Rare Gas Abundances Research Articles

Shock‐induced compaction, melting, and entrapment of atmospheric gases in Martian meteorites

The strongest evidence for a Martian origin of the SNC meteorites is the strong correlation between the rare gas abundances and isotopic compositions in shock‐induced melt pockets, and those measured for the Martian atmosphere. However, the formation of melt pockets and the entrapment of atmospheric gases remain poorly understood. Here we report the presence in the melt pockets of three Martian meteorites of the hollandite‐structured high‐pressure polymorph of feldspar. These occurrences set constraints on the continuum pressure (21–25 GPa), the local temperature increase (2000–2500 K) and the energy delivered during shock. We then test a mechanism for melt pocket formation by compaction of pre‐existing porous precursors. The model explains the local temperature increase required for melting and the presence of an atmospheric component in pores that were connected with the Martian atmosphere prior to the shock event.

D’Orbigny: A non-igneous angritic achondrite?

D’Orbigny is the sixth and by far the largest angrite known. Its bulk chemical and mineral chemical compositions, rare gas abundances and oxygen and rare gas isotope compositions fit the compositional ranges known from other angrites. It is, however, peculiar with respect to three features: the abundance of hollow shells, the presence of abundant open druses and the abundant presence of glasses. The shape, structure and texture of D’Orbigny and its mineral and bulk chemical compositions indicate an unusual genesis under changing redox conditions. In our view, data and observations are incompatible with an igneous origin of this rock but are suggestive of a complex growth and metasomatism scenario. The sequence of events apparently began with the formation of spheres of a phase which later vanished and therefore is unknown but could have been CaS. On top of these spheres (sizes from < 1–30 mm) olivine-anorthite intergrowths precipitated forming compact shells and fluffy protrusions. Aggregation of these objects plus occasional large plates made of the same intergrowths led to formation of a highly porous object with abundant large open space between the olivine-anorthite intergrowths. The aggregate also included previously formed olivines, olivinite rocks and Al-spinels. The latter carry highly porous decomposition rims of Cr-enriched Al-spinel and record mildly oxidizing conditions prevailing very early in D’Orbigny’s history. Conditions changed (with falling T?) and became oxidizing causing the phase(s) that constituted the spheres to become unstable. Their breakdown liberated large amounts of Ca and trace elements which at least in part re-precipitated by reacting with Si and Mg from the vapor to form augites that grew into the open space thus forming augite druses. Also, some of the preexisting olivine was converted into augite, which is very rich in refractory lithophile trace elements (abundances ∼ 10 × CI). Augites grew mainly under oxidizing conditions leading to atomic Fe/(Fe+Mg) ratios of about 0.44. Finally, conditions became highly oxidizing and strongly mobilized Ca from a source that apparently became unstable. The high partial pressures of Ca and Fe (and also Ti) led to precipitation of Ca-olivine and kirschsteinite (∼Fo1La20 and ∼Fo1La33, respectively) and of titaniferous aluminous hedenbergite—atomic Fe/(Fe+Mg) ∼ 0.97. Ulvöspinel and sulfides were also precipitated. Because the original phase(s) forming the early spheres vanished during these oxidizing events, the shells remained empty. In this scenario, D’Orbigny provides us with a record of changing conditions ranging from extremely reducing to highly oxidizing and with a record of the formation of an achondritic rock from a chondritic source. Angrites bear many similarities with CAIs, texturally, mineralogically and chemically. Possibly, they can be seen as CAIs, which grew larger than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. They reproduce most of the textures and structures of CAIs: crystallized liquids (Asuka 881371, LEW 87051), metasomatic granoblastic rocks (LEW 86010, Angra dos Reis?) and aggregates (D’Orbigny). In addition, all angrites record metasomatic alterations, subsolidus processing after formation, also similar to what is recorded by most CAIs. Obviously, they missed the alkali metasomatic event recorded by many CAIs but they record a siderophile—lithophile element separation event that is not recorded by CAIs.

Vesiculation and vesicle loss in mid-ocean ridge basalt glasses: He, Ne, Ar elemental fractionation and pressure influence

Sarda and Graham (1990) proposed that in mid-ocean ridge basalts (MORBs), degassing occurs through equilibrium vesiculation followed by various extents of vesicle loss. This model predicts that in a bulk sample of MORB glass with vesicles, the rare gases represent a binary mixture between a vesicle component and a component dissolved in the melt. As vesiculation is expected to produce very different rare gas concentrations and elemental ratios in gas and melt, binary mixing systematics should be recorded in the MORB rare gas abundance data. Indeed, a large range of 4He/40Ar∗ ratios was known to exist, but these binary mixing systematics remained elusive because helium was used as a proxy for rare gas abundance because helium is not affected by air addition. Here we show that using Ar instead of He, the 4He/40Ar∗ ratio is higher where the Ar concentration is lower, as expected from simple binary mixing systematics.Taking advantage of the growing Ne database, we further show that the predicted binary mixing is recorded by the He-Ar and He-Ne couples, provided He concentration is not used to trace vesicle abundance. This is because a significant part of helium remains in the melt due to its higher solubility. In contrast, Ar or Ne concentrations, which can both be corrected for air addition, clearly trace vesicles and yield binary mixing patterns that hold for ridges worldwide. The model of vesiculation and vesicle loss thereby finds geochemical support in the rare gas abundance data.The He-Ne-Ar concentration data is best explained by assuming the ratio of helium to neon or argon solubility is about 5 to 15 times higher than values measured in 1 bar laboratory experiments, due to higher He and lower Ne and Ar solubilities. We propose that this is a pressure effect, and vesiculation mainly occurs during magma ascent in the mantle after melting.

Basin scale natural gas source, migration and trapping traced by noble gases and major elements: the Pakistan Indus basin

He, Ne and Ar concentrations, He and Ar isotopic ratios, carbon isotopic ratios and chemical compositions of hydrocarbon gases were measured in natural gas samples from gas-producing wells in the Indus basin, Pakistan, where no oil has ever been found. 3He/ 4He ratios are in the range 0.01–0.06 Ra (Ra is the atmospheric value of 1.38×10 −6) indicating the absence of mantle-derived helium despite the Trias extension. 40Ar/ 36Ar ratios range from 296 to 800, consistent with variable additions of radiogenic argon to atmospheric, groundwater-derived argon. Rare gas concentrations show large variations, from 6×10 −5 to 1×10 −3 mol/mol for 4He and from 3×10 −7 to 3×10 −5 mol/mol for 36Ar. In general, 36Ar concentrations are high compared to literature data for natural gas. CO 2 and N 2 concentrations are variable, ranging up to 70 and 20%, respectively. Mantle-derived He is not observed, therefore CO 2 and N 2 are not mantle-derived either. Hydrocarbon gas maturity is high, but accumulation efficiency is small, suggesting that early-produced hydrocarbons, including oil, were lost as well as mantle helium. This is consistent with the generally late, Pliocene, trap formation, and explains the high N 2 concentrations, since N 2 is the final species generated at the end of organic matter maturation. Based on δ 13C data, CO 2 originates from carbonate decomposition. Very elevated 20Ne/ 36Ar ratios are found, reaching a maximum of 1.3 (compared to 0.1–0.2 for air-saturated water and 0.5 for air), and these high values are related to the lowest rare gas concentrations. We suggest that this highly fractionated signature is the trace of the past presence of oil in the basin and appeared in groundwater. We propose a model where oil–water contact is followed by gas–water contact, both with Rayleigh distillation for rare gas abundance ratios, thereby generating the fractionated 20Ne/ 36Ar signature in groundwater first and transferring it to gas later. Assuming the gas–water contact occurred shallower than present reservoir depths, this model explains the generally high 36Ar concentrations and low CH 4/ 36Ar ratios compared to other studies on younger basins. It thus provides a historical perspective on fluid transfer in a sedimentary basin, where a gas accumulation may have been buried to greater depth since formation. Rare gas and major element data point to mixing between two gas pulses produced successively. The very CO 2–N 2-rich gases are terminal products of organic matter maturation which have been trapped after important migration. This gas was followed by a more typical thermogenic gas which mixed with it.

The evolution of terrestrial volatiles: a view from helium, neon, argon and nitrogen isotope modelling

In this contribution, we have developed an evolutionary model in order to identify and quantify processes which were able to reproduce rare gas and nitrogen isotopic abundances in the main terrestrial reservoirs. The following processes appear to have played an important role in the history of terrestrial volatiles. During accretion, impact degassing could have released ≈95% of the initial rare gas abundances, which were quite similar to those typical of solar wind-implanted gases. After the major phase of accretion and core segregation (4.50 Ga ago), some parts of the upper mantle were partially melted by giant impact(s) and experienced vigorous convection and solubility-controlled degassing. More than 99% of the volatile species initially presented in the upper mantle–atmosphere reservoirs were lost during this period (from 4.50 to 4.30 Ga ago). This loss was accompanied by elemental and isotopic fractionation of residual atmospheric constituents. The atmosphere became retentive for Xe at 4.40 Ga but degassing, loss and fractionation of lighter rare gases and nitrogen might have taken place some time afterwards. Therefore each gas might have undergone fractionation to various extent at different times. After closure of the atmosphere (at ≈4.30 Ga) for all but the lightest (H, He) constituents, the lower mantle supplied the upper mantle with minute amounts of parent incompatible elements, rare gases and nitrogen, between ∼1% (4.3 Ga ago) and ∼0.2% (at present) of their total amount in the lower mantle per Ga. The post-atmosphere closure flux of liquid silicates from the upper mantle, analogue to the present-day MOR flux of basaltic melts, decreased by a factor of ∼100, from ∼5·10 18 (4.3 Ga ago) to 6·10 16 g a −1 (at present). The ratio of 36Ar(now)/ 36Ar(4.3 Ga) um ∼10 −4 illustrates the total degree of upper mantle degassing yielded by this flux. This rate of degassing corresponds to a present-day ratio of 40Ar/ 36Ar um >10 6, if no fluxes from the lower mantle and the crustal–atmospheric reservoirs had operated, and the ratios of radiogenic over primordial species could have been higher in the past than those at present. The model postulates that nitrogen trapped in the Earth–Atmosphere system was initially depleted in 15N relative to present-day atmospheric composition (ATM). Atmospheric escape enriched the ancient atmosphere in 15N, resulting in a δ 15N isotopic composition of +2.5 parts per mil (ATM) 4 Ga ago. Subsequent degassing of mantle nitrogen allowed this element to reach its present-day composition in air. Because the upper mantle is extremely depleted in volatile elements, their transfer from the lower mantle is sufficient to maintain primordial rare gases and nitrogen abundances in the upper mantle approximately at a steady state. Decays of parent radioactive elements, U um , Th um , and K um , contribute radiogenic nuclides. Recycling fluxes of (sub)surface materials into the upper mantle transfer surface volatiles, ∼6% of surface-derived (atmosphere+sediments) N and ∼0.03% of atmospheric 36Ar ah per Ga. Consequently, the isotopic composition of mantle nitrogen varied from its initial δ 15N value of −30 parts per mil to its present-day upper mantle value of −5 parts per mil. The consequence of nitrogen recycling in the Earth–Atmosphere system is therefore partial re-homogenisation of N-isotopes, but this process was not efficient enough to have erased early heterogeneity. Thus the present contribution proposes early formation of the Earth's atmosphere by a combination of degassing, dissipation, and fractionation processes. Primordial rare gases and nitrogen were set in this reservoir around 4.3 Ga ago and further mantle outgassing has contributed less than 3% of these species since that time.

Mantle neon and atmospheric contamination

The apparent distinction between atmospheric and mantle 20Ne 22Ne ratios may provide a technique to quantify air contamination in mantle-derived materials. In the absence of mantle nuclear reactions, which produce either 20Ne or 22Ne in substantial quantities, it is likely that the entire mantle is characterized by a single, uniform 20Ne 22Ne ratio; a value of around 12.5 is suggested by analyses of MORBs, OIBs, diamonds and xenoliths. If this premise is correct, then any measured 20Ne 22Ne ratios in mantle samples that are lower than this must result from addition of an air component, with 20Ne 22Ne= 9.8 . This is most likely a syn- or post-eruptive contaminant. The degree of air contamination inferred from 20Ne 22Ne ratios is generally small for diamonds, but is increasingly significant for MORBs and OIBs; many OIB's may carry > 90% air neon. We calculated “air-neon corrected” 21Ne 22Ne and 40Ar 36Ar ratios for the highly degassed MORB mantle and for the less degassed (high 3He 4He ) “plume” reservoir. The inferred MORB composition is indistinguishable from measurements of some gas-rich glasses. The calculated plume composition is similar to the least air-like measurements from ocean islands, but is less air-like than has been proposed previously. This plume composition is not consistent with a completely undegassed reservoir. From these corrected mantle compositions, we calculated the relative time-integrated rare gas abundances in the mantle, using a simple evolutionary model, which simultaneously considers the isotopic compositions of He, Ne, Ar and Xe. The model shows that both MORB and plume reservoirs have evolved with nearly solar elemental abundances. This provides strong support to suggestions based on Ne isotopes that the Earth accreted with gases nearly solar in composition. Importantly, the inferred mantle Ne/Ar ratios are much higher than atmospheric, which is consistent with simultaneous fractionation of both the atmospheric neon isotope ratio and the Ne/Ar ratio by massive hydrodynamic escape. Mixing between MORB and plume reservoirs (with our calculated elemental and isotopic abundances), plus varying amounts of added air, can account for the rare gases in nearly all mantle-derived rocks.

Mid-ocean ridge popping rocks: implications for degassing at ridge crests

The vesicle size distribution (VSD) and rare gas abundances in popping rocks from 14°N on the Mid-Atlantic Ridge provide constraints on the behavior of volatiles during ridge crest volcanism. These popping rocks, which contain 16–18 volume percent vesicles, are rare mid-ocean ridge basalt (MORB) magmas which appear to have retained much of their volatile inventory. The logarithm of vesicle population density displays the same linear correlation with decreasing size in two of the samples studied. This implies that continuous and simultaneous nucleation and bubble growth have occurred during magma ascent, with no significant perturbations due to accumulation, coalescence or loss of bubbles. In contrast, most MORB magmas display low vesicularities and we suggest that they have suffered some degree of pre-eruptive vesicle loss. We tentatively propose that large vesicles are produced by coalescence when MORB melt is at rest in chambers and conduits, and may be lost during early gas-rich episodes. Most MORB would represent residual liquids which erupt after vesicle loss has occurred, whereas popping rocks would represent a rare case where physical sorting of vesicles from melt did not occur, because storage in a magma chamber did not occur. The rare gas concentrations in the studied popping rocks are the highest yet measured in glassy ridge basalts ([He] > 50 μccSTP/g). The rare gas abundance pattern of these popping rocks probably resembles the pattern for non-vesiculated MORB magma and potentially reflects that of the depleted mantle source. This pattern is similar to the “mean MORB” pattern (computed from MORB glasses with 40Ar/ 36Ar > 10,000) although a higher enrichment in He (and possibly Ne) compared to the heavier rare gases is observed in MORB. The overall similarity in abundance patterns for MORB and popping rocks indicates that vesiculation and vesicle loss do not fractionate the Ar Kr Xe relative abundances from those in non-vesiculated magma, and that the modern flux ratios of these gases at ridges are similar to their elemental ratios in the depleted mantle. The degassing flux of He at ridge crests estimated from the MORB He deficit relative to popping rocks is comparable to the flux derived from the 3He budget for the abyssal ocean. This suggests that degassing at ridges may be strongly influenced by the dynamics and style of submarine volcanism.

Elemental and isotopic abundances of rare gases in natural gases obtained by a quadrupole mass spectrometer.

An attempt was made to determine both elemental and isotopic abundances of rare gases in natural gases by means of a conventional quadrupole mass spectrometer (QMS). Elemental abundances and isotopic ratios of Ar, Kr and Xe on the atmospheric concentration level were successfully measured by the present method. Reproducibility of 10–20% was obtained for 4He, 20Ne, 36Ar, 84Kr and 132Xe measurements. Isotopic ratios of Ar, Kr and Xe were determined within about 2% experimental errors, except for 78Kr/84Kr, 124Xe/132Xe and 126Xe/132Xe. Sensitivity and reproducibility of the measurements were examined by repeating runs of the atmospheric air. In order to verify the applicability of the present method to terrestrial natural gas analyses, eight typical samples of natural gases in Japan, with differences in major compositions, were measured. The values observed by a QMS in this work are generally in good agreement with those obtained by magnetic deflection mass spectrometers. Enrichment of radiogenic 40Ar was observed in some CO2-rich natural gas samples. Observed Kr and Xe isotopic ratios in N2-rich gases and CH4-rich gases coincided with the atmospheric ratios within experimental error.

Rare gas isotopes and mass fractionation: An indicator of gas transport into or from a magma

A simple model of mass fractionation may explain the isotopic ratios of rare gases in volcanic materials. Single-stage mass fractionation of atmospheric rare gases predicts an upper limit for 20Ne/ 22Ne of 10.3 and a lower limit for 40Ar/ 36Ar of 280. The rare gas data in volcanic materials seem to support this interpretation. Relatively low 40Ar/ 36Ar ratios, as low as 282, have been observed in recent Japanese volcanic rocks. Such a low 40Ar/ 36Ar ratio may be explained by mass fractionation of the atmospheric value if the rare gases represent those which were transported into the magma chamber with other volatile elements. Both the amounts and the fractionated rare gas abundance pattern of lighter elements which are observed in pumices from the recent eruption of Mt. Usu, Southern Hokkaido, Japan, suggest the possibility of air injection into its magma chamber. Thus, the fractionation of rare gases in volcanic materials may be a common occurrence, and it must be considered in models for the origin of isotopic differences between rare gases in volcanic materials and the atmosphere.

Helium and xenon in deep-sea basalts as a measure of magmatic differentiation

There has been much recent interest in the rare gas abundances of terrestrial materials, due mainly to the discovery of ‘excess’ radiogenic Ar and He trapped in deep-sea glasses1–4. The abundances of these gases indicated that such materials might effectively trap the internal Earth's atmosphere, and evidence of trapped primordial gases has by now been found not only in deep-sea glasses5–10, but also in xenolithic inclusions11–15, deep ocean waters16, CO2 well gas17–19, volcanic gas20 and diamonds21. The rare gas elemental abundances measured in these samples may not quantitatively reflect the deep-Earth pattern. Several elemental fractionation processes have been suggested, ranging from selective enrichment of the lighter gases in an upward-rising magma22,23 to posteruptive diffusion loss which would selectively deplete the lighter gases24. Each of these processes will affect concentrations of the helium and xenon isotopes in a characteristic manner, thus leaving in the glasses an isotopic fingerprint of what has occurred. New data are reported here which indicate extremely small concentrations of fissiogenic xenon in deep-sea basalts containing large concentrations of trapped excess radiogenic helium and argon. The resulting high 4Her/136Xef ratios are diagnostic of frac-tionation during the magmatic processes leading to eruption rather than of posteruptive loss of gas (which would show the opposite effect). The results indicate that studies of volatiles in general and of the rare gases in particular in such mantle-derived samples, even in those with no subsequent alteration or contamination, may be profoundly influenced by mass-dependent pre-eruptive effects. The consequences of this fractionation for the deep-Earth K/U ratio are discussed.

Palaeoatmospheric argon in Rhynie chert

THE isotopic compositions and relative abundances of rare gases in the Earth's atmosphere impose rigid constraints on models of atmospheric evolution. Appreciable quantities of argon from the Devonian atmosphere have been extracted from silicified fossil plants and the isotopic composition of this argon implies that the Earth must have been strongly outgassed early in its history.

Implications of the Viking results for volatile outgassing from Earth and Mars

TUREKIAN and Clark1 have recently discussed an inhomogeneous accumulation model for terrestrial planets, have compared the outgassing product from a veneer of carbonaceous chondrites with the abundances of volatiles on Earth and predicted the amounts of various gases for Venus. The recent measurements of rare gas abundances on Mars by Viking2–4 have prompted us to test such a model for Mars. We find that not only the relative abundance of H, C, N, 20Ne, 36A, Kr and Xe but their total amounts can be best explained if the outer layers of Earth and Mars were made of material similar to ordinary chondrites (LL) as suggested by Anders5, rather than carbonaceous chondrites as assumed by a number of authors in recent years.

Rare gas fractionation patterns in terrestrial samples and the Earth‐atmosphere evolution model

Rare gas abundances in various terrestrial materials can be classified into three distinct fractionation patterns: type 1 shows a progressive enrichment in the heavier gases (shales, natural gases, holocrystalline submarine basalts, seawater and groundwater), type 2 shows a large enrichment of Ne and a slight apparent enrichment of Xe (glassy margins of submarine basalts, a subaerial basalt, an olivine xenolith from Hawaii), and type 3 shows large enrichments of Ne, Kr, and Xe relative to Ar (thucolite and a shale sample). The type 1 pattern is formed by the low‐temperature adsorption of rare gases usually via the rare gases dissolved in seawater. The type 2 pattern is interpreted as a combination of high‐temperature diffusion, solubility, and bulk melting in the mantle. The type 3 pattern probably represents a hybrid pattern. These patterns are consistent with an earth‐atmosphere evolution model which assumes (1) that the present rare gas abundances in the atmosphere closely approximate the rare gas inventory of the whole earth except for xenon and (2) that the average rare gas abundance pattern in the solid earth is roughly similar to that in the atmosphere except for Xe.

Isotopic Analysis of Rare Gases with a Laser Microprobe

A ruby-pulsed laser and high-sensitivity mass spectrometer are used to analyze isotopic abundances of rare gases from microgram samples of polished sections. The feasibility of the technique is demonstrated by the analysis of primordial helium and neon from the Kapoeta and Fayetteville meteorites.

Cosmogenic Rare Gas Ratios in Iron Meteorites

Rare gas abundances in iron meteorites are compared with cross section ratios from high energy bombardments. A meteoritic low-energy flux (below 1 Gev) of about 13 times the high energy flux (above 1 Gev) is found. About half the helium is produced by this low energy flux.