Noble Gas Composition Research Articles

Tracing partial melting and subduction-related metasomatism in the Kamchatkan mantle wedge using noble gas compositions

We determined noble gas composition of minerals separated from mantle-derived xenoliths hosted by andesites in the active Avacha volcano, Kamchatka peninsula, Russia in order to better constrain the provenance and nature of fluids involved in partial melting and metasomatism in the mantle wedge. The lithospheric mantle beneath Avacha mainly consists of spinel harzburgites produced by high degrees of melt extraction. Data on coarse olivine separated from seven harzburgite xenoliths constrain fluid regime during flux melting in arc settings. Pyroxenes from two websterite veins cross-cutting the harzburgites characterize post-melting metasomatism by subduction-related melts or fluids. 3He/4He-ratios of 5.2±0.6 to 8.1±0.3 RA obtained on both olivines and pyroxenes overlap the highest values reported for volcanic rocks from Kamchatka and fall into the typical range of continental lithospheric mantle worldwide. This rules out significant contributions of slab-derived radiogenic 4He*. The highest 40Ar/36Ar ratios are 400; Ne and Xe isotope ratios are indistinguishable from those in the air. We consider the slab as the initial source of a major portion of these ‘atmospheric’ gases. Element composition of noble gases in olivine differs markedly from that in vein pyroxene indicating that the composition of the fluid phase involved in partial melting was distinct from that during metasomatism. In particular, the harzburgites and veins define distinct linear trends on plots of 3He/36Ar vs. 40Ar/36Ar and of 132Xe/36Ar vs. 40Ar/36Ar. Estimates of ‘mantle’ 132Xe/36Ar values by extrapolating 40Ar/36Ar to 40000 yield unrealistically high values of 0.5–0.8 (olivine) and 4–5 (vein pyroxene) ruling out a simple two-component mixing of mantle and atmospheric noble gases. Rather a two-stage mixing process applies: (1) Changes in relative proportions of slab-derived element-fractionated atmospheric gases and ‘mantle’ produce two hybrid mixtures dominated by atmospheric gases. This may reflect interaction of slab fluids with mantle wedge fluids on a regional scale within the melting zone. (2) Subsequently mixing of these mantle-atmosphere “hybrids” occurred shortly before the entrapment of the fluids and possibly represents locally restricted compositional variations within the source region of the xenoliths.

The noble gas systematics of late-orogenic H 2O–CO 2 fluids, Mt Isa, Australia

The noble gases (He, Ne, Ar, Kr and Xe) are powerful geochemical tracers because they have distinctive isotopic compositions in the atmosphere, crust and mantle. This study illustrates how noble gases can be used to trace fluid origins in high-temperature metamorphic and mineralising environments; and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously. We report data for H 2O and CO 2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the Proterozoic Mt Isa Inlier of Australia. Sulphide fluid inclusions are dominated by crustal 4He. However, co-variations in fluid inclusion 20Ne/ 22Ne, 21Ne/ 22Ne, 40Ar/ 36Ar and 136Xe/ 130Xe indicate noble gases were derived from three or more reservoirs. In most cases, the fluid inclusions elemental noble gas ratios (e.g. Ne/Xe) are close to the ranges expected in sedimentary and crystalline rocks. However, the elemental ratios have been modified in some of the samples providing evidence for independent pulses of CO 2, and interaction of CO 2 with high-salinity aqueous fluids. Compositional variation is attributed to mixing of: (i) magmatic fluids (or deeply sourced metamorphic fluids) characterised by basement-derived noble gases with 20Ne/ 22Ne ∼ 8.4, 21Ne/ 22Ne ∼ 0.4, 40Ar/ 36Ar ∼ 40,000 and 136Xe/ 130Xe ∼ 8; (ii) basinal–metamorphic fluids with a narrow range of compositions including near-atmospheric values and (iii) noble gases derived from the meta-sedimentary host-rocks with 20Ne/ 22Ne ∼ 8–9.8, 21Ne/ 22Ne < 0.1, 40Ar/ 36Ar < 2500 and 136Xe/ 130Xe ∼ 2.2. These data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochemical reservoirs in Mt Isa’s largest ore deposits. In addition the data show how noble gases in fluid inclusions can provide information on fluid origins, the composition of the crust’s major lithologies, fluid–rock interactions and fluid–fluid mixing or immiscibility processes.

He, Ne and Ar in peridotitic and eclogitic paragenesis diamonds from the Jwaneng kimberlite, Botswana—Implications for mantle evolution and diamond formation ages

We have undertaken helium, neon and argon step-heating, isotopic analyses of eleven polycrystalline diamonds of known peridotite/eclogite paragenesis from the Jwaneng kimberlite pipe, Botswana. In contrast to the findings of crustal noble gases in framesites from the same kimberlite pipe (Honda et al., 2004. Unusual noble gas compositions in polycrystalline diamonds: preliminary results from the Jwaneng kimberlite, Botswana. Chem. Geol. 203, 347–358.), the Jwaneng polycrystalline diamonds appear to contain similar noble gas isotopic compositions (particularly Ne) to those representing a mantle source for MORBs. This implies that the Jwaneng polycrystalline diamonds may have formed in recent times, possibly close to the time of kimberlite emplacement at ~235Ma. In contrast, Jwaneng framesites could be as old as gem diamonds (mineral inclusion ages of ~2.9Ga). Furthermore, the data indicate that the sub-continental mantle lithosphere in the region has heterogeneous Ne isotopic compositions, or that these compositions changed over time from crustal Ne (as observed in the framesites) to MORB-like (as observed in the polycrystalline diamonds).

Noble gases in anhydrous mantle xenoliths from Tasmania in comparison with other localities from eastern Australia: Implications for the tectonic evolution

Ultramafic xenoliths from Tasmanian basalts were analysed for elemental and isotopic compositions of noble gases, for greater understanding of the evolution of the underlying subcontinental lithosphere. The noble gas isotopic compositions indicate contributions of radiogenic and MORB-like components to the xenoliths. The MORB-like component is characterized by 3He/ 4He ratios between 7 and 9 Ra while the radiogenic component shows lower 3He/ 4He ratios than those in MORB (8.75 ± 2.14 Ra, Graham, 2002). The identified radiogenic component in xenoliths from Tasmania may stem from subduction events, during which the mantle wedge and subcontinental lithospheric mantle were metasomatised by U- and Th-rich fluids. As subduction processes have played important role in the evolution of Eastern Australia during the Paleozoic, the radiogenic component is probably associated with this event. Incorporation of MORB-like noble gases in the subcontinental lithospheric mantle beneath Tasmania is likely related to extensional rifting and associated opening of the Tasman Sea in the Late Mesozoic and Cenozoic. The identified noble gas components of the xenoliths are consistent with the geochemical character of the host basalt based on the radiogenic isotope and trace element systematics. This agreement and the petrographic observations (e.g. fluid inclusion trails connected with the reaction rim of the minerals close to the host basalt, spongy texture) indicate that fluids (noble gases, CO 2) most likely propagated from the host basalts to the xenoliths forming fluid inclusions. The observed noble gas isotopic heterogeneity in mantle-derived xenoliths from Tasmanian to North Queensland reflects isotopic heterogeneity at regional scale in the subcontinental lithospheric mantle beneath eastern Australia.

Chronology and shock history of the Bencubbin meteorite: A nitrogen, noble gas, and Ar–Ar investigation of silicates, metal and fluid inclusions

We have investigated the distribution and isotopic composition of nitrogen and noble gases, and the Ar–Ar chronology of the Bencubbin meteorite. Gases were extracted from different lithologies by both stepwise heating and vacuum crushing. Significant amounts of gases were found to be trapped within vesicles present in silicate clasts. Results indicate a global redistribution of volatile elements during a shock event caused by an impactor that collided with a planetary regolith. A transient atmosphere was created that interacted with partially or totally melted silicates and metal clasts. This atmosphere contained 15N-rich nitrogen with a pressure ⩾3 × 10 5 hPa, noble gases, and probably, although not analyzed here, other volatile species. Nitrogen and noble gases were re-distributed among bubbles, metal, and partly or totally melted silicates, according to their partition coefficients among these different phases. The occurrence of N 2 trapped in vesicles and dissolved in silicates indicates that the oxygen fugacity ( f O2) was greater than the iron–wüstite buffer during the shock event. Ar–Ar dating of Bencubbin glass gives an age of 4.20 ± 0.05 Ga, which probably dates this impact event. The cosmic-ray exposure age is estimated at ∼40 Ma with two different methods. Noble gases present isotopic signatures similar to those of “phase Q” (the major host of noble gases trapped in chondrites) but elemental patterns enriched in light noble gases (He, Ne and Ar) relative to Kr and Xe, normalized to the phase Q composition. Nitrogen isotopic data together with 40Ar/ 36Ar ratios indicate mixing between a 15N-rich component (δ 15N = +1000‰), terrestrial N, and an isotopically normal, chondritic N. Bencubbin and related 15N-rich meteorites of the CR clan do not show stable isotope (H and C) anomalies, precluding contribution of a nucleosynthetic component as the source of 15N enrichments. This leaves two possibilities, trapping of an ancient, highly fractionated atmosphere, or degassing of a primitive, isotopically unequilibrated, nitrogen component. Although the first possibility cannot be excluded, we favor the contribution of primitive material in the light of the recent finding of extremely 15N-rich anhydrous clasts in the CB/CH Isheyevo meteorite. This unequilibrated material, probably carried by the impactor, could have been insoluble organic matter extremely rich in 15N and hosting isotopically Q-like noble gases, possibly from the outer solar system.

Interior textures, chemical compositions, and noble gas signatures of Antarctic cosmic spherules: Possible sources of spherules with long exposure ages

Abstract– The interior texture and chemical and noble gas composition of 99 cosmic spherules collected from the meteorite ice field around the Yamato Mountains in Antarctica were investigated. Their textures were used to classify the spherules into six different types reflecting the degree of heating: 13 were cryptocrystalline, 40 were barred olivine, 3 were porphyritic A, 24 were porphyritic B, 9 were porphyritic C, and 10 were partially melted spherules. While a correlation exists between the type of spherule and its noble gas content, there is no significant correlation between its chemical composition and noble gas content. Fifteen of the spherules still had detectable amounts of extraterrestrial He, and the majority of them had 3He/4He ratios that were close to that of solar wind (SW). The Ne isotopic composition of 28 of the spherules clustered between implantation‐fractionated SW and air. Extraterrestrial Ar, confirmed to be present because it had a 40Ar/36Ar ratio lower than that of terrestrial atmosphere, was found in 35 of the spherules. An enigmatic spherule, labeled M240410, had an extremely high concentration of cosmogenic nuclides. Assuming 4π exposure to galactic and solar cosmic rays as a micrometeoroid and no exposure on the parent body, the cosmic‐ray exposure (CRE) age of 393 Myr could be computed using cosmogenic 21Ne. Under these model assumptions, the inferred age suggests that the particle might have been an Edgeworth‐Kuiper Belt object. Alternatively, if exposure near the surface of its parent body was dominant, the CRE age of 382 Myr can be estimated from the cosmogenic 38Ar using the production rate of the 2π exposure geometry, and implies that the particle may have originated in the mature regolith of an asteroid.

An attempt to characterize phase Q: Noble gas, Raman spectroscopy and transmission electron microscopy in residues prepared from the Allende meteorite

We have prepared a HF–HCl residue and its oxidized residue of the Allende meteorite and have measured the elemental concentrations and the isotopic compositions of noble gases. In the HF–HCl reside, noble gases are enriched in colloidal fraction compared to the non-colloidal fraction by a factor of 2–4. The heavy noble gases were evidently lost after the oxidization, indicating that phase Q (carrier of planetary heavy noble gases) was removed by the oxidation. The Raman spectroscopic parameters show that the colloidal fraction of the HF–HCl residue is more amorphous compared to the non-colloidal fraction. As the ion irradiation converts carbon into a more amorphous form, our result indicates that the “plasma model” is more plausible than the “labyrinth model” as the origin of phase Q. TEM (Transmission Electron Microscope) observations also show such a trace of ion irradiation. While the TEM observations did not show any large difference between the HF–HCl residue and its oxidized residue, the Raman spectroscopic parameters changed discretely resulting from the oxidization. This observation indicates that the oxidization not only dissolved and removed oxidized carbon, but also changed the carbon structure itself to a more amorphous (disordered) state. The Raman spectroscopic results indicate the possibility that release of Q-gas during oxidation is not accompanied by mass loss and that the release of Q-gas simply resulted from rearrangement of carbon structure during oxidation.

Seawater-derived noble gases and halogens preserved in exhumed mantle wedge peridotite

Here we show how the Higashi-akaishi peridotite body in the Sanbagawa metamorphic belt, a sliver of the former mantle wedge of the eastern Eurasian plate margin, has exhumed subduction fluid noble gases and halogens with a marine pore-fluid signature from a depth of ∼ 100 km. Previous work has only considered that water subduction into the mantle wedge occurs via decomposition of hydrated minerals in altered oceanic crust and sediment. The striking similarities of the observed noble gas and halogen compositions with marine pore fluids require subduction and closed system retention of marine pore fluid to at least 100 km. The Higashi-akaishi peridotites appear to have frozen-in and preserved a previously unseen part of the deep water recycling process, requiring a reassessment of the dominant transport mechanism and source of water in subduction zones. Indeed, a small proportion of marine pore fluid, preserved in the downgoing hydrous peridotite, can account for the dominant heavy noble gas isotopic and elemental composition observed in the convecting mantle.

Source materials for inception stage Hawaiian magmas: Pb‐He isotope variations for early Kilauea

New noble gas and radiogenic isotopic compositions are presented for tholeiitic, transitional, and alkalic rocks from the submarine Hilina region on the south flank of Kilauea, Hawaii. The 3He/4He ratios for undegassed glass and olivine separates (11–26 Ra) contrast with those of postshield and rejuvenated alkalic lavas, consistent with the alkalic and transitional basalts at Hilina corresponding to early Kilauea magmas. Most early Kilauea samples contain highly radiogenic Pb isotopes compared with other Hawaiian rocks and therefore derive from a Hawaiian plume end‐member source (here referred to as the Hilina component) distinctive in that respect. Besides radiogenic Pb isotopes, the Hilina component has relatively low 3He/4He (&lt;12 Ra) among the Hawaiian magmas. Hawaiian inception stage magmas, including Hilina, Loihi, and deep Hana Ridge (east Maui), define a linear array in 206Pb/204Pb‐3He/4He isotope space, indicating that mixing between the Hilina and Loihi components (or their melts) dominates magmatism at the leading edge of the Hawaiian plume. The Hilina component's isotopic characteristics can be derived from young subduction‐recycled crust or metasomatised mantle. The isotopic differences between the geographically discriminated Kea and Loa trend volcanic chains, observed in shield stage lavas, are also seen in the inception stage magmas, suggesting that proportions of melts derived from the Hilina and Loihi components were different between the Kea and Loa trend volcanoes.

Noble gas study of the Saratov L4 chondrite

Abstract– We have determined the elemental abundances and the isotopic compositions of noble gases in a bulk sample and an HF/HCl residue of the Saratov (L4) chondrite using stepwise heating. The Ar, Kr, and Xe concentrations in the HF/HCl residue are two orders of magnitude higher than those in the bulk sample, while He and Ne concentrations from both are comparable. The residue contains only a portion of the trapped heavy noble gases in Saratov; 40 ± 9% for 36Ar, 58 ± 12% for 84Kr, and 48 ± 10% for 132Xe, respectively. The heavy noble gas elemental pattern in the dissolved fraction is similar to that in the residue but has high release temperatures. Xenon isotopic ratios of the HF/HCl residue indicate that there is no Xe‐HL in Saratov, but Ne isotopic ratios in the HF/HCl residue lie on a straight line connecting the cosmogenic component and a composition between Ne‐Q and Ne‐HL. This implies that the Ne isotopic composition of Q has been changed by incorporating Ne‐HL (Huss et al. 1996) or by being mass fractionated during the thermal metamorphism. However, it is most likely that the Ne‐Q in Saratov is intrinsically different from this component in other meteorites. The evidence of this is a lack of correlation between the isotopic ratio of Ne‐Q and petrologic types of meteorites (Busemann et al. 2000). A neutron capture effect was observed in the Kr isotopes, and this process also affected the 128Xe/132Xe ratio. The 3He and 21Ne exposure ages for the bulk sample are 33 and 35 Ma, respectively.

Noble gases in enstatite chondrites released by stepped crushing and heating

Abstract–Enstatite chondrites (ECs) were subjected to noble gas analyses using stepped crushing and pyrolysis extraction methods. ECs can be classified into subsolar gas‐carrying and subsolar gas‐free ECs based on the36Ar/84Kr/132Xe ratios. For subsolar gas‐free ECs, elemental ratios, and Xe isotopic compositions indicate that Q gas is the dominant trapped component, the Q gas concentration can be correlated with the petrologic type, reasonably explained by gas release from a common EC parental material during subsequent heating. Atmospheric Xe with sub‐Q elemental ratios is found in Antarctic E3s at 600–800 °C and through crushing. The132Xe released in these fractions accounts for 30–60% of the bulk concentrations. Hence, the sub‐Q signature is generally due to contamination of elementally fractionated atmosphere. Subsolar gas is mainly released (up to 78% of the bulk36Ar) at 1300–1600 °C and through crushing, suggesting that enstatite and friable phases are the host phases. Subsolar gas is isotopically identical to solar gas, but elementally fractionated. These observations are consistent with a previous study, which suggested that subsolar gas could be fractionated solar wind having been implanted into chondrule precursors (Okazaki et al. 2001). Unlike subsolar gas‐free ECs, the primordial gas concentrations of subsolar gas‐carrying ECs are not simply correlated with the petrologic type. It is inferred that subsolar gas‐rich chondrules were heterogeneously distributed in the solar nebula and accreted to form subsolar gas‐carrying ECs. Subsequent metamorphic and impact‐shock heating events have affected noble gas compositions to various degrees.

HELIUM ISOTOPE COMPOSITION OF FLUID INCLUSIONS HOSTED IN MASSIVE SULFIDES FROM MODERN SUBMARINE HYDROTHERMAL SYSTEMS

Noble gas compositions of fluid inclusions hosted in sulfides from two modern submarine hydrothermal sites of contrasting tectonic settings have been analyzed. The JADE Field (central Okinawa Trough, Japan) is located in an active intracontinental back-arc basin whereas the SO 134 field is related to the central spreading ridge of the North Fiji Basin, which represents an active, highly mature intra-ocean back-arc system. Previous conventional studies of fluid inclusions have revealed compelling evidence for subcritical phase separation of the hydrothermal fluids at both sites. Elemental abundances of Ne, Ar, Kr, and Xe in fluid inclusions in all studied samples of chalcopyrite and sphalerite from both hydrothermal sites indicate that the heavier noble gases are derived from ambient seawater. In contrast, helium concentrations are 1 to 2 orders of magnitude higher than expected for equilibrium solution in seawater, indicating that helium is predominantly derived from a magmatic source beneath the hydrothermal systems. The 3 He/ 4 He ratios in sphalerite-hosted fluid inclusions from the JADE Field and the SO 134 field are similar to those reported from related vent fluids and show that fluid inclusions reliably record the helium isotope ratio of the original hydrothermal fluid. However, in the North Fiji Basin, earlier “black smoker” activity and the formation of Fe-rich “Kies-type”sulfides seems to have been related to a more MORB-like volcanism, whereas the younger recent “white smoker” hydrothermal activity in the North Fiji Basin can be attributed to a hot spot magmatism in the study area.

Noble gas and carbon isotopic signatures of Cape Verde oceanic carbonatites: Implications for carbon provenance

We discuss noble gas (He, Ne, Ar, Kr, and Xe) and C isotope signatures of carbonatites from the Cape Verde Archipelago. These are the first noble gas compositions ever reported for oceanic carbonatites. The noble gas analyses were performed by crushing the calcite and apatite separates. Some of the analyzed calcites present low 4He/ 3He ratios (down to 46,700; R/Ra up to 15.5) that cannot be explained by the addition of cosmogenic 3He, demonstrating that carbonatite magmas came from a reservoir characterised by low time-integrated (U + Th)/ 3He. Such a reservoir is thought to be localised in the deep lower mantle, constraining the depth of origin of the Cape Verde plume. In contrast, apatite samples return highly radiogenic 4He/ 3He signatures due to their high Th and U contents. An in situ source for 4He in these apatites is further supported by air-like or lower 20Ne/ 22Ne ratios and relatively high 21Ne/ 22Ne ratios (up to 0.0485), which result from 21Ne (and 22Ne) formation by nucleogenic reactions. Some apatites plot to the left of the MORB line in the neon three-isotope diagram. This is explained by mass-fractionation processes since these apatites are also characterised by 38Ar/ 36Ar ratios lower than the air value or even than the range of values usually used to characterise the MORB-OIB field. Considering that carbonate recycling, which could be characterised by high levels of Te and Ba, would simultaneously increase with time the 129Xe and 130Xe, the observed 129Xe anomalies ( 129Xe/ 130Xe up to 6.84) cannot be explained by models calling upon crustal carbonate recycling. We attribute these anomalies to an ancient mantle origin by decay of the now extinct 129I. Moreover, experimental work has suggested that crustal carbonates are unlikely to be transported to deep lower mantle depths as a consequence of their removal by melting reactions at subduction. Thus, our noble gas data are indicative of a non-recycled origin for carbon, endorsing the role of primordial carbon in the genesis of Cape Verde carbonatites. This conclusion is supported by typical mantle δ 13 C values (− 8.0 to − 4.2‰) that are lighter than those characterising crustal inorganic carbonates. The fact that He is the noble gas element characterised by the highest solubility in magmas, the faster diffusion and the lower mineral/melt partition coefficients is taken as an explanation for its decoupling from the other noble gases, with He being the one better retaining a deep lower mantle signal.

Noble gas compositions and water contents in tektites from Hainan Island

Noble gas compositions and water contents in tektites from Hainan Island

Noble Gas Diffusion in Silicate Glasses and Melts

Studies of noble gas diffusion and solubility provide information on the internal structure of silicate glasses, i.e., the size distribution and interconnection of cavities within the framework (Doremus 1994). Additionally, such diffusion data are of interest for understanding degassing kinetics of glasses and melts. Water and carbon dioxide, the most important volatiles in magmas, can be present in silicate melts both as unreacted (molecular) species and as dissociated species (hydroxyl groups or carbonate groups, respectively) (e.g., Holloway and Blank 1994; Kohn 2000). The contributions of the different species to the overall transport of the volatiles are difficult to separate. In the case of rhyolitic melts it was found that the diffusivity of Ar is similar to that of molecular CO2 (Watson 1994; Behrens and Zhang 2001). Assuming that this similarity is a general property for silicate melts, Nowak et al. (2004) analyzed the diffusion behavior in synthetic rhyolitic to basaltic melts and estimated the relative abundance of carbonate and molecular CO2 in the melt. Knowledge of noble gas diffusivity is also of importance for interpretation of geochemical findings both on short and on large scales. For instance, the isotopic compositions of noble gases have been extensively used to identify long-lived heterogeneities within the Earth’s mantle (Allegre et al. 1986; Graham 2002). The noble gases He, Ne and Ar are highly incompatible elements, which will be preferentially incorporated into a melt during melting of the mantle, so at partial melt fractions relevant to mid-ocean ridges a primary melt will inherit the relative noble gas abundance pattern of the mantle source (Burnard et al. 2004). Helium, on the other hand, is a geochemical tracer for mantle signatures in rocks. Measurements of both helium abundance and the 3He/4He ratios in volcanic rocks, gases and waters …

Noble gas composition of the solar wind as collected by the Genesis mission

We present the elemental and isotopic composition of noble gases in the bulk solar wind collected by the NASA Genesis sample return mission. He, Ne, and Ar were analyzed in diamond-like carbon on a silicon substrate (DOS) and 84,86Kr and 129,132Xe in silicon targets by UV laser ablation noble gas mass spectrometry. Solar wind noble gases are quantitatively retained in DOS and with exception of He also in Si as shown by a stepwise heating experiment on a flown DOS target and analyses on other bulk solar wind collector materials. Solar wind data presented here are absolutely calibrated and the error of the standard gas composition is included in stated uncertainties. The isotopic composition of the light noble gases in the bulk solar wind is as follows: 3He/ 4He: (4.64 ± 0.09) × 10 −4, 20Ne/ 22Ne: 13.78 ± 0.03, 21Ne/ 22Ne: 0.0329 ± 0.0001, 36Ar/ 38Ar 5.47 ± 0.01. The elemental composition is: 4He/ 20Ne: 656 ± 5, and 20Ne/ 36Ar 42.1 ± 0.3. Genesis provided the first Kr and Xe data on the contemporary bulk solar wind. The preliminary isotope and elemental composition is: 86Kr/ 84Kr: 0.302 ± 0.003, 129Xe/ 132Xe: 1.05 ± 0.02, 36Ar/ 84Kr 2390 ± 150, and 84Kr/ 132Xe 9.5 ± 1.0. The 3He/ 4He and the 4He/ 20Ne ratios in the Genesis DOS target are the highest solar wind values measured in exposed natural and artificial targets. The isotopic composition of the other noble gases and the Kr/Xe ratio obtained in this work agree with data from lunar samples containing “young” (∼100 Ma) solar wind, indicating that solar wind composition has not changed within at least the last 100 Ma. Genesis could provide in many cases more precise data on solar wind composition than any previous experiment. Because of the controlled exposure conditions, Genesis data are also less prone to unrecognized systematic errors than, e.g., lunar sample analyses. The solar wind is the most authentic sample of the solar composition of noble gases, however, the derivation of solar noble gas abundances and isotopic composition using solar wind data requires a better understanding of fractionation processes acting upon solar wind formation.

Noble gases in spinel peridotite xenoliths from Mt Quincan, North Queensland, Australia: Undisturbed MORB-type noble gases in the subcontinental lithospheric mantle

Noble gas elemental and isotopic compositions have been determined for a suit of fresh spinel peridotite xenoliths from Mt Quincan, North Queensland (Northeast Australia). The observed helium, argon and xenon isotopic compositions are very similar to those reported for mid-ocean ridge basalts (MORB) in that they have following features: (1) 3He/ 4He ratios covering a narrow range from 8 Ra to 10 Ra, (2) 40Ar/ 36Ar (300–4000) and the 3He/ 36Ar (0.0012–0.07; corrected for some recent elemental fractionation) values that plot on the well-established MORB line, and (3) correlated excesses in 129Xe and 136Xe with regard to atmospheric xenon. MORB-like noble gas compositions suggest fluids ultimately derived from the asthenopshere are present in the subcontinental lithospheric mantle beneath northeastern Australia. The absence of any subduction-related noble gas signature precludes the preservation of fluids in the xenoliths added during Paleozoic subduction at the eastern margin of Australia. We also note that there is no mantle plume noble gas signature found in the Mt Quincan xenoliths, despite its location close to the suspected hotspot track across eastern Australia. These features are consistent with Sr and Nd isotopic characteristics of Mt Quincan xenoliths [Handler, M.R., Bennett, V.C., Carlson, R.W. 2005. Nd, Sr and Os isotope systematics in young, fertile peridotite xenoliths from northern Queensland, Australia: A unique view of depleted MORB mantle? Geochim. Cosmochim. Acta 69, 5747–5763.]. Thus, the fluid-entrapment is likely associated with subsequent rifting tectonic settings when asthenospheric fluid (and magma) could rise into the subcontinental lithospheric mantle and was able to overwrite any pre-existing signatures or even replace it with “MORB”-like mantle by physical removal of previously metasomatised lithospheric mantle. Additionally, differing from noble gas compositions previously considered typical for the subcontinental lithospheric mantle, the north Queensland data highlight the existence of noble gas isotopic heterogeneity in the subcontinental lithospheric mantle on a global scale.

Diffusive fractionation of noble gases in mantle with magma channels: Origin of low He/Ar in mantle-derived rocks

By crushing olivine and pyroxene phenocrysts in volcanic rocks from Kyushu Island, Japan, we determined 3He/ 4He of 3–7 Ra and 40Ar/ 36Ar of up to 1750. These values are lower than those of MORB. 4He/ 40Ar⁎ (down to 0.1) is much lower than the production ratio of 4He/ 40Ar⁎ (1–5), where an asterisk denotes correction for the atmospheric contribution. Such values are typical of mantle-derived samples from the island arcs and active continental margins. Although the origin of the low 3He/ 4He and 40Ar/ 36Ar of subcontinental mantle has been widely discussed, low 4He/ 40Ar⁎ has been given little attention. Actually, 3He/ 4He and 4He/ 40Ar⁎ of phenocrysts overlap with those of subcontinental mantle xenoliths. Although noble gas compositions of phenocrysts are affected considerably by diffusive fractionation in ascending magma, they have little effect on the noble gases in the mantle xenoliths because it takes 100 years for He/Ar fractionation of ca. 15% for a mantle xenolith with 5 cm diameter. Therefore, the low 4He/ 40Ar⁎ of the mantle xenoliths is inferred to result from another kinetic fractionation in the mantle. During generation and migration of magma in the mantle, lighter noble gases diffuse rapidly out into the magma. This diffusive fractionation can explain low 4He/ 40Ar⁎ and somewhat low 3He/ 4He in the residual mantle. Furthermore, the combination of the diffusive fractionation and subsequent radiogenic ingrowth explain the fact that data from subcontinental mantle xenoliths have extremely low 3He/ 4He and various 4He/ 40Ar⁎. Consequently, 4He/ 40Ar⁎ and 3He/ 4He in mantle-derived materials are proposed as indicators of the degree of noble gas depletion of the source mantle.

Iridium concentration and noble gas composition of Cretaceous-Tertiary boundary clay from Stevns Klint, Denmark

The Cretaceous-Tertiary (K-T) boundary about 65 million years ago records a mass extinction event caused by a bolide impact. K-T boundary clay collected from Stevns Klint, Denmark was investigated in this work. Iridium concentrations of eight clays across the K-T boundary were determined using a multiple gamma-ray analysis system after neutron activation. Anomalously high Ir concentrations were detected in five marl samples, with the highest concentration being 29.9 ppb. Four samples were analyzed for all noble gases. No extraterrestrial Ar, Kr, and Xe were discovered in any of the samples, although most of the 3He which was detected was extraterrestrial. Solar-like Ne was observed only in the sample SK4, which had an Ir concentration of 14.3 ppb, indicating the presence of micrometeorites. The solar-like Ne clearly did not originate from an asteroid/comet associated with the bolide impact, as that asteroid is thought to have been extremely large. Also, because there was no sign of a high accretion rate of micrometeorites at the boundary it could not be ascertained whether the solar-like Ne was related to a catastrophic event that led to the extinction of the dinosaurs.

Noble gas compositions of individual chondrules from the SaU 290 CH3 chondrite

Noble gas compositions of individual chondrules from the SaU 290 CH3 chondrite