Variable190Os/184Os ratio in acid residues of iron meteorites

In residual materials obtained on dissolution of iron meteorites in 2M H₂SO₄, the ratio of¹⁹⁰Os/¹⁸⁴Os has been measured by radiochemical neutron activation analysis. Most residues have a normal isotopic ratio (to within ±2%). However, in some residues both positive and negative deviations in the isotopic ratio are seen. The most spectacular deviations are in the insoluble fragments (nuggets) from Sikhote Alin iron meteorite where the¹⁹⁰Os/¹⁸⁴Os ratio is about 50% of the normal value. The new results confirm our earlier observations that iron meteorites contain pre-solar grains.     

Total nitrogen in meteorites

Total N has been measured in a number of meteorites by neutron activation analysis using the reaction N¹⁴(n, p)C¹⁴. From each meteorite a number of chips have been analysed to investigate the variation of N contents in a sample. Many meteorites are found to contain a heterogeneous distribution of N. Eighteen chondrites, mostly of the classes C3, H4, H5, L4, L5, L6 and LL6, and six achondrites are found to have average N contents of 10–45 ppm. These do not show any clear-cut dependence of N on petrological group. However, the inherent heterogeneity or the fact that from most meteorite classes only single falls were studied might be responsible for this lack of correlation. In Cold Bokkeveld (C2) N is high (420 ppm). Unlike C, N content of ureilites is low (26 ppm). Nitrogen is enriched in the non-magnetic as compared to the magnetic fractions in H-group chondrites. Analyses of sieved Bjurböle phases show no enrichment of N in finer matrix material, nor any depletion in chondrules. In two gas-rich meteorites, Kapoeta and Assam, there is no excess N in the dark phases. Nine iron meteorites and three mesosiderites were analysed. Twenty analyses of Canyon Diablo and seven of Odessa establish a very heterogeneous N distribution in these meteorites.     

I-Xe and 40Ar-39Ar analyses of silicate from the Eagle Station pallasite and the anomalous iron meteorite Enon

Silicate from two unusual iron-rich meteorites were analyzed by the I-Xe and ⁴⁰Ar-³⁹Ar techniques, Enon, an anomalous iron meteorite with chondritic silicate, shows no loss of radiogenic ⁴⁰Ar at low temperature, and gives a plateau age of 4.59 ± 0.03 Ga. Although the Xe data fail to define an I-Xe correlation (possibly due to a very low iodine content), the inferred $\text{Pu}\text{U}$ ratio is more than 2σ above the chondritic value, and the Pu abundance derived from the concentration of Pu-fission Xe is 6 times greater than the abundance inferred for Cl meteorites. These findings for Enon, coupled with data for IAB iron meteorites, suggest that presence of chondritic silicate in an iron-rich meteorite is diagnostic of an old radiometric age with little subsequent thermal disturbance. The Eagle Station pallasite, the most ¹⁶O-rich meteorite known, gives a complex ⁴⁰Ar-³⁹Ar age pattern which suggests a recent (?0.85 Ga) severe thermal disturbance. The absence of excess ¹²⁹Xe, and the low trapped Ar and Xe contents, are consistent with this interpretation. The similarity between ⁴⁰Ar-³⁹Ar data for Eagle Station and for the olivine-rich meteorite Chassigny lends credence to the previous suggestion of a connection between Chassigny and pallasites, in the sense that similar processes operating at similar times on different parent bodies may have been involved in the formation of olivine in both types of meteorites.     

High precision iron isotope measurements of meteoritic material by cold plasma ICP-MS

The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ⁵⁶Fe and ± 0.25 ‰ for δ⁵⁸Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ⁵⁶Fe_chondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.     

A search for nickel isotopic anomalies in iron meteorites and chondrites

We report Ni isotopic data, for ^58,60-62Ni, on (1) FeNi metal and sulfides in different groups of iron meteorites, (2) sulfides and a whole rock sample of the St. Séverin chondrite, and (3) chondrules from the Chainpur chondrite. We have developed improved, Multiple-Collector, Positive ion Thermal Ionization Mass Spectrometric (MC-PTIMS) techniques, with Ni⁺ ionization efficiency at 1‰, and chemical separation techniques for Ni which reduce mass interferences to the 1 ppm level, so that no mass interference corrections need be applied, except for ⁶⁴Ni (from ⁶⁴Zn, at the 0.1‰ level), for which we do not report results. We normalize the data to ⁶²Ni/⁵⁸Ni to correct for mass dependent isotope fractionation. No evidence was found for resolved radiogenic or general Ni isotope anomalies at the resolution levels of 0.2 and 0.5 εu (εu = 0.01%) for ⁶⁰Ni/⁵⁸Ni and ⁶¹Ni/⁵⁸Ni, respectively. From the ⁵⁶Fe/⁵⁸Ni ratios and ε(⁶⁰Ni/⁵⁸Ni) values, we calculate upper limits for the initial value of (⁶⁰Fe/⁵⁶Fe)₀ of (a) <2.7 × 10⁻⁷ for Chainpur chondrules, (b) <10⁻⁸ for the St. Séverin sulfide, and (c) <4 × 10⁻⁹ for sulfides from iron meteorites. We measured some of the same meteorites measured by other workers, who reported isotopic anomalies in Ni, using Multiple-Collector, Inductively-Coupled Mass Spectrometry. Our results do not support the previous reports of Ni isotopic anomalies in sulfide samples from Mundrabilla by Cook et al. [Cook D. L., Clayton R. N., Wadhwa M., Janney P. E., and Davis A. M. (2008). Nickel isotopic anomalies in troilite from iron meteorites. Geophy. Res. Lett. 35, L01203] and in sulfides from Toluca and Odessa by Quitté et al. [Quitté G., Meier M., Latkoczy C., Halliday A. N., and Gunther D., (2006). Nickel isotopes in iron meteorites-nucleosynthetic anomalies in sulfides with no effects in metals and no trace of ⁶⁰Fe. Earth Planet. Sci. Lett. 242, 16-25]. Hence, we find no need for specialized physical-chemical planetary processes for the preservation of different Ni isotope compositions, between FeNi metal and sulfides in the same iron meteorites, as proposed by the above reports nor for complex astrophysical scenarios to provide the very peculiar Ni isotope anomalies reported by these workers for sulfides.     

The concentration and isotopic composition of hydrogen,carbon and nitrogen in carbonaceous meteorites

Concentrations and isotopic compositions were determined for H₂, N₂ and C extracted by stepwise pyrolysis from powdered meteorites, from residues of meteorites partially dissolved with aqueous HF, and from residues of meteorites reacted with HF-HCl solutions. The meteorites treated were the carbonaceous chondrites, Orgueil, Murray, Murchison, Renazzo and Cold Bokkeveld. Data determined for whole rock samples are in approximate agreement with previously published data. Acidification of the meteorites removed the inorganic sources of H₂, so that H₂ in the HF-HCl acid residues came primarily from insoluble organic matter, which makes up 70–80% fraction of the total carbon in carbonaceous meteorites. The δD in the organic matter differs markedly from previously determined values in organic matter in meteorites. The δD values of organic matter from acid residues of C1 and C2 carbonaceous chondrites range from +650 to + 1150%. The acid residues of the Renazzo meteorite, whose total H₂ has a δD of +930‰, gave a δD value of +2500‰. Oxidation of the HF-HCl residue with H₂O₂ solution removes the high δD and the low δ¹⁵N components. The δ¹³C values range between ?10 and ?21 and δ¹⁵N values range between +40 and ?11. The δ¹⁵N of Renazzo is unusual; its values range between +150 and ?190.There is good correlation between δD and the concentration of H₂ in the acid residues, but no correlation exists between δD, δ¹³C and δ¹⁵N in them. A simple model is proposed to explain the high δD values, and the relationships between δD values and the concentration of H₂. This model depends on the irradiation of gaseous molecules facilitating reaction between ionic molecules, and indicates that an increase in the rate of polymerization and accumulation of organic matter on grains would produce an increase in the deuterium concentration in organic matter.     

I-Xe dating of silicate and troilite from IAB iron meteorites

The IAB iron meteorites may be related to the chondrites: siderophile elements in the metal matrix have chondritic abundances, and the abundant silicate inclusions are chondritic both in mineralogy and in chemical composition. Silicate and troilite (FeS) from IAB irons were analyzed by the I-Xe technique. Four IAB silicate samples gave well-defined I-Xe ages [in millions of years relative to Bjurböle; the monitor error (± 2.5 Myr) is not included]: ?3.7 ± 0.3 for Woodbine, ?0.7 ± 0. 6 for Mundrabilla, +1.4 ± 0.7 for Copiapo, and +2.6 ± 0.6 for Landes. The (¹²⁹Xe/¹³²Xe)_trapped ratios are consistent with previous values for chondrites, with the exception of Landes which has an extraordinary trapped ratio of 3.5 ± 0.2. Both analyses of silicate from Pitts gave anomalous I-Xe patterns.Troilite samples were also analyzed: Pitts troilite gave a complex I-Xe pattern, which suggests an age of +17 Myr; Mundrabilla troilite defined a good I-Xe correlation, which after correction for neutron capture on ¹²⁸Te gave an age of ?10.8 ± 0.7 Myr. Thus, surprisingly, low-melting troilite substantially predates high-melting silicate in Mundrabilla.Abundances of Ga, Ge, and Ni in metal from these meteorites are correlated with I-Xe ages of the silicate; meteorites with older silicates have greater Ni contents. No model easily accounts for this result as well as other properties of IAB irons; nevertheless, these results, taken at face value, overall favor a nebular formation model (e.g. Wasson, 1970, Icarus 12, 407–423). The great age of troilite from Mundrabilla suggests that this troilite formed in a different nebular region from the silicate and metal, and was later mechanically mixed with these other phases.The correlation between the trace elements in the metal and the I-Xe ages of the silicate provides one of the first known instances in which another well-defined meteoritic property correlates with I-Xe ages. In addition, almost all the ¹²⁹Xe in Mundrabilla silicate (etched in acid) was correlated with ¹²⁸Xe. These two results further support the validity of the I-Xe dating method.     

Retention of ion-implanted-xenon in olivine: Dependence on implantation dose

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500°C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 × 10¹⁵ Xe ions/cm². Retention was less at lower doses, with ≥ 50% loss at 1 × 10¹⁴ Xe ions/cm². Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 μm layer for 10⁷ years was calculated as a function of metamorphic temperature. For example, an activation energy of 50 kcal/mole implies Xe retention may be possible for metamorphic temperatures below 500°C.     

Noble-gas-rich separates from ordinary chondrites

Acid-resistant residues were prepared by HCl-HF demineralization of three H-type ordinary chondrites: Brownfield 1937 (H3), Dimmitt (H3,4), and Estacado (H6). These residues were found to contain a large proportion of the planetary-type trapped Ar, Kr, and Xe in the meteorites. The similarity of these acid residues to those from carbonaceous chondrites and LL-type ordinary chondrites suggests that the same phase carries the trapped noble gases in all these diverse meteorite types. Because the H group represents a large fraction of all meteorites, this result indicates that the gas-rich carrier phase is as universal as the trapped noble-gas component itself. When treated with an oxidizing etchant, the acid residues lost almost all their complement of noble gases. In addition, the Xe in at least one oxidized residue, from Dimmitt, displayed isotopic anomalies of the type known as CCFX or DME-Xe, which is characterized by simultaneous excesses of both the lightest and heaviest isotopes. The anomaly in the Dimmitt sample differs from that observed in carbonaceous-chondrite samples, however, in the relative proportions of the light- and heavy-isotope excesses.The results of this study do not show an inverse correlation between trapped ${}^{20}\text{Ne}{}^{36}\text{Ar}$ and trapped ³⁶Ar abundance, as has been reported for acid-resistant residues from LL-chondrites. The results of this work therefore fail to support the hypothesis that meteoritic trapped noble gas abundances were established at the time of condensation.     

Laboratory simulation of meteoritic noble gases. I. Sorption of xenon on carbon: Trapping experiments

We have studied trapping of radioactive ¹²⁷Xe in three types of carbon: carbon black (lamp black  LB), pyrolyzed polyvinylidene chloride (PVDC), and pyrolyzed acridine (C₁₃H₉N). A total of 86 samples were exposed to Xe at T between 100 and 1000°C, for times between 5 min and 240 hours, at p_xe ~ 5 × 10^?7 atm. Excess gas phase and loosely sorbed Xe were pumped away and the remaining, tightly bound Xe was measured by γ-spectrometry.At 100°C,× >90% of the Xe desorbs within a few minutes' pumping but a small amount remains even after 4000 min. Distribution coefficients for this tightly bound Xe are ~1 × 10^?2, 1 and 10 ccSTP/g atm for LB, acridine and PVDC carbons. The tightly bound Xe consists of two components. One occurs over the entire range 100–1000°C, becoming less abundant at high T; it appears to be physisorbed. The other occurs only at T > 500°C and is probably due to volume diffusion. The adsorbed component in LB has an apparent ΔH between ?2.3 and ?5.7 kcal/mole. The diffused component, which occurs in LB and possibly in acridine carbon, has an activation energy Q = 27 ± 8 kcal/mole and a diffusion coefficient D = 1.3 × 10^?17 cm²/sec at 1000°C. These values are comparable to those found for other types of amorphous carbon (Morrisonet al., 1963; Nakai et al., 1960).The low-T component displays two paradoxical features: low ΔH_ads, in the range for Xe physisorbed on carbon, but exceedingly long adsorption or desorption times (~10³ min at 100–400 or 1000°C). Although these long times seem to suggest a high energy process such as chemisorption, our results are best explained by a model that invokes physisorption within a labyrinth of micropores—of atomic dimensions—known to exist in amorphous carbons. The long adsorption/desorption times reflect either the long distances (~5 cm) Xe atoms must migrate by random walk to enter or leave the labyrinth, or the long times needed for Xe atoms to traverse tight spots or constricted pores that connect interior and exterior surfaces of the carbon (activated entry). Both variants of this model predict long equilibration times for the observed ΔH_ads of ?2 to ?6 kcal/mole. Apparently, xenon can be tightly trapped in carbon without resorting to high-energy bonding or to exotic mechanisms.These results suggest that “planetary” type noble gases in meteorites, located at or near grain surfaces of amorphous carbon, may be trapped by adsorption in micropores, whereas components such as CCFXe, which are uniformly distributed in their carrier phases, may be trapped by mechanisms such as volume diffusion or ion implantation.     

Carbonaceous chondrites—I. Characterization and significance of carbonaceous chondrite (CM) xenoliths in the Jodzie howardite

Mineralogical, chemical, textural, and isotopic studies of the abundant carbonaceous inclusions in the Jodzie howardite are consistent with CM characteristics. These CM xenoliths show regolith alteration on a level comparable to the Murray and Murchison meteorites but less than Nogoya, flow-oriented development of phyllosilicates and ‘poorly characterized phases’, and partial oxidation of sulfides. Temperature-programmed pyrolysis mass spectrometry (25°–1400°C) indicates that gas release patterns of volatiles and hydrocarbon components and percent contents of N(0.15), C(2.3) and S(2.4) are typical of CM meteorites. Release of significant amounts of SO₂ is attributed to the thermal breakdown of ‘poorly characterized phases’ (Fe-Ni-C-S-O) that formed during low temperature aqueous alteration in the CM parent body.Noble gas abundances are well within the reported range of CM meteorites. The fact that the Ne composition is typical for ‘solar’ values and the isotopic structure of Xe is ‘planetary’ argues that these gases were entrapped by different mechanisms. Cosmic ray exposure ages for the xenoliths (³He, 5 × 10⁶; ²¹Ne, 6.7 × 10⁶; ³⁸Ar, 6.9 × 10⁶ yr) agree with the reported exposure age for the eucritic host. Volatile abundances, presence of intact organic molecules, and phyllosilicates in the CM xenoliths preclude regolith temperatures in excess of 200°C after CM incorporation. Mixing of the host and xenoliths probably occurred during a low-velocity collision of main belt asteroids.     

Ruthenium endemic isotope effects in chondrites and differentiated meteorites

We report on the abundances of Ru isotopes in (1) iron meteorites, (2) stony-iron meteorites (pallasites), (3) ordinary and carbonaceous chondrites, and (4) in refractory inclusions from the carbonaceous meteorite Allende. We have developed improved Multiple-Collector, Negative-ion Thermal Ionization Mass Spectrometric (MC-NTIMS) techniques for Ru, with high ionization efficiency of 4% and with chemical separation techniques for Ru, which reduce mass interferences to the ppm level, so that no mass interference corrections needed to be applied. Our data were normalized to ⁹⁹Ru/¹⁰¹Ru to correct for mass-dependent fractionation. We find no Ru isotopic effects in the ordinary chondrites and group IAB iron meteorites we have measured. There are significant effects (deficits) in the pure s-process nuclide ¹⁰⁰Ru, in the Allende whole-rock and in refractory inclusions of up to 1.7 parts in 10,000 (εu). There are also endemic deficits in ¹⁰⁰Ru in iron meteorites and in pallasites of up to 1.1 εu. The Ru data suggest a wide spread and large scale heterogeneity in p-, s-, and r-process components resulting in a deficit in s-process nuclides or enhancements in both p- and r-process nuclides, in refractory siderophiles condensing in the early solar nebula. In contrast, the data on bulk Murchison suggest an excess in ¹⁰⁰Ru and in ¹⁰⁴Ru, which are distinct from the rest of the measured patterns. Our results establish the presence of significant isotopic heterogeneity for Ru in the early solar nebula. The observation of endemic Ru effects in planetary differentiates, such as iron meteorites and pallasites, must reflect the siderophile nature of Ru and the preservation in condensing FeNi metal of refractory metal condensate grains formed in the early solar nebula. Once incorporated in the metal phase, the refractory siderophiles remained in the metal phase through the melting and differentiation of planetesimals to form FeNi cores and silicate mantles and crusts.     

Noble gas and carbon abundances of the Haverö, Dingo Pup Donga,and North Haig ureilites

Total carbon determinations on the Haverö, Dingo Pup Donga, and North Haig ureilites yield values of 2.07, 3.17, and 5.58 wt.%, respectively. Haverö and Dingo Pup Donga contain relatively large amounts of trapped Ar, Kr and Xe, which like the carbon content varies with grain size for Haverö. These two meteorites also contain dominant cosmic rayproduced He and Ne, and show ³He exposure ages of ~23 m.y. and ~7 m.y., respectively. North Haig contains much smaller amounts of trapped gases and spallogenic gases, which may result from loss due to terrestrial weathering. The isotopic composition of Xe in five grain size analyses of Haverö and a whole rock analysis of Dingo Pup Donga shows the presence of a major solar-like Xe component. The presence of this solar component adds an additional complication to the concept of forming ureilites from carbonaceous chondrites.     

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio ³⁸Ar/³⁶Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of ⁴⁰Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported ³⁶Ar excesses due to possible decay of extinct ³⁶Cl in the Efremovka meteorite.Trapped ²²Ne in the range of (1.4-2.5) × 10⁻⁸ cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. ²¹Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (¹³²Xe/³⁶Ar)₀, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (¹³²Xe/³⁶Ar)₀ is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ¹⁷O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio ¹²⁹Xe/¹³²Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of ¹²⁹I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.     

Noble gas and oxygen isotope studies of aubrites: A clue to origin and histories

Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peña Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the ⁸¹Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the ⁸¹Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show “exposure at the surface” on their parent body(ies): (i) neutron capture ³⁶Ar, ⁸⁰Kr, ⁸²Kr and/or ¹²⁸Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peña Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peña Blanca Spring and ALHA 78113). The concentrations of ¹²⁸Xe from neutron capture on ¹²⁷I vary among four measured specimens of Cumberland Falls (0.5-76 × 10⁻¹⁴ cm³STP/g), but are correlated with those of radiogenic ¹²⁹Xe, implying that the concentrations of (¹²⁸Xe)_n and (¹²⁹Xe)_rad reflect variable abundances of iodine among specimens. The ratios of (¹²⁸Xe)_n/(¹²⁹Xe)_rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic ¹²⁹Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 × 10⁻⁷, 9.4 × 10⁻¹⁰ and 2.8 × 10⁻¹⁰ cm³STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peña Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely ²⁴⁴Pu-derived, and about 10-20% seems to be ²³⁸U-derived at ¹³⁶Xe. The observed (¹³⁶Xe)_Pu corresponds to 0.019-0.16 ppb of ²⁴⁴Pu, from which the ²⁴⁴Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after ∼4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas signatures, oxygen with chondritic isotopic compositions was found only in a specimen of Cumberland Falls (Δ¹⁷O of ∼0.3‰). The other four aubrites and the other two measured specimens of Cumberland Falls are concurrent with the typical range for aubrites.     

Helium,neon, and argon in the iron meteorites Dongling,Nantan and Ningbo

The light noble gases He, Ne and Ar have been measured in the iron meteorites Dongling, Nantan and Ningbo. Dongling and Ningbo show a deficit of cosmic-ray that produced³He of ca. 30% and 10%, respectively, which is argued to be caused by the loss of³H (tritium) from the meteoroids during the time of their exposure to the cosmic radiation. Nantan has the lowest content of noble gases as yet reported for any iron meteorite. Cosmogenic³He and³⁸Ar are only about 1/5000 of those in Dongling which has particularly interesting implications if the two meteorites belong to the same fall^[2]. In addition, Nantan contains nonspallogenic⁴He which we believe to be of radiogenic origin. This radiogenic⁴He, together with a U-content of 2.6×10⁻¹¹ g/g^[20] yields a⁴He retention age close to the cosmic-ray exposure age of Dongling. If Dongling and Nantan were part of the same meteoroid^[2], this result would indicate that He retention in the meteoroid age were 4,500 Ma, a U-content of less than 7.2×10⁻¹³ g/g is required to explain the non-cosmogenic₄He present. An upper limit to the number of transuranium or superheavy-element atoms which have decayed by α-emission in Nantan since onset of He retention is 2×10¹⁰ per gram.     

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 Xe¹²⁷ and pumped for >9 hrs to remove the most labile gas were examined by etching with HNO₃, 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 HNO₃. 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 cm²/sec at 1000°C.PVDC carbon lost its adsorbed Xe at about the same rate as lampblack on exposure to air or HNO₃, 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 10⁶× 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.     

A 33S Anomaly in the allende meteorite

The isotope ratios ³³S/³²S and ³⁴S/³²S have been measured in sulphur fractions extracted from samples of the meteorites Allende and Eagle Station by leaching at successively greater acid concentrations and higher temperatures. On a three isotope plot of δ³³S$\text{vs}$δ³⁴S most of the data lie on or close to the mass fractionation line. The last fraction of sulphur extracted from a bulk Allende sample lies off the line and has an approximately 1%. excess in the ³³/³²S ratio.Previous searches for anomalous abundance patterns of ³²S, ³³S, ³⁴S and ³⁶S have been reported by HULSTON and THODE (1965a,b), THODE and REES (1971), and REES and THODE (1972). No isotope abundance variations were found, in the meteorite and lunar samples studied, which could not be explained on the basis of either mass dependent isotope fractionation or, in the special case of iron meteorites, cosmic ray production of ³³S and ³⁶S. We report here preliminary results of a renewed search for isotopically anomalous sulphur in which we are concentrating on the Allende and Eagle Station meteorites, both of which contain anomalous oxygen (CLAYTON $\text{et}$$\text{al}$., 1973, 1976). In a first attempt to distinguish between normal sulphur and any possible anomalous sulphur, we have leached both bulk samples and hand separated components of these meteorites with hydrochloric acid.CLAYTON and RAMADURAI (1977) suggested that the presence of isotopically anomalous sulphur would be evidence for the existence of presolar grains which are relics of nucleosynthesis in certain zones of supernova expansion. In particular they suggested that sulphides of titanium are good candidates for isotopic analysis. These are not expected to exist in conventional solar equilibrium condensation sequences, but might be abundant in condensates from silicon burning shells of supernovae. Our chemical procedures were already completed when CLAYTON and RAMADURAI'S suggestions came to our attention and it must be stressed that so far, in all cases but one we have examined only sulphur from sulphides which are decomposed by HC1. Thus we may not have sampled sulphides of the type suggested by CLAYTON and RAMADURAI.All samples of the Allende meteorite were ground finer than 50μm before acid extraction of sulphur. Samples of sulphur were extracted from the various phases of the meteorites by using successively stronger hydrochloric acid leaches, longer times and higher temperatures of reaction. Sulphur initially released as H₂S was successively converted to CdS, Ag₂S and SF₆, this latter compound being analysed mass spectrometrically (THODE and REES, 1971). Analyses of nine SF₆ samples prepared from Ag₂S originally derived from Canyon Diablo troilite were also performed in order to monitor fluorination and mass spectrometry precision and to establish the zero points ofthe isotope variation scales. The results are shown in Table 1. The sulphur contents of the various samples were determined gravimetrically as Ag₂S. The bulk and matrix samples are probably a few percent low because of mechanical losses. The percentages of sulphur in each fraction of a sample extracted during each leaching stage are given in the table. The total sulphur content in the bulk and matrix samples of the Allende meteorite i.e., the sum of the sulphur contents of the individual fractions, varies from 1.8 to 2.08%, the highest percentage being in the matrix. These values compare with about 2 to 2.1% obtained by CLARKE $\text{et}$$\text{al}$. (1970).     

New data on the abundance of palladium in meteorites

The concentration of Pd in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites, 29 iron meteorites and other samples has been determined by stable isotope dilution using solid source mass spectrometry. The Cl chondrite Orgueil gives a ‘cosmic’ abundance for Pd of 1.5 (Si = 10⁶ atoms), in good agreement with the currently accepted value.The concentration of Pd shows little variation among the carbonaceous chondrites, but in ordinary chondrites decreases from the H to L to LL groups. Pd in achondrites is approx 100 times lower than in chondrites. Data for iron meteorites plot around the ‘cosmic’ $\text{Pd}\text{Ni}$ ratio; however the Pd data falls into distinct groups, corresponding to the chemical group classification. These results support the hypothesis that at least two fractionation processes have occurred during the formation of iron meteorites.     

Records of ancient cosmic radiation in extraterrestrial rocks

Recent results on cosmic ray interactions in lunar samples and meteorites resulting in production of stable and radionuclides, particle tracks and thermoluminescence are reviewed. A critical examination of²⁶A1 depth profiles in lunar rocks and soil cores, together with particle track data, enables us to determine the long term average fluxes of energetic solar protons (>10 MeV) which can be represented by (J _s,R _o)=(125, 125). The lunar rock data indicate that this flux has remained constant for 5×10⁵ to 2×10⁶ years. Production rates of stable and radionuclides produced by galactic cosmic rays is given as a function of size and depth of the meteoroid. Radionuclide (⁵³Mn,²⁸Al) depth profiles in meteorite cores, whose preatmospheric depths are deduced from track density profiles are used to develop a general procedure for calculating isotope production rates as a function of meteoroid size. Based on the track density and²²Ne/²¹Ne production rates, a criterion is developed to identify meteorites with multiple exposure history.²²Ne/²¹Ne ratio <1·06 is usually indicative of deep shielded exposure. An examination of the available data suggests that the frequency of meteorites with multiple exposure history is high, at least 15% for LL, 27% for L and 31% for H chondrites. The epi-thermal and the thermal neutron density profiles in different meteorites are deduced from⁶⁰Co and track density data in Dhajala, Kirin and Allende chondrites. The data show that the production profile depends sensitively on the size and the chemical composition of the meteoroid. Cosmic ray-induced thermoluminescence in meteorites of known preatmospheric sizes has been measured which indicates that its production profile is nearly flat and insensitive to the size of the meteoroid. Some new possibilities in studying cosmic ray implanted radionuclides in meteorites and lunar samples using resonance ionisation spectroscopy are discussed.