Atmospheric Noble Gases Research Articles

The effect of the negative pressure of capillary water on atmospheric noble gas solubility in ground water and palaeotemperature reconstruction

Atmospheric-derived noble gases He, Ne, Ar, Kr and Xe dissolved in ground waters record the temperature at the moment of their recharge. Palaeotemperature reconstructions should include all the parameters affecting the soil air–ground water thermodynamic equilibrium. Particularly, the occurrence of excess air, that is ANG in excess compared to the equilibrium concentrations and often mass-fractionated compared to the atmospheric pattern, is likely produced by air entrapment during water infiltration in the nonsaturated zone (NSZ). Here, we show that water persisting in the NSZ has peculiar capillary properties controlled by the internal water pressure, which decreases (even down to negative pressures) with decreasing soil–air humidity. Decreasing water pressure induces mass-dependent increasing solubility of ANG from He to Xe. In addition, in nonsaturated soils where water and air coexist, air entrapment is favored allowing nonfractionated excess air. This capillary approach has been coded in an inverse-method program called Thermo_Inver, and tested on two well-characterized noble gas datasets used to constrain palaeotemperatures in Midwestern United States and Brazil during the last glacial maximum [Quat. Res. 43 (1995) 209; Science 269 (1995) 379]. Results of these simulations highlight that under suitable climatic conditions and in fine-porous media, the capillary pressure of water may be a prominent parameter controlling the addition of ANG to ground waters. Under these conditions, the recharge zone for ANG is the NZS and this water–air coexistence zone easily produces excess air.

Evidence for subduction-related components in the subcontinental mantle from low 3He/ 4He and 40Ar/ 36Ar ratio in mantle xenoliths from Far Eastern Russia

By applying both vacuum crushing and stepwise heating methods for the extraction of the noble gases, we have discovered 3He/ 4He ratios much lower than the atmospheric ratio (∼0.3 R A; R A is the atmospheric 3He/ 4He ratio of 1.4×10 −6) and relatively low 40Ar/ 36Ar ratios (<1000) in olivine separates from some subcontinental mantle-derived xenoliths from Far Eastern Russia. The low 3He/ 4He ratios cannot be explained by the addition of radiogenic 4He generated in-situ after the eruption of magma entraining the xenoliths. Furthermore, petrographic evidence suggests that incorporation of crustal fluids is not likely. Hence, it must reflect a feature of the Far Eastern Russian upper mantle. Spectroscopic and petrographic observations confirm that there are at least two compositionally distinct fluids in these xenoliths; liquid CO 2 inclusions and melt inclusions with shrinkage bubbles. Based on the crushing experiments, it is inferred that the inclusions of liquid CO 2 have a 3He/ 4He ratio similar to that of MORB, and the component with the low 3He/ 4He ratio is derived from the shrinkage bubbles in the melt inclusions. For the present samples, the 40Ar/ 36Ar ratios obtained by crushing were less than 1000, suggesting incorporation of atmospheric components in the source materials. Since low 40Ar/ 36Ar ratios were observed irrespective of the occurrence of the liquid CO 2 inclusions, the atmospheric component exists in the melt inclusions. Ne and Xe isotopes are also consistent with incorporation of atmospheric components. Since the Far Eastern Russia area was located at a subduction zone in the Jurassic–early Cretaceous Period, it is most likely that the melt inclusions displaying atmospheric noble gas characteristics together with low 3He/ 4He ratios have been derived from the Jurassic–Cretaceous subducted slab. Although we cannot exclude the possibility that low 3He/ 4He ratios are due to the existence of minor U-bearing minerals in the lithospheric continental mantle caused by metasomatism, we have no petrographical evidence for such minerals in this area. On the other hand, 3He/ 4He ratios observed in the liquid CO 2 inclusions, which are similar to the MORB-like value, might reflect the general character of the upper mantle. The Far Eastern Russian mantle may therefore be a MORB-like source that has been partly infiltrated by subduction-related fluids.

Atmospheric noble gases in lake sediment pore water as proxies for environmental change

Lake sediment pore water has been proposed as a noble gas archive for paleoenvironmental reconstruction, but appropriate experimental techniques have not been available until recently. Here we present noble gas concentrations measured in the sediment pore water of Lake Issyk‐Kul (Kyrgyzstan) which demonstrate for the first time the value of the sediment pore water archive for paleoclimate reconstruction. The noble gas profiles in the sediment indicate that the salinity of the lake water during the mid‐Holocene was more than twice its present value of 6.0 g/kg, implying a 200‐m lower lake level.

Migration Processes and Volatiles Delivery

Migration processes of comets and asteroids from the outer regions of the solar system, including the Edgeworth–Kuiper belt, are regarded as important mechanisms for the formation and evolution of the inner planets. These minor bodies may be responsible for the delivery of volatile matter to the inner planets and thus be responsible for the origin of life. We estimate that the cumulative mass of icy comets impacting on the Earth during the formation of the giant planets is similar to the mass of water in the Earth oceans, and that Mars acquired more water per unit planet mass than Earth. We find that these cometary objects mostly evolved from typical near-Earth orbits and Encke-type orbits with aphelia located inside the orbit of Jupiter, and played a greater role than those with Jupiter-crossing orbits. The relative importance of comets and chondrites in the delivery of volatiles is constrained by the observed fractionation patterns of atmospheric noble gas abundance.

Atmospheric xenon radioactive isotope monitoring

The Comprehensive Nuclear Test Ban Treaty (CTBT) organisation is implementing a world-wide monitoring network in order to check that the State Signatories comply with the treaty. One of the monitoring facilities consists of an atmospheric noble gas monitoring equipment. According to the requirements annexed in the treaty, the French Atomic Energy Commission (CEA) developed a device, called SPALAX TM, which automatically extracts xenon from ambient air and makes in situ measurements of the activities of four xenon radioisotopes ( 131mXe, 133mXe, 133Xe, 135Xe). The originality of this device is noticeable essentially in the gas sample processing method: thanks to the coupling of a gas permeator and of a noble gas specific adsorbent, it can selectively extract and concentrate xenon to more than 3×10 E 6. This process is carried out continuously without cryogenic cooling, without any regeneration time. The detection of the xenon radioactive isotopes is done automatically by high spectral resolution γ spectrometry, a robust technology well-suited for on-field instrumentation. In the year 2000, a prototype was involved in an international evaluation exercise directed by the CTBT organisation (CTBTO). This exercise demonstrated that the SPALAX TM equipment perfectly met the requirements of the CTBTO for such systems. On the basis of the continuous 24-h resolution record of the atmospheric xenon radioactive isotopes concentrations, the SPALAX TM system also demonstrated that ambient levels of 133Xe can fluctuate quickly from less than the detection limit to over 40×10 −3 Bq m −3. In order to build an industrial version of this equipment, the CEA entered into a partnership with a French engineering company (S.F.I., Marseille, France), which is now able to produce an industrial version of SPALAX TM, i.e. more compact and more efficient than the prototypes. The 133Xe minimum detectable concentration is 0.15×10 −3 Bq m −3 air per 24 h sampling cycle.

Dating ultra-deep mine waters with noble gases and 36Cl, Witwatersrand Basin, South Africa

Concentrations and isotopic ratios of dissolved noble gases, 36Cl, δD and δ 18O in water samples from the ultra-deep gold mines (0.718 to 3.3 km below the surface) in the Witwatersrand Basin, South Africa, were investigated to quantify the dynamics of these ultra deep crustal fluids. The mining activity has a significant impact on the concentrations of dissolved gases, as the associated pressure release causes the degassing of the fissure water. The observed under saturation of the atmospheric noble gases in the fissure water samples (70–98%, normalized to ASW at 20°C and 1013 mbar) is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature. Corrections for degassing result in 4He concentrations as high as 1.55 · 10 −1cm 3STP 4He g −1, 40Ar/ 36Ar ranging between 806 and 10331, and 134Xe/ 132Xe and 136Xe/ 132Xe ratios above 0.46 and 0.44, respectively. Corrected 134(136)Xe/ 132Xe and 134(136)Xe/ 4He-ratios are consistent with their production ratios, whereas the nucleogenic 4He/ 40Ar, and 134(136)Xe/ 40Ar ratios generally indicate that these gases are produced in an environment with an average [U + Th]/K-content 2–3 times above that of crustal average. In two scenarios, one considering only accumulation of in situ produced noble gases, the other additionally crustal flux components, the model ages for 14 individual water samples range from 13 to 168 Ma and from 1 to 23 Ma, respectively. The low 36Cl-ratios of (4–37) · 10 −15 and comparatively high 36Cl-concentrations of (8–350) · 10 −15 atoms 36Cl l −1 reflect subsurface production in secular equilibrium indicating an age in excess of 1.5 Ma or 5 times the half-life of 36Cl. In combination, the results suggest residence times of the fluids in fissures in this region (up to 3.3 km depth) are of the order of 1–100 Ma. We cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk.

Thermodynamic properties of solutions in metastable systems under negative or positive pressures

Metastable systems are created when the interface between the atmosphere (in which P atm = 1 bar) and water forms a spherical meniscus either concave toward the air (water filling capillaries, wherein P water < P atm) or convex toward the air (fog water droplet, wherein P water > P atm). Soil water, undergoing negative pressure (“capillary potential”) remains bound to the solid matrix (instead of flowing downward) by the capillary meniscus, concave toward the undersaturated dry atmosphere. The positive counterpart of tensile water in soils is the pressurized water contained in fine droplets suspended in oversaturated humid air, as in clouds. All these systems are anisobaric domains the phases of which have different pressures. Geochemical consequences of such characteristics are assessed here by calculating the consequences of the positive or negative water potential on the equilibrium constants of reactions taking place in stretched or pressurized aqueous solutions. Thermodynamic properties of aqueous species are obtained by using the TH model, used explicitly for positive pressures but extrapolated to negative ones for soil solutions. It appears that soil water dissolves gases, offering an alternative explanation of the observed enrichment of atmospheric noble gases in groundwater and of carbonic gas in the unsaturated zone below the root zone. Water droplets obviously show the opposite behavior, that is, a decreasing dissolutive capability with decreasing droplet size (water pressure increases), inducing some climatic consequences. An application of this approach to the solid-solution equilibria is performed by comparing experimental solubility of amorphous silica in unsaturated media on the one hand, to theoretical calculations taking account of the negative water pressure on the other hand. This comparison outlines the potential complexity of anisobaric situations in nature and the necessity to develop a suitable approach for solid pressure.

Degassing and contamination of noble gases in Mid‐Atlantic Ridge basalts

New He, Ne, Ar and CO2 stepped‐crushing data from the Mid‐Atlantic Ridge show that contamination of basalts by atmospheric noble gases involves three or more components: unfractionated air, fractionated air with high 36Ar/22Ne (≥45) and fractionated air with low 36Ar/22Ne (≤5). In addition, the magmatic noble gases trapped in these basaltic glasses are variably fractionated such that 4He/40Ar* (where the asterisk indicates corrected for atmospheric contamination based on all 36Ar being atmospheric in origin) is in the range 3–12. Single samples have a range in 4He/40Ar* with the highest ratios in the final crush steps, consistent with the most fractionated (highest 4He/40Ar*) volatiles trapped in the smallest vesicles. It is not possible to distinguish between batch and Rayleigh degassing mechanisms. The complexities of the contamination and magmatic fractionation processes means that it is not possible to estimate 40Ar/36Ar of the mantle source to these basalts other than it must be higher than the highest ratio measured (26,200 ± 5200). Noble gas/CO2 ratios are also variable. While some CO2 adsorption during crushing exaggerates the variations in He/CO2 and Ar/CO2, we show that it is not possible to account for the entire variation as an analytical artefact: some of the variation is present in the vesicles. Variations in He/CO2 cannot be attributed to solubility controlled degassing because of the broadly similar solubilities of He and CO2 in tholeiitic magmas. The large range in He/CO2 in these glasses (factor of 10) is not accompanied by indications of major changes in melting regime or source region chemistry, therefore is thought to reflect late‐stage (magmatic) fractionation of CO2 from the noble gases. It is not possible to identify an explicit mechanism, although both CO2 reduction (e.g., to hydrocarbons or graphite) and kinetic CO2‐noble gas fractionation could account for the variations.

Experimental investigations on the formation of excess air in quasi-saturated porous media

The formation of an excess of dissolved gas (“excess air”) in quasi-saturated media was studied by analyzing and interpreting dissolved noble gas concentrations in laboratory column experiments. Using quartz sand filled columns of 1 m length, two different experimental designs were realized. In the first, groundwater recharge was simulated by a unidirectional vertical water flow through the columns. In the second, groundwater level fluctuations in an aquifer zone without active infiltration were reproduced by cyclic water level fluctuations in the columns. The reproducible generation of excess air under these defined, near natural conditions was successful. Partial or complete dissolution of air bubbles entrapped in the quartz sand could be identified as the mechanism responsible for the generation of excess air. Depending on the experimental design, supersaturation of the dissolved atmospheric noble gases ranging between 1.4% ΔNe and 16.2% ΔNe was found. The measured noble gas patterns were interpreted using inverse modeling and conceptual gas exchange models and were compared to results of numerical simulations of gas bubble dissolution in water filled soil columns. The gas composition in most of the samples resembles either unfractionated pure atmospheric excess air or is fractionated in accordance with closed-system equilibration between entrapped air and surrounding water. In addition to the amount of entrapped air, the hydrostatic pressure exerted on the entrapped air bubbles is the dominating parameter responsible for the total amount of dissolved air. The composition of the excess air component is controlled by the water flow regime, the bubble size distribution, the initially dissolved gas concentrations and the initially entrapped gas composition.

Multiple atmospheric noble gas components in hydrocarbon reservoirs: a study of the Northwest Shelf, Delaware Basin, SE New Mexico

The Northwest Shelf of the Delaware Basin, SE New Mexico is the site of several large and productive oil and gas fields. The most productive reservoirs are located in the late Pennsylvanian Morrow and early Permian Abo formations. Production from the latter more important play is predominately from fluvial Abo red beds of the Pecos Slope Field. The oxidizing conditions implied by the reddish color of the formation require an external hydrocarbon source. To test the existing migration model for the region and constrain the location of potential hydrocarbon sources, we measured the elemental and isotopic composition of noble gases produced along with the hydrocarbons. We found the hydrocarbons to be highly enriched in radiogenic 4He, 40*Ar and nucleogenic 21*Ne [F( 4He) = 44,000–250,000; 40Ar/ 36Ar = 400–3145; 21Ne/ 22Ne = 0.044–0.071]. The greatest enrichments occur in the Pecos Slope gas fields. The hydrocarbons also contain three independent nonradiogenic noble gas components each with an atmospheric isotopic composition. One component is most likely air-saturated water (ASW). The second component is enriched in the heavy noble gases [F( 130Xe) > 8.5] and is derived from the hydrocarbon sources. The third component is enriched in Ne [F( 20Ne) > 0.8] that we believe is degassed from sources within the reservoirs. This component is correlated with but decoupled from the dominant source of radiogenic 4He and 40*Ar. Very high concentrations of 4He (up to ∼1% by volume) in the Pecos slope reservoirs require a source external to the reservoirs, such as the underlying Precambrian basement granites and sedimentary equivalents. Structural buckles cutting through the Pecos field may act as high flux vertical pathways for the radiogenic 4He. If the hydrocarbons in the Pecos slope fields have migrated northward from the deeper Delaware Basin, as suggested by compositional trends, then perhaps the buckles also play an important role in the distribution and filling of the Pecos slope reservoirs.

Neutron Capture Isotopes in the Martian Regolith and Implications for Martian Atmospheric Noble Gases

Impact-produced glasses in some martian meteorites have trapped significant amounts of the recent martian atmosphere. From literature data, we estimate that ∼9% of the trapped 80Kr in these meteorites was produced from neutron capture on 79Br. Estimates of neutron fluences made from 80Kr and 149Sm for bulk samples of meteorite EET79001 indicate that 80Kr excesses in the impact glass were not produced in situ. Theoretical calculations independently predict production of a large neutron-capture component of 80Kr and 36Ar in the martian regolith, and part of this component presumably escaped into the martian atmosphere. These calculations were made by using the Los Alamos High-Energy Transport Code to calculate the fluxes of galactic cosmic ray (GCR)-produced thermal neutrons as a function of depth in the uppermost 500 g cm −2 of the martian surface, and by adopting average Cl, Br, and I concentrations of the upper martian surface of ∼0.3%, ∼20 ppm, and ∼0.5 ppm, respectively. Combining these data with the appropriate neutron-capture cross sections, we calculate Mars global production rates of 80Kr n=2.4×10 16atoms sec −1, 36Ar n=5.5×10 18 atoms sec −1, and 128Xe n=3×10 13 atoms sec −1. Calculated global production rates of spallogenic 80Kr sp, and 36Ar sp, are smaller by factors of ∼770 and ∼29, respectively. It would require ∼330 Myr to produce an amount of 80Kr n equivalent to the amount inferred to be present today in the martian atmosphere (∼2.5×10 32 atoms). Production of these neutron-capture components probably has occurred over the past ∼4 Gyr, as only an atmospheric pressure substantially higher than today's would appreciably decrease the neutron flux in the regolith. Thus, most of the neutron-capture noble gases produced over time probably remain in the martian regolith and would make sensitive indicators of the time period a sample has resided near the martian surface. Assuming mixing of the martian surface to an average depth of 100 m, the predicted average regolith concentrations of 80Kr n, 36Ar n, and 128Xe n are ∼4×10 −9 cm −3 g −1, ∼1×10 −6 cm 3 g −1, and ∼5×10 −12 cm 3 g −1, respectively. If similar fractions of these neutron-capture isotopes have escaped into the atmosphere, they would comprise ∼3% and ∼0.2% of the present atmospheric inventories of 36Ar and 128Xe, respectively. The fractional excess of 80Kr n in ancient martian meteorite ALH84001 appears similar to that in shock-glass phases of young shergottite meteorites. If ALH84001 acquired its atmospheric gases ∼4 Gyr ago, this implies that, prior to that time, halogens were greatly concentrated at the martian surface by crustal formational and weathering processes, impacts efficiently degassed the regolith, and Mars did not have a significant atmosphere to shield the surface.

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

Low 14C activities in groundwater of the confined part of the Aquia aquifer in southeastern Maryland suggest that most of this water infiltrated at least 30,000 years ago. However, radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange, possibly with secondary calcite deposits in the formation, leading to overestimated 14C ages. Whereas the geochemistry of the Aquia aquifer complicates the application of the widely used 14C dating method, the accumulation of radiogenic He seems to provide a viable alternative for establishing a chronology. The quasi-linear increase of He concentrations with flow distance observed in the Aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic He. U and Th concentrations in Aquia sand were measured in order to determine the accumulation rate of 4He with sufficient confidence to establish a He time scale.Concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration. These noble gas temperatures (NGTs) clearly show the presence of water that infiltrated under much cooler conditions than at present. NGTs are correlated with chloride concentrations, corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal. In contrast, the stable isotope ratios δ180 and δD do not provide a clear record of past climatic changes in the Aquia aquifer and the correlation between NGTs and stable isotope ratios is weak. The NGT record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum (LGM) were (9.0 ± 0.6) °C colder than during the Holocene. This difference is slightly lower than estimates derived from pollen data for this region, but considerably larger than the rather uniform cooling of about 5°C indicated by noble gas studies in more southern locations of North America. The larger cooling is ascribed to the influence of the Laurentide ice sheet, which at its maximum extension came as close as 250 km to our study site.

Noble Gases in Lakes and Ground Waters

In contrast to most other fields of noble gas geochemistry that mostly regard atmospheric noble gases as ‘contamination,’ air-derived noble gases make up the far largest and hence most important contribution to the noble gas abundance in meteoric waters, such as lakes and ground waters. Atmospheric noble gases enter the meteoric water cycle by gas partitioning during air/water exchange with the atmosphere. In lakes and oceans noble gases are exchanged with the free atmosphere at the surface of the open water body. In ground waters gases partition between the water phase and the soil air of the quasi-saturated zone, the transition between the unsaturated and the saturated zone. Extensive measurements have shown that noble gas concentrations of open waters agree well with the noble gas solubility equilibrium according to (free) air/(free) water partitioning, whereby the aquatic concentration is directly proportional to the respective atmospheric noble gas abundance (Henry law, Aeschbach-Hertig et al. 1999b). In applications in lakes and ground waters the gas specific Henry coefficient can simplifying be assumed to depend only on temperature and salinity of the water. Hence the equilibrium concentrations of noble gases implicitly convey information on the physical properties of the water during gas exchange at the air/water interface, i.e., air pressure, temperature and salinity of the exchanging water mass. The ubiquitous presence of atmospheric noble gases in the meteoric water cycle defines a natural baseline, which masks other noble gas components until their abundance is sufficiently large that these components can be separated against the natural atmospheric background. For most classical geochemical aspects this typical feature of natural waters may look at first sight as a disadvantage. In fact it turns out to be advantageous because in most cases the noble gas abundance in water can be understood as a binary mixture of two …

Concomitant occurrence of primordial and recycled noble gases in the Earth’s mantle

Abstract Helium and argon concentrations and isotopic compositions have been determined for a suite of fresh alpine-type peridotites from the Horoman ultramafic complex, Hokkaido, Japan. Crushing and heating gas extractions on the samples yielded quite uniform 3He/4He and 4He/40Arna(=non-atmospheric) ratios of 7.7–9.2 Ra and 1.5–3.4, respectively. These values are consistent with those suggested for a mantle source for mid-oceanic ridge basalts (MORBs) (∼8 Ra, 1.4–4.5, respectively). In contrast to these apparent affinities with a MORB component, the observed 40Ar/36Ar ratios (330–470) appeared to be significantly lower than the suggested MORB source value of >40 000, indicating that virtually all 36Ar resides in fluid inclusions should be of atmospheric origin. Intriguingly, the amounts of 36Ar in the samples showed a clear positive correlation with those of 3He (=the isotope which can safely be regarded as a primordial mantle component). The correlation between primordial and atmospheric components cannot be explained by shallow-level atmospheric contamination. Rather, we infer that an atmospheric component is of recycled origin which had introduced into the mantle by the subducting slab when the complex was at a mantle wedge setting. Infiltration of metasomatic fluid or melt produced in the mantle wedge above the subducting slab would result in fluid inclusions being formed with noble gas signatures like those observed in the samples. This would be the first compelling evidence that some fraction of recycled atmospheric noble gases derived from the subducting slab could have been preserved in the mantle without returning to the atmosphere through arc volcanism.

MARTIAN VOLATILES: ISOTOPIC COMPOSITION, ORIGIN, AND EVOLUTION

Information about the composition of volatiles in the Martian atmosphere and interior derives from Viking spacecraft and ground-based measurements, and especially from measurements of volatiles trapped in Martian meteorites, which contain several distinct components. One volatile component, found in impact glass in some shergottites, gives the most precise measurement to date of the composition of Martian atmospheric Ar, Kr, and Xe, and also contains significant amounts of atmospheric nitrogen showing elevated 15N/14N. Compared to Viking analyses, the 36Ar/132Xe and 84Kr/132Xe elemental ratios are larger in shergottites, the 129Xe/132Xe ratio is similar, and the 40Ar/36Ar and 36Ar/38Ar ratios are smaller. The isotopic composition of atmospheric Kr is very similar to solar Kr, whereas the isotopes of atmospheric Xe have been strongly mass fractionated in favor of heavier isotopes. The nakhlites and ALH84001 contain an atmospheric component elementally fractionated relative to the recent atmospheric component observed in shergottites. Several Martian meteorites also contain one or more Martian interior components that do not show the mass fractionation observed in atmospheric noble gases and nitrogen. The D/H ratio in the atmosphere is strongly mass fractionated, but meteorites contain a distinct Martian interior hydrogen component. The isotopic composition of Martian atmospheric carbon and oxygen have not been precisely measured, but these elements in meteorites appear to show much less variation in isotopic composition, presumably in part because of buffering of the atmospheric component by larger condensed reservoirs. However, differences in the oxygen isotopic composition between meteorite silicate minerals (on the one hand) and water and carbonates indicate a lack of recycling of these volatiles through the interior. Many models have been presented to explain the observed isotopic fractionation in Martian atmospheric N, H, and noble gases in terms of partial loss of the planetary atmosphere, either very early in Martian history, or over extended geological time. The number of variables in these models is large, and we cannot be certain of their detailed applicability. Evolutionary data based on the radiogenic isotopes (i.e., 40Ar/36Ar, 129Xe/132Xe, and 136Xe/132Xe ratios) are potentially important, but meteorite data do not yet permit their use in detailed chronologies. The sources of Mars’ original volatiles are not well defined. Some Martian components require a solar-like isotopic composition, whereas volatiles other than the noble gases (C, N, and H2O) may have been largely contributed by a carbonaceous (or cometary) veneer late in planet formation. Also, carbonaceous material may have been the source of moderate amounts of water early in Martian history.

Lunar surface exposure models for meteorites Elephant Moraine 96008 and Dar al Gani 262 from the Moon

Abstract— We derived the cosmic‐ray and solar particle exposure history for the two lunar meteorites Elephant Moraine (EET) 96008 and Dar al Gani (DaG) 262 on the basis of the noble gas isotopic abundances including the radionuclide 81Kr. For EET 96008, we propose a model for the exposure to cosmic rays and solar particles in three stages on the Moon: an early stage ∼500 Ma ago, lasting less than 9 Ma at a shallow shielding depth of 20 g/cm2, followed by a stage when the material was buried, without exposure, until it was exposed in a recent stage. This recent stage, at a shielding depth in a range of 200–600 g/cm2, lasted for ∼26 Ma until ejection. This model is essentially the same as that previously found for lunar meteorite EET 87521; thus, pairing of the two Elephant Moraine lunar meteorites that were recovered on the same icefield in Antarctica is confirmed by our data. The cosmic‐ray‐produced isotopes, the trapped solar and lunar atmospheric noble gases, as well as the radionuclide 81Kr observed for the DaG 262 lunar meteorite are consistent with a one‐stage lunar exposure history. The average burial depth of the Dar al Gani material before ejection was within a range of 50–80 g/cm2. The exposure to cosmic rays at this depth lasted 500–1000 Ma. This long residence time for Dar al Gani at relatively shallow depth explains the high concentrations of implanted solar noble gases.

Sub sea floor boiling of Red Sea brines: new indication from noble gas data

Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (Ne, Ar atm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30% relative to the initial concentrations in ambient Red Sea Deep Water without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henry’s Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in He MORB and Ar MORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.

Cool glacial temperatures and changes in moisture source recorded in oman groundwaters

Concentrations of atmospheric noble gases (neon, argon, krypton, and xenon) dissolved in groundwaters from northern Oman indicate that the average ground temperature during the Late Pleistocene (15,000 to 24,000 years before present) was 6.5 degrees +/- 0.6 degrees C lower than that of today. Stable oxygen and hydrogen isotopic groundwater data show that the origin of atmospheric water vapor changed from a primarily southern, Indian Ocean source during the Late Pleistocene to a dominantly northern, Mediterranean source today. The reduced northern water vapor source is consistent with a drier Last Glacial Maximum through much of northern Africa and Arabia.

Analysis of deep-water exchange in the Caspian Sea based on environmental tracers

In order to quantify deep-water exchange in the Caspian Sea, the world's largest inland water body, water samples were analyzed for the transient tracers H 3 , He 3 , He 4 , CFC-11, CFC-12 and atmospheric noble gases. Measurements of temperature, salinity (calculated from conductivity for the ionic composition of Caspian Sea water), and dissolved oxygen were employed to investigate the processes responsible for deep-water renewal. The Caspian Sea consists of two deep basins, the southern and central basins, separated by a sill, and a shallow northern basin. The deep water (below 200 m) accounts for almost 60% of the total water mass. Below 200 m the concentrations of H 3 and He 3 are much lower in the southern basin than at the same depths in the central basin, but this is not the case for either of the CFCs. However, apparent water ages calculated from H 3 – He 3 and from CFC-12 concentrations are the same for the deep water of the southern and central basins, and yield deep-water exchange rates of approximately 7% per year for each of the two basins. This implies volume fluxes across the 200-m level of about 2220 km 3 yr −1 within the southern basin and 770 km 3 yr −1 within the central basin. Based on the apparent water ages, the oxygen depletion in the deep water is estimated to be about 0.35 mg l −1 yr −1. The processes responsible for deep-water exchange have not yet been identified conclusively. However, vertical temperature and salinity gradients observed during two expeditions, in September 1995 and 1996, suggest that within the southern and central basins large-scale convection cannot be triggered by seasonal cooling alone, but requires the surface water to be cold/saline or to contain high suspended sediment loads. In the central basin the increase in salinity occurring during ice formation in early winter is possibly sufficient to cause convection. In late summer, the horizontal transport of water from the upper 170 m of the central basin into the southern basin results in mixing down to 400 m. In winter this process might result in convection down to the maximum depth. Alternatively, the data are also consistent with the hypothesis that rare events cause large-scale convection down to the maximum depth in the southern and in the central basin simultaneously, followed by slight mixing that mainly affects only the top 500 m. According to apparent water ages from below 500 m, the last such major mixing event could have occurred in 1976 shortly before the water level of the Caspian Sea began to rise.

Noble gas record, collisional history, and pairing of CV, CO, CK, and other carbonaceous chondrites

Abstract— Concentration and isotopic composition of the light noble gases as well as of 84Kr, 129Xe, and 132Xe have been measured in bulk samples of 60 carbonaceous chondrites; 45 were measured for the first time. Solar noble gases were found in nine specimens (Arch, Acfer 094, Dar al Gani 056, Graves Nunataks 95229, Grosnaja, Isna, Mt. Prestrud 95404, Yamato (Y) 86009, and Y 86751). These meteorites are thus regolith breccias. The CV and CO chondrites contain abundant planetary‐type noble gases, but not CK chondrites. Characteristic features of CK chondrites are high 129Xe/132Xe ratios. The petrologic type of carbonaceous chondrites is correlated with the concentration of trapped heavy noble gases, similar to observations shown for ordinary chondrites. However, this correlation is disturbed for several meteorites due to a contribution of atmospheric noble gases, an effect correlated to terrestrial weathering effects.Cosmic‐ray exposure ages are calculated from cosmogenic 21Ne. They range from about 1 to 63.5 Ma for CO, CV, and CK classes, which is longer than exposure ages reported for CM and CI chondrites. Only the CO3 chondrite Isna has an exceptionally low exposure age of 0.15 Ma. No dominant clusters are observed in the cosmic‐ray exposure age distribution; only for CV and CK chondrites do potential peaks seem to develop at ∼9 and ∼29 Ma. Several pairings among the chondrites from hot deserts are suggested, but 52 of the 60 investigated meteorites are individual falls.In general, we confirm the results of Mazor et al. (1970) regarding cosmic‐ray exposure and trapped heavy noble gases. With this study, a considerable number of new carbonaceous chondrites were added to the noble gas data base, but this is still not sufficient to obtain a clear picture of the collisional history of the carbonaceous chondrite groups. Obviously, the exposure histories of CI and CM chondrites differ from those of CV, CO, and CK chondrites that have much longer exposure ages. The close relationship among the latter three is also evident from the similar cosmic‐ray exposure age patterns that do not reveal a clear picture of major breakup events. The CK chondrites, however, with their wide range of petrologic types, form the only carbonaceous chondrite group which so far lacks a solar‐gas‐bearing regolith breccia. The CK chondrites contain only minute amounts of trapped noble gases and their noble gas fingerprint is thus distinguishable from the other groups. In the future, more analyses of newly collected CK chondrites are needed to unravel the genetic and historic evolution of this group. It is also evident that the problems of weathering and pairing have to be considered when noble gas data of carbonaceous chondrite are interpreted.