectrum data derived from stepwise heating of 10 related samples, have been used to constrain the limits of modification of primary gas compositions in fluid inclusions and the possible extent of the loss of radiogenic Ar. The observed variations in the isotopic compositions of He, Ne, and Ar released by stepwise heating have been used to identify the residence sites and determine the diffusion coefficients of the gases in the mineral. The data suggest that the only modification to primary gas compositions after entrapment in fluid inclusions and formation of the mineral is due to radiogenic and nucleogenic processes which affect the noble gas isotopic compositions. Three gas retention sites are recognized in alunite: (1) primary fluid inclusions, (2) crystal structure OH sites, and (3) crystal structure sulfate sites. Alunite undergoes OH loss at <500 °C, and K-SO 4 structural decomposition occurs at >600 °C. Fluid inclusions generally are ≤1 μm in diameter and have variable but high vapor/liquid ratios. The gases in inclusion fluids are quantitatively released in vacuo by heating at 200 °C for ∼1 h. In the inclusion fluids, H 2O is 32 mol% of total gas, H 2S/SO 2 ranges from approximately 4 to 2, and N 2/Ar from 0.3 to 96.3. The presence of large amounts of H 2 and CO indicates disequilibrium among the gas species in the fluids. Helium abundance is 214 ppm. Helium from fluid inclusions ( R/Ra=19.5) makes up about 4% of the total helium, whereas He ( R/Ra=0.2–2.0) from the crystal structure makes up about 65% of the total. Argon from fluid inclusions has 40Ar/ 36Ar=584–629 and that from crystal structure sites is >9.6×10 4. Most gases are released from fluid inclusions at 200 °C, whereas most Ar (≥95%) is released between 525 and 725 °C. Argon released from fluid inclusions at 200 °C has 38Ar/ 36Ar=0.0–0.064. In contrast, Ar released from the matrix of the mineral at high temperature has 38Ar/ 36Ar=3.6–14.7. This difference suggests that, since the formation of the alunite at 1.87 Ga, traces of Cl in the mineral structure have undergone Cl ( n, γ) and 41K ( n, α) in situ reactions with neutrons derived from U–Th. The amount of 36Ar production from Cl nucleogenic reactions used in correcting for atmospheric 40Ar typically increases the calculated age by 1–5 m.y., which is generally an insignificant component of the determined Ar age. Decay of U–Th over this time contributes 4He ( α) buildup in the crystal structure K-SO 4 sites. Atmospheric corrected excess 21Ne/ 22Ne=0.028–0.409 indicates that nucleogenic Ne was also produced via ( α, n) reactions in matrix sites. Diffusion coefficients and activation energies for the diffusion of Ar and He, as determined using Arrhenius plots, indicate two distinct groups definable by their differences in activation energies. Argon log D o=2.45 and 15.33, with activation energies of 225 and 465 kJ mol −1, respectively; the diffusion of He in alunite is quantified with log D o=−4.33 and E=106.8 kJ mol −1. Model calculations of simplistic 1/ e-folding times and diffusion distance–time curves indicate that He should remain in alunite for millions of years at ≤100 °C, whereas at <200–220 °C, the alunite will retain Ar almost indefinitely. The data demonstrate why alunite is suitable for Ar geochronological applications and also show that, unless the alunite is subjected to metamorphic deformation, the inclusion fluids should retain their primary compositions.

Cosmic‐ray prdocution rates of helium, neon and argon isotopes in H chondrites based on chlorine‐36/argon‐36 ages

Abstract— We present the concentrations and isotopic compositions of He, Ne, and Ar for nonmagnetic fractions and bulk samples of 17 H chondrites which were recently investigated for their 36Cl‐36Ar cosmic‐ray exposure ages (Graf et al., 2001). All selected meteorites are observed falls with cosmic‐ray exposure ages close to the 7 Ma peak. The rare gas data are consistent with 10Be and 36C1 production rates in the metal phase. Remarkably, only 1 out of the 17 H chondrites, Bath, shows clear indications for a complex exposure history. Based on rare gas concentrations and 36Cl‐36Ar exposure ages, 21Ne production rates as a function of 22Ne/21 Ne and a mean 38Ar production rate are determined. The results confirm model calculations which predict that the relationship between 21Ne production rates and 22Ne/21Ne is ambiguous for high shielding. Besides the mean 38Ar production rate we also give production rate ratios P(38Ar from Ca)/P(38Ar from Fe). They vary between 10 and 77, showing no significant correlation with 38Ar concentrations or 22Ne/21Ne. By investigating the metal separates, Graf et al. (2001) found significant 3He deficits for 6 out of the 17 meteorites. For the nonmagnetic fractions and bulk samples investigated here, the data points in a 3He/21Ne vs. 22Ne/21Ne diagram plot in the area defined by most of the H chondrites. This means that 3He deficits in the metal phase are much more pronounced than in silicate minerals and we will argue that 3H diffusive losses in meteorites should be the rule rather than the exception. The 21Ne exposure ages, calculated on the basis of modeled 21Ne production rates, confirm the assumption by Graf et al. (2001) that the H5 chondrites with low 3He/38Ar in the metal formed in a separate event than those with normal 3He/38Ar ratios. The data can best be interpreted by assuming that the prominent 7 Ma exposure age peak of the H chondrites is due to at least two events about 7.0 and 7.6 Ma ago.

Primordial helium isotope signature from Plio–Quaternary alkaline basalts in Yemen

Abstract Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south‐western Arabian Peninsula. Very high 3He/4He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial 3He/4He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high 3He/4He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous‐like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.

Effects of recycled materials involved in a mantle source beneath the southwest Japan arc region: evidence from noble gas, Sr, and Nd isotopic systematics

Isotopic compositions of He, Ar, Kr, Xe, Sr, and Nd and concentrations of noble gases are reported for phenocrysts, megacrysts, and ultramafic xenoliths in Cenozoic basalts from southwest (SW) Japan and compared with those in lavas and tectonic emplaced peridotite from northeast (NE) Japan arc. The mantle source beneath SW Japan arc indicate that the old subducted oceanic crust and sediment (enriched mantle, EM1) is the major source of the mantle. Some mantle source demonstrates mixing of depleted mantle (mid-ocean ridge basalts (MORB) mantle) and recycled materials (EM2). The isotopic data of ultramafic rocks (Tsuyama lherzolite) from SW Japan arc indicate the contribution of a Samoan-like plume component. This suggests that some mantle source beneath the SW Japan arc formed by upwelling of a deep-mantle superplume. The mixing processes among the depleted mantle, recycled materials, and mantle plume may have produced a mantle heterogeneity beneath the SW Japan arc region. On the other hand, ultramafic rocks and olivine phenocrysts in basalt from NE Japan arc have simple binary mixing trends between depleted mantle (MORB mantle) and subducted sediment components (EM2 component).

Rare gas systematics on the southernmost Mid‐Atlantic Ridge: Constraints on the lower mantle and the Dupal source

Concentrations and isotopic compositions of He, Ne, Ar, Kr, and Xe have been measured for mid‐ocean ridge basalt glasses from the Mid‐Atlantic Ridge Discovery section, centered at 47°30′S, thus extending the database for the 50°–53°S Shona section [Moreira et al, 1995]. The 44°–53°S part of the Mid‐Atlantic Ridge includes the Discovery and Shona bathymetrie and geochemical ridge anomalies [Douglass et al, 1999], which also appear clearly in the rare gas isotopic record. In addition to air, present at the surface or possibly mantle recycled, three source components are identified, upper mantle, primitive plume, and a Dupal‐related component. He and Ne isotopes indicate a very primitive source for both the Discovery and Shona plumes, which must originate in deep, poorly degassed mantle. Ne and Ar, corrected from air based on Ne systematics, reveal very consistent along‐strike He, Ne, and Ar isotopic patterns, also consistent with Xe data. These systematics provide evidence that plume argon has low 40Ar/36Ar and plume Xe low isotopic ratios relative to degassed mantle. A segment of the Discovery ridge anomaly shows a Dupal‐type, low 206Pb/204Pb component named LOMU (low μ, where μ = 238U/204Pb) by Douglass et al. [1999], and has radiogenic 4He/3He and 21Ne/22Ne, relatively elevated 20Ne/22Ne, mildly radiogenic 40Ar/36Ar, and low Xe isotopic ratios, possibly representing the Dupal rare gas signature. Interpretations of this component as either recycled oceanic crust, or delaminated subcontinental lithosphere are consistent with the rare gas systematics. In the former case, a maximum subduction age of 500 Ma can be calculated. In the latter case, the sublithospheric mantle should have a 40K/36Ar ratio 2–5 times lower than the convective mantle and a 238U/3He ratio 2–3 times higher.

Geophysically constrained mantle mass flows and the 40Ar budget: a degassed lower mantle?

It has been inferred previously that the lower mantle is much less degassed than the upper mantle, by about two orders of magnitude, based on estimates of the amount of 40Ar expected to have been generated during earth history. Such a gas-rich lower mantle would severely limit the permissible mass flow rate into the upper mantle. However a gas-rich lower mantle conflicts with evidence from refractory trace elements and their isotopes that most of the mantle has been processed, and with increasingly strong geophysical evidence for a large mass flow between the upper and lower mantle. Neither is it implied any longer by isotopic compositions of He, Ne and Ar from oceanic island basalts, which are nearly as radiogenic (within factors of 2–4) as those from mid-ocean ridge basalts. Here the budgets for mantle He, Ne and Ar are developed from geophysical and other geochemical constraints, but without assuming the total 40Ar content of the silicate earth to be known. These budgets permit the lower mantle to be only slightly less degassed than the upper mantle, though they show that the degree of lower mantle degassing inferred in this way depends strongly on poorly-constrained entrainment and degassing efficiencies of mantle plumes. A degassed lower mantle requires either (1) that the earth has 50% less 40K than is usually estimated, (2) that 40Ar is sequestered in the core, or (3) that 40Ar has been lost from the earth entirely. 40Ar in the core is hard to reconcile with chemical systematics. A small amount of argon loss from the earth during the late heavy meteorite bombardment is plausible, but 50% loss is difficult to justify at this stage. A 50% lower K/U ratio in the earth would remove the discrepancy and may not be outside the range of uncertainties. All three hypotheses need to be considered, and some combination of them may apply.

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.

Fluxes and sources of volatiles discharged from Kudryavy, a subduction zone volcano, Kurile Islands

The Kudryavy volcano, a 996-m-high basaltic-andesite cone on the northeastern shore of Iturup Island in the Kuriles, erupted last in 1883 and has since been in a persistent state of high-temperature, >900°C fumarolic activity. Its flux of SO 2, measured by COSPEC, is 73±15 t/d, or 416 Mmol/a. In combination with the chemical composition of the parent gas supplying the high-temperature vents and the isotopic compositions of He and C, it allows the evaluation of contributions from major source components, such as the mantle, the crust, and subducted sediments and carbonate. The 3He/ 4He ratio of 6.7 R A corresponds to a 84% mantle origin and a flux of 2200 mol/a of mantle He. At a He concentration of 2200 mol/Mt, the mass of mantle material required to generate this flux is 1.0 Mt/a. The same mass produces a flux of 0.025 mol/a of 3He and of 50 Mmol/a of mantle CO 2 at a CO 2/ 3He ratio of 2·10 9. In conjunction with the C-isotopic composition of fumarolic CO 2 of −7.2‰, about 12% of the CO 2 are derived from the mantle, 67% from marine carbonate in subducted, altered oceanic crust, 21% are of subducted organic sedimentary origin. The flux of 280 Mmol/a of carbonate-derived CO 2 requires 0.41 Mt/a of oceanic crust with a CO 2 content of 3 wt%, and 0.35 Mt/a of sedimentary material to supply the organic CO 2 flux of 86 Mmol/a. Nitrogen from the mantle contributes at most 2% to the total N 2 flux of 5.4 Mmol/a. Assuming N to be derived from the subducted sediments, its concentration there is 460 mg/kg. The total volume of mantle and subducted material required to maintain the flux of volatiles over the 100 a period of high-temperature fumarolic activity of Kudryavy is 0.07 km 3. Steady-state release of volatiles from the depth of arc magma generation to the fumaroles and continuously high heat flow from the mantle are proposed as the main process supporting the long-term high-temperature degassing at Kudryavy. In this steady-state system, the calculated volatile fluxes are balanced over time by volatiles originating from subducted sediments, hydrothermally altered oceanic crust below the Kudryavy volcano and the mantle wedge. This has significant implications for volatile cycling from the Earth's crust and mantle to the atmosphere.