Solubilities of noble gases in magnetite: implications for planetary gases in meteorites

Solubilities of noble gases in magnetite were determined by growing magnetite in a noble-gas atmosphere between 450 and 700°K. Henry's law is obeyed at pressures up to 10^?2 atm for He, Ne, Ar and up to 10^?5 atm for Kr, Xe, with the following distribution coefficients at 500° (cc STP g^?1 atm^?5): He 0.042, Ne 0.016, Ar 3.6, Kr 1.3, Xe 0.88, some 10²–10⁵ times higher than previous determinations on silicate and fluoride melts. Apparent heats of solution in kcal/mole are: He ?2.42 ±0.12, Ne ?2.20 ±0.10, Ar ?15.25 ±0.25, Kr ?13.0 ±0.3, Xe ?12-5 ± 0.5. These values, too, stand in sharp contrast with earlier determinations on melts which were small and positive, but are comparable to the values for clathrates. Presumably the gases are held in anion vacancies.Extrapolation of the magnetite data to the formation temperature of C1 chondrites, 360°K, shows that the Ar_p³⁶ content of Orgueil magnetite could be acquired by equilibrium solubility at a total nebular pressure of 4 × 10^?6 atm. In the absence of data for silicates (the principal host phase of planetary gas), an attempt is made to estimate the solubilities required to account for planetary gases in meteorites. These values do not appear grossly unreasonable in the light of the magnetite data, when structural differences between the two minerals are taken into account. It seems that equilibrium solubility may be able to account for four features of planetary gas: elemental ratios, amounts, correlations with other volatiles and retentive siting. It cannot account for the isotopic fractionation of planetary gas, however.     

Crystal-melt partitioning of noble gases (helium, neon, argon, krypton, and xenon) for olivine and clinopyroxene

Mineral-melt partition coefficients of all noble gases (^min/meltD_i) have been obtained for olivine (ol) and clinopyroxene (cpx) by UV laser ablation (213 nm) of individual crystals grown from melts at 0.1 GPa mixed noble gas pressure. Experimental techniques were developed to grow crystals virtually free of melt and fluid inclusions since both have been found to cause profound problems in previous work. This is a particularly important issue for the analysis of noble gases in crystals that have very low partition coefficients relative to coexisting melt and fluid phases. The preferred partitioning values obtained for the ol-melt system for He, Ne, Ar, Kr, and Xe are 0.00017(13), 0.00007(7), 0.0011(6), 0.00026(16), and , respectively. The respective cpx-melt partition coefficients are 0.0002(2), 0.00041(35), 0.0011(7), 0.0002(2), and . The data confirm the incompatible behaviour of noble gases for both olivine and clinopyroxene but unlike other trace elements these values show little variation for a wide range of atomic radius. The lack of dependence of partitioning on atomic radius is, however, consistent with the partitioning behaviour of other trace elements which have been found to exhibit progressively lower dependence of ^min/meltD_i on radius as the charge decreases. As all noble gases appear to exhibit similar ^min/meltD_i values we deduce that noble gases are not significantly fractionated from each other by olivine and clinopyroxene during melting and fractional crystallisation. Although incompatible, the partitioning values for noble gases also suggest that significant amounts of primordial noble gases may well have been retained in the mantle despite intensive melting processes. The implication of our data is that high primordial/radiogenic noble gas ratios (³He/⁴He, ²²Ne/²¹Ne, and ³⁶Ar/⁴⁰Ar) characteristic of plume basalt sources can be achieved by recycling a previously melted (depleted) mantle source rather than reflecting an isolated, non-degassed primordial mantle region.     

Regional groundwater focusing of nitrogen and noble gases into the Hugoton-Panhandle giant gas field, USA

The Hugoton-Panhandle giant gas field, located across SW Kansas and the Texas and Oklahoma panhandles in the USA, is the case type example of high nitrogen concentrations in a natural gas being linked with high helium concentrations. We collected 31 samples from producing wells in a north-south traverse of the 350-km-long field. The samples reflect the previously observed north-south change in ⁴He/N₂, with values changing from 0.020 to 0.049 respectively. ³He/⁴He, ²¹Ne/²²Ne, and ⁴⁰Ar/³⁶Ar vary between 0.14-0.25 Ra, 0.0373-0.0508, and 818-1156 respectively, and are caused by quantifiable contributions from mantle, crustal, and atmosphere-derived sources. The atmosphere-derived ²⁰Ne/³⁶Ar ratios are indistinguishable from groundwater values. The crustal ⁴He/²¹Ne* and ⁴He/⁴⁰Ar* ratios show a 60% excess of ⁴He compared to predicted production ratios in the crust and are typical of noble gases released from the shallow crust. The mantle ³He/N₂ and groundwater-recharge ³⁶Ar/N₂ ratios enable us to rule out significant magmatic or atmosphere contributions to the gas field N₂, which is dominantly crustal in origin.Correlated ²⁰Ne/N₂ and ⁴He/N₂ shows mixing between two distinct crustal N₂ components. One N₂ component (N₂*) is associated with the crustal ⁴He and groundwater-derived ²⁰Ne, and the other with no resolvable noble gas contribution. Measured δ¹⁵N_N2 values vary from +2.7‰ to +9.4‰. The N₂* and non-He-associated N₂ endmembers are inferred to have δ¹⁵N_N2 = −3‰ and +13‰ and contribute from between 25-60% and 75-40% of the nitrogen respectively. The non-He-associated nitrogen is probably derived from relatively mature organic matter in the sedimentary column. The δ¹⁵N_N2* value is not compatible with a crystalline or high-grade metamorphic source and, similar to the ⁴He, is inferred to be from a shallow or low metamorphic-grade source rock. ⁴He mass balance requires a regional crustal source, its association with significant magmatic ³He pointing to a tectonically active source to the west of the Hugoton system. The volume of groundwater required to source the ²⁰Ne in the gas field demonstrates the viability of the groundwater system in providing the collection, transport, and focusing mechanism for the ⁴He and N₂*. The N₂*/²⁰Ne ratio shows that the N₂* transport must be in the aqueous phase, and that the degassing mechanism is probably contact between the regional groundwater system and the preexisting reservoir hydrocarbon gas phase.     

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 × 10⁵ 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 N₂ 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 (δ¹⁵N = +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.     

The noble gas systematics of late-orogenic H2O-CO2 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 H₂O and CO₂ 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 CO₂ 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.     

Noble gases,nitrogen and cosmic ray exposure age of the Sulagiri chondrite

The Sulagiri meteorite fell in India on 12 September 2008,LL6 chondrite class is the largest among all the Indian meteorites.Isotopic compositions of noble gases(He,Ne,Ar,Kr and Xe) and nitrogen in the Sulagiri meteorite and cosmic ray exposure history are discussed.Low cosmogenic(~(22)Ne/~(21)Ne)_c ratio is consistent with irradiation in a large body.Cosmogenic noble gases indicate that Sulagiri has a 4πcosmic-ray exposure(CRE) age of 27.9 ± 3.4 Ma and is a member of the peak of CRE age distribution of IX chondrites.Radiogenic ~4He and ~(40)Ar concentrations in Sulagiri yields the radiogenic ages as 2.29 and4.56 Ca,indicating the loss of He from the meteorite.Xenon and krypton are mixture of Q and spallogenic components.     

Sorption of noble gases by solids,with reference to meteorites. III. Sulfides,spinels, and other substances; on the origin of planetary gases

To simulate trapping of noble gases by meteorites, we reacted 15 FeCr or FeCrNi alloy samples with CO, H₂O or H₂S at 350–720 K, in the presence of noble gases. The reaction products, including (Fe,Cr)₂O₃, FeCr₂S₄, FeS, C, and Fe₃C, were analyzed by mass spectrometry, usually after chemical separation by selective solvents. Three carbon samples were prepared by catalytic decomposition of CO or by dehydration of carbohydrates with H₂SO₄.The spinel and carbon samples were similar to those of earlier studies (Yang et al., 1982 and Yang and Anders, 1982), with only minor effects attributable to the presence of Ni. All samples sorted substantial amounts of noble gases, with distribution coefficients of 10^?1–10^?2 cm³ STP/g atm for Xe. On the basis of release temperature three gas components were distinguished: a generally dominant physisorbed component (20–80% of total), and two more strongly bound, chemisorbed and trapped components. Judging from the elemental pattern, the adsorbed components were acquired at the highest noble gas partial pressure encountered by the sample—atmosphere or synthesis vessel.Sulfides, particularly daubréelite, showed three distinctive trends relative to chromite or magnetite: the high-T component was larger, 30–70% of the total; $\text{Ne}\text{Xe}$ ratios were higher, by up to 10², possibly due to preferential diffusion of Ne during synthesis. In one synthesis, at relatively high P, the gases were sorbed with only minimal elemental fractionation, presumably by occlusion.Most of the features of primordial noble gases can be explained in terms of the data and concepts presented in the three papers of this series. The elemental fractionation pattern of Ar, Kr, Xe in meteorites, terrestrial rocks, and planets resembles the adsorption pattern on the solids studied: carbon, spinels, Sulfides, etc. The variation in $\text{Ne}\text{Ar}$ ratio may be explained by preferential diffusion of Ne. The high release temperature of meteoritic noble gases may be explained by transformation of physisorbed to chemisorbed gas, as observed in some experiments. The ready loss of meteoritic heavy gases on surficial oxidation (“Phase Q”) is consistent with adsorption, as is the high abundance. Extrapolation of the limited laboratory data suggests that the observed amounts of noble gases could have been adsorbed from a solar gas at 160–170 K and 10^?6–10^?5 atm, i.e. in the early contraction stages of the solar nebula. The principal unsolved problem is the origin of isotopically anomalous, apparently mass-fractionated noble gases in the Earth's atmosphere and in meteoritic carbon and chromite.     

Low solubility of He and Ar in carbonatitic liquids: Implications for decoupling noble gas and lithophile isotope systems

In order to asses the importance of carbonatitic liquids in transporting noble gases in the mantle, the solubilities of He and Ar in carbonatitic liquids were estimated from analyses of calcium-potassium carbonate glasses that had been synthesized at 1 bar and temperatures between 850 and 950 °C under He or Ar enriched atmospheres. Despite poor reproducibility related to difficulties synthesizing carbonatite glass, we have been able to estimate He and Ar solubilities in carbonatite liquids to be 1 × 10⁻⁸ and 2 × 10⁻⁹ mol g⁻¹ at 1 bar respectively (with ?50% uncertainty). Despite the significant uncertainties on these estimates, it is clear that the noble gases are not massively soluble in carbonatite liquids (within error, these solubilities are identical to their equivalent solubilities in tholeiitic melts). Assuming the results of these low pressure experiments can be applied to mantle conditions, it seems unlikely that carbonatite metasomatism per se transports noble gases within the mantle. It is nevertheless possible that partitioning of lithophile trace elements (including the important radioelements, U, K and Th) and noble gases between a carbonatitic melt and a silicate melt could effectively decouple lithophile and noble gas isotope systematics because the carbonatitic melt expressedly does not transport noble gases, yet is known to efficiently transport incompatible trace elements.     

Volatilization kinetics of silicon carbide in reducing gases: an experimental study with applications to the survival of presolar grains in the solar nebula

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO₂ (cristobalite or glass) were determined in H₂-CO₂, CO-CO₂, and H₂-CO-CO₂ gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wüstite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin (<1 μm thick), continuous layer of cristobalite. SiC exposed to IW-6.0 lacks such a layer (or its thickness is <0.01 μm), although some SiO₂ was found within pits and along incised grain boundaries.In H₂-CO₂ gas mixtures above ∼IW-3, the similarity of the SiC volatilization rate and of its dependence on temperature and fO₂ to that for SiO₂ suggests that SiC volatilization is controlled by volatilization of a SiO₂ layer that forms on the surface of the SiC. With decreasing log fO₂ from ∼IW-3 to ∼IW-6, the SiC volatilization rate is constant at constant temperature, whereas that for SiO₂ increases. The independence of the SiC volatilization rate from the gas composition under these conditions suggests that the rate-controlling step is a solid-solid reaction at the internal SiC/SiO₂ interface. For gas compositions more reducing than ∼IW-6, the SiC volatilization rate increases with decreasing fO₂, with both bare SiC surfaces and perhaps silica residing in pits and along incised grain boundaries contributing to the overall reaction rate.If the volatilization mechanism and reaction rate in the solar nebula were the same as in our H₂-CO₂ experiments at IW-6.0, then estimated lifetimes of 1-μm-diameter presolar SiC grains range from several thousand years at ∼900°C, to ∼1 yr at 1100°C, ∼1 d at 1300°C, and ∼1 h at 1400°C. The corresponding lifetimes for 10-μm SiC grains would be an order of magnitude longer. If the supply of oxidants to surfaces of presolar SiC grains were rate limiting—for example, at T > 1100°C for P^tot= 10⁻⁶ atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.     

Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity

The role of the oxygen fugacity on the incorporation of nitrogen in basaltic magmas has been investigated using one atmosphere high temperature equilibration of tholeiitic-like compositions under controlled nitrogen and oxygen partial pressures in the [C-N-O] system. Nitrogen was extracted with a CO₂ laser under high vacuum and analyzed by static mass spectrometry. Over a redox range of 18 oxygen fugacity log units, this study shows that the incorporation of nitrogen in silicate melts follows two different behaviors. For log fO₂ values between −0.7 and −10.7 (the latter corresponding to IW − 1.3), nitrogen dissolves as a N₂ molecule into cavities of the silicate network (physical solubility). Nitrogen presents a constant solubility (Henry’s) coefficient of 2.21 ± 0.53 × 10⁻⁹ mol g⁻¹ atm⁻¹ at 1425°C, identical within uncertainties to the solubility of argon. Further decrease in the oxygen fugacity (log fO₂ between −10.7 and −18 corresponding to the range from IW − 1.3 to IW − 8.3) results in a drastic increase of the solubility of nitrogen by up to 5 orders of magnitude as nitrogen becomes chemically bounded with atoms of the silicate melt network (chemical solubility). The present results strongly suggest that under reducing conditions nitrogen dissolves in silicate melts as N³⁻ species rather than as CN⁻ cyanide radicals. The nitrogen content of a tholeiitic magma equilibrated with N₂ is computed from thermochemical processing of our data set as

     

Laser microprobe for the study of noble gases and nitrogen in single grains: A case study of individual chondrules from the Dhajala meteorite

A laser microprobe capable of analysing nitrogen and noble gases in individual grains with masses less than a milligram is described. It can be used in both continuous wave (CW) mode, useful for stepwise heating of an individual grain, as well as in pulsed mode, useful for ablating material from a small selected area of a sample, for gas extraction. We could achieve low blanks (in ccSTP units) for 4He(4.8 x 10{-12}),²²Ne(1.0 x 10{-12}),³⁶Ar(1.0 x10 ^-13),⁸⁴Kr(2.9 x 10{-14}),¹³² Xe(2.6 x 10{-14}), and N (87 pg), using this system. Preliminary data for individual chondrules from the Dhajala meteorite show that noble gases and nitrogen from grains as small as 170 microgram can be analysed using the present laser microprobe setup. The amount of trapped neon in Dhajala chondrules is very small, and nitrogen in the chondrules is isotopically heavier as compared to the bulk meteorite.     

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (> 200 km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (³He/⁴He) ratios (up to 9.1 Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing ³He/⁴He values in the SCLM range (³He/⁴He < 7 Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (Y_N and Yb_N, respectively) indicate that fractional partial melting might have affected the local mantle by < 3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of ⁴He/⁴⁰Ar* (⁴⁰Ar* is corrected for atmospheric contamination) ranges from 4.9 × 10^− 4 to 3.6 × 10^− 1, which is below the typical production ratio of the mantle (⁴He/⁴⁰Ar* = 1–5); this range is however compatible with this fractional partial melting. The variable ³He/⁴He and ⁴He/⁴⁰Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO₂-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.     

The origin of rare gases on Earth: The noble gas ‘subduction barrier’ revisited

The origin of the Earth's atmosphere can be constrained by the study of noble gases in oceanic basalts. If it is clear that the mantle is degassed and formed part of the present atmosphere, it has been proposed that an important subduction of atmospheric noble gases in the mantle occurred during Earth's history, altering the primordial signature of the solid Earth. This subduction process has been suggested on the basis of the measurements of light xenon isotopes in CO₂ well gases. Moreover, the fact that the ³⁸Ar/³⁶Ar ratio is atmospheric in all oceanic basalts, even for uncontaminated samples (e.g. with high ²⁰Ne/²²Ne), may also suggest that a massive subduction of atmospheric argon occurred, if the primitive Earth had a solar-like ³⁸Ar/³⁶Ar. This also implies that the atmosphere suffered a massive gas loss accompanied by mass fractionation (e.g. hydrodynamic escape) after mantle degassing or that a late veneer with an atmospheric composition occurred. Such a hypothesis is explored for rare gases, by developing a model in which degassing and subduction of atmospheric noble gases started ∼4.4 Ga ago. In the model, both radiogenic and non-radiogenic isotopic ratios are used (e.g. ³⁸Ar/³⁶Ar and ⁴⁰Ar/³⁶Ar; ¹²⁴Xe/¹³⁰Xe and ¹²⁹Xe/¹³⁰Xe) to constrain the subduction flux and the degassing parameters. It is shown that subduction and massive contamination of the entire mantle is possible, but implies that the ⁴⁰Ar/³⁶Ar and the ¹²⁹Xe/¹³⁰Xe ratios were higher in the past than today, which is not observed in Archean samples. It also implies that the sediments and the altered oceanic crust did not loose their noble gases during subduction or that the contaminated mantle wedge is mixed by the convective mantle. Moreover, such a model has to apply to the oceanic island source, since this later shows the same signature of argon and xenon non-radiogenic isotopic ratios. A scenario where the isotopic compositions of the argon and xenon were settled before or during accretion is therefore preferred to the subduction.     

Dissolved noble gases in the porewater of lacustrine sediments as palaeolimnological proxies

This paper presents results from the numerical modelling of the transport of atmospheric noble gases (He, Ne, Ar, Kr, Xe), tritiated water and ³He produced by radioactive decay of ³H, in unconsolidated lacustrine sediment. Two case studies are discussed: (1) the evolution of ³H and ³He concentrations in the sediment porewater of Lake Zug (Switzerland) from 1953 up to the present; and (2) the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present. (1) Modelled ³H and ³He porewater concentrations are compared with recent data from Lake Zug. An estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical ³H and ³He concentrations in the water column are unknown. (2) The air/water partitioning of atmospheric noble gases is sensitive to water temperature and salinity, and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes. We investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes. Numerical modelling shows that heavy noble gases in sediment porewater, because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations, can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr. Noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes.     

Petrology,phase equilibria modelling,noble gas chronology and thermal constraints of the El Pozo L5 meteorite

We present the results of physical properties, petrography, bulk chemistry, mineral compositions, phase relations modelling and Noble gases study of the meteorite El Pozo. The petrography and mineral compositions indicate that the meteorite is an L5 chondrite with a low shock stage of S2-S3. Heterogenous weathering was preferentially along shock structures. Thermobarometric calculations indicate thermal equilibrium conditions between 768?°C and 925?°C at ～4 to 6?kb, which are substantially consistent with the petrological metamorphism type 5. A pseudosection phase diagram is relatively consistent with the mineral assemblage observed and PT conditions calculated. Temperature vs. fO₂ diagram shows that plagioclase compositional stability is very sensitive to Tschermack substitution in orthopyroxene, clinopyroxene and X_An plagioclase during the high temperature metamorphic process. Based on noble gases He, Ne, Ar and K contents a cosmogenic exposure age CRE of 1.9?Myr was calculated. The ²¹Ne would be totally cosmogenic, with no primordial Ne. The ²¹Ne/²²Ne value (0.97) is higher than solar value. According to the cosmogenic Ne content, we argue that El Pozo chondrite originally had a pre-atmospheric mass of 9–10?kg, which would have been produced by a later collision after the recognized collision of the L-chondrite parent body ～470?Ma ago.     

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

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

Isotopic fractionation of noble gases by diffusion in liquid water: Molecular dynamics simulations and hydrologic applications

Despite their great importance in low-temperature geochemistry, diffusion coefficients of noble gas isotopes in liquid water (D) have been measured only for the major isotopes of helium, neon, krypton and xenon. Data on the diffusion coefficients of minor noble gas isotopes are essentially non-existent and so typically have been estimated by a kinetic-theory model in which D varies as the inverse square root of the isotopic mass (m): D ∝ m^−0.5. To examine the validity of the kinetic-theory model, we performed molecular dynamics (MD) simulations of the diffusion of noble gases in ambient liquid water. Our simulation results agree with available experimental data on the solvation structure and diffusion coefficients of the major noble gas isotopes and reveal for the first time that the isotopic mass-dependence of all noble gas self-diffusion coefficients has the power-law form D ∝ m^−β with 0 < β < 0.2. Thus our results call into serious question the widespread assumption that the ‘square-root’ model can be applied to estimate the kinetic fractionation of noble gas isotopes caused by diffusion in ambient liquid water. To illustrate the importance of this finding, we used the diffusion coefficients determined in our MD simulations to reconsider the geochemical modeling of ²⁰Ne/²²Ne and ³⁶Ar/⁴⁰Ar isotopic ratios in three representative hydrologic studies. Our new modeling results indicate that kinetic isotopic fractionation by diffusion may play a significant role in noble gas transport processes in groundwater.     

Noble gases and nitrogen in Martian meteorites Dar al Gani 476, Sayh al Uhaymir 005 and Lewis Cliff 88516: EFA and extra neon

Meteorite “finds” from the terrestrial hot deserts have become a major contributor to the inventory of Martian meteorites. In order to understand their nitrogen and noble gas components, we have carried out stepped heating experiments on samples from two Martian meteorites collected from hot deserts. We measured interior and surface bulk samples, glassy and non-glassy portions of Dar al Gani 476 and Sayh al Uhaymir 005. We have also analyzed noble gases released from the Antarctic shergottite Lewis Cliff 88516 by crushing and stepped heating. For the hot desert meteorites significant terrestrial Ar, Kr, Xe contamination is observed, with an elementally fractionated air (EFA) component dominating the low temperature releases. The extremely low Ar/Kr/Xe ratios of EFA may be the result of multiple episodes of trapping/loss during terrestrial alteration involving aqueous fluids. We suggest fractionation processes similar to those in hot deserts to have acted on Mars, with acidic weathering on the latter possibly even more effective in producing elementally fractionated components. Addition from fission xenon is apparent in DaG 476 and SaU 005. The Ar-Kr-Xe patterns for LEW 88516 show trends as typically observed in shergottites - including evidence for a crush-released component similar to that observed in EETA 79001. A trapped Ne component most prominent in the surface sample of DaG 476 may represent air contamination. It is accompanied by little trapped Ar (²⁰Ne/³⁶Ar > 50) and literature data suggest its presence also in some Antarctic finds. Data for LEW 88516 and literature data, on the other hand, suggest the presence of two trapped Ne components of Martian origin characterized by different ²⁰Ne/²²Ne, possibly related to the atmosphere and the interior. Caution is recommended in interpreting nitrogen and noble gas isotopic signatures of Martian meteorites from hot deserts in terms of extraterrestrial sources and processes. Nevertheless our results provide hope that vice-versa, via noble gases and nitrogen in meteorites and other relevant samples from terrestrial deserts, Martian secondary processes can be studied.     

Rare gas isotopes and parent trace elements in ultrabasic-alkaline-carbonatite complexes, Kola Peninsula: identification of lower mantle plume component

During the Devonian magmatism (370 Ma ago) ∼20 ultrabasic-alkaline-carbonatite complexes (UACC) were formed in the Kola Peninsula (north-east of the Baltic Shield). In order to understand mantle and crust sources and processes having set these complexes, rare gases were studied in ∼300 rocks and mineral separates from 9 UACC, and concentrations of parent Li, K, U, and Th were measured in ∼70 samples. ⁴He/³He ratios in He released by fusion vary from pure radiogenic values ∼10⁸ down to 6 × 10⁴. The cosmogenic and extraterrestrial sources as well as the radiogenic production are unable to account for the extremely high abundances of ³He, up to 4 × 10⁻⁹ cc/g, indicating a mantle-derived fluid in the Kola rocks. In some samples helium extracted by crushing shows quite low ⁴He/³He = 3 × 10⁴, well below the mean ratio in mid ocean ridge basalts (MORB), (8.9 ± 1.0) × 10⁴, indicating the contribution of ³He-rich plume component. Magnetites are principal carriers of this component. Trapped ³He is extracted from these minerals at high temperatures 1100°C to 1600°C which may correspond to decrepitation or annealing primary fluid inclusions, whereas radiogenic ⁴He is manly released at a temperature range of 500°C to 1200°C, probably corresponding to activation of ⁴He sites degraded by U, Th decay.Similar ⁴He/³He ratios were observed in Oligocene flood basalts from the Ethiopian plume. According to a paleo-plate-tectonic reconstruction, 450 Ma ago the Baltica (including the Kola Peninsula) continent drifted not far from the present-day site of that plume. It appears that both magmatic provinces could relate to one and the same deep-seated mantle source.The neon isotopic compositions confirm the occurrence of a plume component since, within a conventional ²⁰Ne/²²Ne versus ²¹Ne/²²Ne diagram, the regression line for Kola samples is indistinguishable from those typical of plumes, such as Loihi (Hawaii). ²⁰Ne/²²Ne ratios (up to 12.1) correlate well with ⁴⁰Ar/³⁶Ar ones, allowing to infer a source ⁴⁰Ar/³⁶Ar ratio of about 4000 for the mantle end-member, which is 10 times lower than that of the MORB source end-member. In (³He/²²Ne)_PRIM versus (⁴He/²¹Ne)_RAD plot the Kola samples are within array established for plume and MORB samples; almost constant production ratio of (⁴He/²¹Ne)_RAD ≅ 2 × 10⁷ is translated via this array into (³He/²²Ne)_PRIM ∼ 10. The latter value approaches the solar ratio implying the non-fractionated solar-like rare gas pattern in a plume source.The Kola UACC show systematic variations in the respective contributions of in situ-produced radiogenic isotopes and mantle-derived isotopes. Since these complexes were essentially plutonic, we propose that the depth of emplacement exerted a primary control on the retention of both trapped and radiogenic species, which is consistent with geological observations. The available data allow to infer the following sequence of processes for the emplacement and evolution of Kola Devonian UACC: 1) Ascent of the plume from the lower mantle to the subcontinental lithosphere; the plume triggered mantle metasomatism not later than ∼700 to 400 Ma ago. 2) Metasomatism of the lithosphere (beneath the central part of the Kola Peninsula), including enrichment in volatile (e.g., He, Ne) and in incompatible (e.g., U, Th) elements. 3) Multistage intrusions of parental melts, their degassing, and crystallisation differentiation ∼370 Ma ago. 4) Postcrystallisation migration of fluids, including loss of radiogenic and of trapped helium. Based on model compositions of the principle terrestrial reservoirs we estimate the contributions (by mass) of the plume material, the upper mantle material, and the atmosphere (air-saturated groundwater), into the source of parent melt at ∼2%, 97.95%, and ∼0.05%, respectively.