Meteoritic Xenon Research Articles

Xe isotopic abundances in enstatite meteorites and relations to other planetary reservoirs

Reported Xe isotopic abundances in enstatite chondrites (EC's) show some variability, and this makes comparisons to other solar system reservoirs rather difficult. In contrast, we find uniform Xe isotopic abundances in the EC chondrite Abee for a variety of clasts, except for 128Xe and 129Xe, the isotopes affected by neutron capture in I and by extinct 129I. We report averages for the studied clasts which are consistent within error limits with OC‐Xe and with the Q‐Xe signature. On the other hand, the elemental abundance ratios Ar/Xe are variable between clasts. A strongly reducing environment which is indicated for enstatite meteorites was generally assumed to be consistent with conditions existing in the early inner solar system. Xe isotopic abundances in SNC meteorites from Mars and also those in some terrestrial wells show that distinct isotopic reservoirs coexisted on the same planets. In particular, the Xe isotopic signatures in terrestrial well gases show the presence of a minor distinct component in two of the reported four well gases. These authors suggested that the extra component represents solar Xe, but we show that also a meteoritic xenon reservoir of the Abee‐Xe structure is an option. The reported Xe data in Ar‐rich (subsolar) EC's show isotopic abundances slightly lighter than those in Abee‐Xe, but the relative abundances of Ar, Kr, and Xe indicate only a minor component of elementally unfractionated solar Xe. The elemental ratios suggest rather a different origin for these gases: the loading of solar particles into grain surfaces during exposure at elevated temperatures during accretion of matter in the inner solar system. A model of this type was suggested for the accretion of gases now observed in the atmosphere on Venus. We note that disks of crystalline silicates (including enstatite and olivine) have been observed in T Tauri stars during their early evolution.

Chemically fractionated fission-xenon in meteorites and on the earth

This is a report on the nature of isotopically anomalous xenon, which has been detected in two Ca-Al-rich inclusions of the Allende carbonaceous chondrite. It is extremely enriched in 132Xe, 129Xe, and to a lesser extent in 131Xe. Similar large excesses of 132Xe as well as of 131Xe, 134Xe, and 129Xe have previously been found in material processed in a natural nuclear reactor (Oklo phenomenon). Excess of these isotopes had also been encountered in MORB-glasses, in an ancient Greenland anorthosite. Thus, this Xe-type, which had previously been termed “alien” ( Jordon et al., 1980a) does not seem to be unique. To determine the origin of “alien” Xe, we analysed Xe 1. (a) in neutron irradiated pitchblende and in the irradiation capsule, 2. (b) in non-irradiated extremely fine-grained pitchblende (so-called Colorado-type deposit), and 3. (c) in sandstone taken from the epicentre of an atomic explosion. In addition, the isotopic composition of xenon released by stepwise degassing and after selective dissolving of rocks from the Oklo natural reactor was determined. The results of these dedicated experiments demonstrate that the formation of alien Xe is due to the migration of the radioactive precursors of the stable isotopes 134Xe, 132Xe, 131Xe, and 129Xe. Due to this reason we now call it CFF-Xe—Chemically Fractionated Fission Xenon. Prerequisites for its formation are the simultaneous prevalence of two conditions: 1. (1) fission (of 238U, 235U, and/ or 244Pu) and 2. (2) a physicochemical environment (temperature, pressure, fluidity) at which the precursors of xenon (mainly Te and I) are mobile. Taking into account the occurrence of xenon in meteorites and terrestrial rocks, not all excesses of 129Xe in mantle rocks and natural gases are necessarily connected with the decay of primordial 129I.

Laboratory simulation of meteoritic noble gases. II. Sorption of xenon on carbon: Etching and heating experiments

Sixteen amorphous carbon (lampblack) samples that had been exposed to Xe 127 and pumped for >9 hrs to remove the most labile gas were examined by etching with HNO 3, for comparison with the release pattern of meteoritic xenon. Samples originally exposed at 100–200°C lost 90% of their Xe very readily, when the surface had been etched to a mean depth of only ~0.2 Å. This suggests that the Xe is adsorbed mainly at rare sites that are unusually reactive to HNO 3. The adsorbed Xe survived several months' storage in vacuum, but on exposure to air, part of it was lost within a few hours, while the remainder persisted without measurable exchange. Samples exposed at 800–1000°C had a similar adsorbed component, as well as a second, tightly bound component extending to a mean depth of up to 30 Å; this component had apparently diffused into the carbon during exposure. The (microscopic) diffusion coefficient for graphitic crystallites is 5 × 10 −20 cm 2/sec at 1000°C. PVDC carbon lost its adsorbed Xe at about the same rate as lampblack on exposure to air or HNO 3, though it differs from lampblack in being non-graphitizable and more porous. It had only a small diffused component, however. The most tightly bound part of the Xe adsorbed on lampblack resembles planetary Xe in most characteristics: surface siting, etchability, persistence in vacuum, and lack of exchange with atmospheric Xe. The Xe concentrations—if interpreted as equilibrium distribution coefficients—are some 10 6× too small to account for meteoritic Xe, but it appears that equilibrium had not been reached by any of the samples, even after 1 day's exposure to Xe. If the uptake of Xe is controlled by rate rather than equilibrium, then the high noble gas concentrations in meteorites may simply reflect the much longer uptake times in the solar nebula. It seems likely that the trapping mechanisms discussed here can also explain two other features: elemental fractionations of noble gases, and the close correlation between planetary Xe and CCFXe.

Isotopically anomalous xenon in meteorites - A new clue to its origin

The CCF xenon component in primitive meteorites, which has been attributed either to fission of a superheavy element or to nucleosynthesis in a supernova, does not show the large enrichment in Xe-129 (from decay of 16 Myr I-129) expected for supernova ejecta. Although this problem can be circumvented by ad hoc assumptions, a fission origin of CCFXe seems more likely.

Isotopic compositions of calcium, barium, neodymium and xenon in meteorites.

Isotopic anomalies observed in meteorites for Ca, Ba, Nd and Xe are explained as due to the alteration of the isotopic ratios by a combined effect of mass fractionation, neutron-capture and cosmic-ray irradiation processes, which took place prior to and during the period of formation of the solar system. The neutron-capture process appears to have occurred at a temperature higher than that of the earth's surface.

Trapped xenon in meteorites

Xenon in meteorites can be resolved into a mixture of component X and trapped xenon with the following composition,124Xe:126Xe: 128 Xe: 130Xe: 131Xe: 132Xe:134Xe: 136Xe = 0.0276: 0.0248: 0.501: 1.00: 5.04: 6.19: 2.31: 1.90. This trapped meteoritic xenon is distinct from xenon trapped in the average carbonaceous chondrite which is shown to represent the average composition of the total xenon in meteorites containing various mixtures of component X and trapped meteoritic xenon.

Xenon in natural gases

Analyses of noble gases from four natural gases reveal radiogenic He 4 and Ar 40, excess Ne 21 and Ar 38 from α-induced reactions and an excess of the β-unshielded xenon isotopes with an abundance pattern unlike that from the spontaneous fission of U 238. The pattern of excess Xe 129, Xe 131, Xe 132, Xe 134 and Xe 136 in these gas samples is such that a mixture of this xenon with the primordial xenon in meteorites could produce the abundance pattern of atmospheric xenon for all isotopes heavier than Xe 128. It is suggested that this strange “fission-yield” pattern is the result of a mechanism suggested earlier by Cameron: The addition of β-shielded xenon isotopes to the atmosphere by the solar wind. By modifying Cameron's hypothesis so that solar-wind xenon contains excess Xe 124 and Xe 126 in addition to Xe 128 and Xe 130, the addition of this solar wind xenon to atmospheric and meteoritic xenon can account for the general isotopic anomaly pattern observed across meteoritic xenon, the very low fission yields of Xe 131 and Xe 132 reported in xenon-rich meteorites and the high fission yields of Xe 131 and Xe 132 which have been calculated for xenon in gas wells and in the earth's atmosphere.

Noble gases in the Fayetteville meteorite

Noble gases were extracted from the dark portions of Fayetteville by heating the sample to successively higher temperatures up to 1600°C. Analyses of the neon evolved at each temperature indicate that this gas is a mixture of at least three components. It is shown that the isotopic composition of neon observed could be produced from a mixture of primordial neon, cosmogenic neon, and highly mass-fractionated primordial neon. The isotopic composition of helium, neon and argon indicate a primordial component for which He 3: He 4 ≤ 0.00031, Ar 38: Ar 36 ≤ 0.177, Ne 20: Ne 22 ≥ 12.6, and Ne 21: Ne 22 ≥ 0.033. The xenon spectra show an excess of the heavy xenon isotopes with a mass yield curve intermediate between that observed in Pasamonte and that in the earth's atmosphere. It is shown that this atmosphere-like xenon component is not due to contamination but is definitely a component of meteoritic xenon.

Fission and spallation xenon in meteorites

Xenon isotope abundance data from other investigations have been analyzed with the aid of an IBM 7040 computer to determine by a least-squares method the best values for the isotope ratios of the fission and spallation xenon. The isotope ratios of Murray were used as the trapped or primordial component. The values obtained were: fission component (normalized to Xe136 = 1.00) Xe131 = 0.38, Xe132 = 1.05, and Xe134 = 0.95; spallation or cosmogenic component (normalized to Xe130 = 1.00) Xe124 = 0.64, Xe126 = 1.07, Xe128 = 1.53, Xe131 = 3.73, and Xe132 = 0.62.

Isotopic anomalies of krypton and xenon in Canyon Diablo graphite

The abundance and isotopic composition of xenon and krypton have been determined in Canyon Diablo graphite. Large excesses of 80Kr, 82Kr and 128Xe are attributed to neutron capture on bromine and iodine. The graphite contains one of the largest excesses of 129Xe observed in meteoritic xenon. The xenon spectra do not display the ‘general anomaly pattern’ which characterizes xenon in chondrites.

On the origin of fissiogenic xenon in meteorites

Rowe and Kuroda [1965] and Rowe and Bogard [1966] measured the abundance of xenon in the Pasamonte achondrite. They found that the fissiogenic isotopes Xe131,132, 134, 136 were more abundant than could be accounted for on the basis of spontaneous fission of U238. Fleischer et al. [1965] measured the number density of fossil fission tracks in the Moore County achondrite. They found a greater density than could be explained on the basis of U238 spontaneous fission.

Xenology

Work in the field of xenon isotope abundances in meteorites is discussed in the context of current theories about the formation of the heavy elements and the early history of the solar system. Using a model in which the special (Xe129*) anomaly is due to decay of I129 produced in the principal galactic synthesis, we conclude that meteorites Abee and Richardton began to retain radiogenic xenon 52 m.y. after the cessation of nucleosynthesis. The xenon-retentive phase (probably chondrules, from results obtained by Merrihue) of the Bruderheim meteorite was formed 17 m.y. earlier, a result which depends very little on the model chosen for the origin of the I129. An iron meteorite appears to have cooled about 200 m.y. later than Abee. The general anomalies in meteorites, including carbonaceous chondrites, are not fixed but vary with release temperature in a heating experiment, suggesting that components of different isotopic composition are being fractionated in the release of the gas. A search for fissionogenic xenon in meteorites has yielded very definite evidence of two components of xenon in some meteorites, especially convincing results being presented for chondrules from the Bjurböle meteorite.

Meteoritic krypton and barium versus the general isotopic anomalies in meteoritic xenon

The general isotopic anomalies in meteoritic xenon are described in detail. Where superior isotopic analyses exist, the xenon anomalies appear to be the same for all meteorites. In other cases there is fair evidence that the xenon examined is a mixture of “meteoritic” and contaminating atmospheric xenon. Two superior krypton analyses for carbonaceous chondrites show no anomalies which are significant in comparison with those for xenon. Barium from the Richardton chondrite is of normal isotopic composition. Cyclotron deuterons produce no Xe 124 in a tellurium target, although the other xenon isotopes, which are in excess in meteorites, are produced. A number of possible mechanisms for producing the general anomalies are discussed and found wanting. One of them, due to Kuroda and Cameron, calls for excess terrestrial fission xenon and for transfer of solar xenon to the atmosphere. It thus involves reasonable processes, but, as we show, requires unreasonable yields for spontaneous fission. A mechanism we propose calls for excess meteoritic fission xenon and for gross mass fractionation of terrestrial xenon. It thus produces the observed anomalies accurately, but by somewhat unlikely processes.

The origin of atmospheric xenon

Part of the differences in isotopic composition of atmospheric and meteoritic xenon can be attributed to irradiation of solar xenon by neutrons during the deuterium-burning phase of solar contraction, followed by accretion of the irradiated xenon into the atmosphere. This problem is examined here more carefully following a new measurement of the reaction rate for capture of protons by deuterium by Griffiths, Lal, and Scorfe. It is concluded that the majority of atmospheric xenon, apart from that produced by the decay of extinct radioactivities, was once in the sun during the deutrium-burning phase.

Xenon in the Bruderheim meteorite

Gas samples used to determine the isotopic composition of xenon, obtained by vacuum melting samples of the Bruderheim meteorite, were analyzed in a ‘static’ mass spectrometer. The xenon content was accurately determined by means of calibrated quantities of Xe128 introduced into the high vacuum extraction system prior to melting. Samples of atmospheric xenon of approximately the same size as the meteoritic xenon samples were prepared from aliquots of air and used for comparison purposes. The results of the mass spectrometric analyses are presented in Tables 1 and 2. Table 1 gives the content of ‘atmospheric’ xenon and ‘excess’ Xe129, with the subtraction of atmospheric xenon from meteoritic xenon based on Xe130. Table 2 gives a comparison between the isotopic abundances of meteoritic xenon and atmospheric xenon expressed in the following way: δi=(XeiXe130)meteorite−(XeiXe130)atmosphere

The record in the meteorites: 6. On the chronology of the early solar system

An attempt is made to account for the differences between the isotopic composition of terrestrial and meteoritic xenon reported by Reynolds. The two chief mechanisms proposed (other than the decay of I129 to Xe129) are the production of Xe131–136 by spontaneous fission of extinct nuclides in the earth, and the production of Xe124–128 by nuclear spallation reactions in the early history of the solar nebula. About 9.6 per cent of the Xe136 in the earths's atmosphere appears to have arisen from the spontaneous fission of 76-million-year Pu244, as proposed by Kuroda. The Pu244-Xe136 decay interval of the earth is 290 m.y., and its I129-Xe129 decay interval may be estimated as ≥210 m.y. Thus, the earth appears to be 100–200 m.y. younger than the meteorites. Possible errors in these determinations are discussed. A dating method, similar to the Pb207-Pb206 method and based upon the two decay systems I129-Xe129 and Pu244-Xe136, is proposed and the appropriate equations are given. The initial solar-system ratios of I129/I127 and Pu244/U238, which can be determined by this method, would provide a crucial test of models of nucleosynthesis.