The variation of cosmogenic Kr and Xe in a core from Estherville mesosiderite: direct evidence that the lunar131Xe anomaly is a depth effect

Kr and Xe were measured by a stepwise heating technique in three samples of a drill core in the “Minnesota” fragment of the Estherville mesosiderite. The cosmogenic⁷⁸Kr/⁸³Kr decreased from the “top” sample to the “bottom” sample(“top” = 0.163 ± 0.005, “bottom” = 0.151 ± 0.005) while the cosmogenic¹³¹Xe/¹²⁶Xe ratio increased(“top” = 5.58 ± 0.35, “bottom” = 6.92 ± 0.17). Cosmic-ray track studies have shown that the “top” sample was indeed closer to the preatmospheric surface than the “bottom” sample by ～ 10 cm. This is the first direct evidence, in a sample of known geometry, that the cosmogenic¹³¹Xe/¹²⁶Xe ratio increases as a function of depth, and as such, confirms the hypothesis that the lunar¹³¹Xe anomaly is a bona fide depth effect due to resonance neutron capture in¹³⁰Ba.     

Ar,Kr, and Xe in terrestrial materials

³⁶Ar, ⁸⁴Kr, and ¹³²Xe abundances along with Kr and Xe isotopic compositions are reported for two African shales and samples of chert from the Gunflint Formation in Canada. It is observed that these data and similar abundance data from other terrestrial materials show correlations between ³⁶Ar-⁸⁴Kr and between ⁸⁴Kr-¹³²Xe. The elemental ratios defined by all the above data differ significantly from and show greater variations than those measured in carbonaceous chondrites, a well-investigated class of extraterrestrial material. Physical absorption and equilibrium solubility acting on the atmosphere are investigated as explanations for the observations. The Kr and Xe isotopic spectra are also consistent with a mass-dependent incorporation process acting on a reservoir of atmospheric composition.     

Radiogenic, fissiogenic and nucleogenic noble gases in zircons

The concentrations and isotopic compositions of argon, krypton and xenon have been determined in a grain size suite of zircons separated from pyroxene syenite of the Botnavatn Igneous Complex, southwestern Norway. The UPb systematics of these zircons has been studied previously.Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of²³⁸U and a component with atmospheric isotopic composition. From correlation diagrams the fissiogenic component is determined to be:⁸³Kr :⁸⁴Kr :⁸⁶Kr = (4.6 ± 1.3) : (11.0 ± 2.0) : 100 and¹²⁹Xe :¹³¹Xe :¹³²Xe :¹³⁴Xe :¹³⁶Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100. The fissiogenic¹³⁶Xe/⁸⁶Kr is 6.0 ± 0.4.The Ar isotopic composition shows radiogenic⁴⁰Ar and a small excess of³⁸Ar. The excess³⁸Ar of about 1 × 10⁻¹¹ cm³ STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of²³⁸U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed³⁸Ar concentrations.UXe ages are (1.19 ± 0.07) Ga, in agreement with UPb ages. However, if the recoil loss of fissiogenic Xe is considered the UXe ages of these zircons are about 1.53 Ga, which is comparable with the KAr ages and some RbSr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of²³⁸U prevents to decide which age is the true crystallization age.     

87Rb-87Sr chronology of enstatite meteorites

A⁸⁷Rb-⁸⁷Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ⁸⁷Rb= 1.42 × 10^?11yr^?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the⁸⁷Sr/⁸⁶Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the⁸⁷Rb-⁸⁷Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the⁸⁷Rb-⁸⁷Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.     

Planetary-type rare gases in an upper mantle-derived amphibole

All twenty-three stable rare gas isotopes have been measured in a mantle-derived amphibole, kaersutite. The elemental abundance pattern of the rare gases is similar to the “planetary” rare gas pattern as defined by carbonaceous chondrites. The³He/⁴He ratio, (4.9 ± 0.6) × 10^?5, is suggestive of primordial He degassing from the mantle. Excess²¹Ne is present. The measured⁴⁰Ar/³⁶Ar ratio,400 ± 5, may represent a mantle⁴⁰Ar/³⁶Ar ratio <240 when corrected for radiogenic⁴⁰Ar. The heavy isotopes of Kr and t0he Xe isotopes are within error of the atmosphere values.     

On the distribution of noble gases in Allende: a differential oxidation study

We have developed a closed-system combustion technique and utilized it to progressively oxidize a gas-rich, highly carbonaceous acid residue and a fine-grained (<4 μm) matrix sample from the Allende C3V meteorite and analyze the released gases mass spectrometrically. For the residue complete gas mobilization occurs at temperatures below 600°C. The temperature interval over which most of the gases are released coincides with that for combustion of most of the carbon. Release is primarily due to chemical attack rather than thermal activation of the gases. There are somewhat different oxidation thresholds for the heavy gases (Ar, Kr, and Xe) and the light gases (He, Ne), indicating chemically different sites for the two groups. Relative enhancement of isotopically anomalous components near 600°C is as large as in any open-system oxidation method. Differential combustion of the matrix sample reveals three distinct outgassing peaks, the first matching the release from the carbonaceous residue (“combustibles”), the second attributed to sulfides, and the third tentatively assigned to silicates. They comprise about 53%, 7% and 40% of the total heavy gases respectively. While the “sulfides” exhibit a small fission-like component, the Xe in the “silicates” appears isotopically uniform with roughly AVCC composition. The “combustibles” of the matrix contain relatively less excess¹²⁹Xe than the residue, perhaps indicating that ～10% of the total¹²⁹Xe in the residue was acquired from “silicates” by redistribution during acid treatment. We cannot rule out the possibility that planetary gases assigned to “sulfides” or “silicates” actually reside in carbonaceous phases somehow sheltered within soluble mineral assemblages, or in acid-soluble carbonaceous phases resistant to oxidation. Integrated releases below and above 600°C during the matrix combustion exhibit virtually identical heavy gas elemental composition, implying similar fractionation during planetary gas entrapment in various materials or in the same material in various environments.     

Primordial129Xe in meteorites

Xenon isotopic analyses by stepwise heating are presented for two neutron-irradiated chondrites, Arapahoe (L5) and Bjurböle (L4). The iodine-xenon formation age of Arapahoe is the oldest yet observed, 9.9 ± 0.8 m.y. before that of Bjurböle. It is thus unlikely that younger ages found in carbonaceous chondrite magnetite record the condensation of the solar nebula. The composition of trapped xenon in Arapahoe is normal except for a deficiency of¹²⁹Xe, where we infer ¹²⁹/Xe¹³²Xe= 0.56 ? 0.04, well below the apparent primordial solar system value. This need not conflict with higher values in other metamorphosed meteorites since growth of¹²⁹Xe from decay of¹²⁹I in xenon-depleted environments can be substantial. The contrast with apparent average solar system composition cannot be easily explained, however, since there is no way to generate one composition from the other. The simplest way to achieve low¹²⁹Xe seems to be to suppose that before decay to¹²⁹Xe r-process production at mass 129 condensed into dust as¹²⁹I, and that Arapahoe's parent body formed in a region of the solar system substantially depleted of this dust before any isotopic homogenization by vaporization of the remaining dust. Arapahoe is not unique in having trapped¹²⁹Xe-deficient xenon, nor in any other respect yet observed, so some such history evidently characterizes major groups of meteorites.     

I-Xe age and trapped Xe components of the Murray (C-2) chondrite

A neutron-irradiated bulk sample of the Murray (C-2) carbonaceous chondrite was etched with H₂O₂ and then divided into colloidal and non-colloidal fractions. The H₂O₂ treatment removed ～80% of the trapped Xe and greatly increased variations in the¹²⁹Xe/¹³²Xe ratio measured in stepwise heating. The colloid showed very little excess¹²⁹Xe, but the anti-colloid gave a fairly good I-Xe correlation corresponding to formation 3.7 ± 2.1 m.y. after Bjurböle.Variations in the trapped Xe component were also observed; most notably the 550°C anti-colloid fraction has large deficiencies relative to AVCC at the heavy isotopes. A tentative decomposition suggests U-Xe, a “primitive” trapped component, as the dominant component with minor contributions from H-Xe, L-Xe, and S-Xe (s-process nucleosynthesis). The identification of U-Xe rests primarily on the agreement of themeasured¹³⁴Xe/¹³⁶Xe ratio with U-Xe. This observation lends support to proposals for such a “primitive” trapped Xe component and demonstrates that at least some carbonaceous chondrite phases sampled a xenon reservoir nearly devoid of H-Xe.     

Evidence for244Pu fission tracks in hibonites from Murchison carbonaceous chondrite

We have developed a technique for revealing nuclear tracks in the mineral hibonite (CaAl₁₂O₁₉), found in the refractory inclusions from carbonaceous chondrites. The tracks in hibonitesfrom Murchison carbonaous chondrite are dominated by fission tracks from²⁴⁴Pu (constituting more than 90% of the total). The measured uranium contents in these crystals range from 1.2 to 62 ppb. We deduce that the average value for the²⁴⁴Pu/²³⁸U ratio in most of the Murchison hibonites at the time of track retention is0.022 ± 0.011.     

Fission tracks in a white inclusion of the Allende chondrite — Evidence for244Pu

Fossil fission tracks have been found in a coarse-grained white inclusion of the Allende chondrite. Tracks are present in excess of those produced by²³⁸U spontaneous fission and cosmic rays. The ratio of excess tracks to²³⁸U tracks is ～20, intermediate to ratios previously observed in meteorites but much lower than might be expected in light of the high initial²⁴⁴Pu/U ratio measured in these inclusions from Xe isotope ratios.     

The Xe-Xe spectrum technique,a new dating method

A new method to date uranium-bearing minerals exclusively by means of a mass spectrometric determination of Xe and/or Kr isotopic ratios has been developed and experimentally tested. It is based on the compositional differences between Xe produced by spontaneous fission of²³⁸U in nature and Xe from²³⁵U fission induced by thermal neutrons in a nuclear reactor. Xe is extracted in 5–10 release fractions at successively higher temperatures. This relates the radiogenic Xe contained in various structural elements of different retention characteristics to the respective U concentrations and allows to account for natural Xe losses, which are quite common. A monazite monitor mineral of known age is included in each irradiation and its Xe isotopes are measured in the same way.The samples analyzed include uranium oxides, REE phosphates, tantalum niobates, zircon and others.The results and comparisons with reported ages obtained by other means demonstrate the applicability of the new dating method for minerals with U contents up to a few percent. In age spectra the low-temperature release fractions of some samples indicate radiogenic Xe losses, while at high temperatures age plateaux can be obtained.     

The abundance of barium in stony meteorites

The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=10⁶ abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.     

The case for an unfractionated244Pu/238U ratio in high-temperature condensates

The coarse-grained, Ca-rich inclusions in the Allende meteorite are the highest-temperature condensates from the cooling solar nebula and, as such, the oldest solid objects in the solar system. All refractory elements with condensation points above the accretion temperature of the inclusions whose concentrations in them have been measured are seen to be present in the inclusions in unfractionated proportion to one another relative to C1 chondrites when data are averaged for a large number of inclusions. Observational data for U and theoretical data for both U and Pu suggest that these elements exhibited refractory behavior in the solar nebula. An experiment is proposed in which fissiogenic Xe and U contents are measured in a suite of these inclusions to obtain the²⁴⁴Pu/²³⁸U ratio of the solar system at the time of initial condensation with an uncertainty of ±15%.     

14C-39ArMe correlations in chondrites and their pre-atmospheric size

Cosmogenic¹⁴C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of¹⁴C.In eight cases³⁹Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR₀ of the meteoroids and depth of burial of the samples investigated. Values ofR₀ between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published¹⁴C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.     

Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth

Sm-Nd and Lu-Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm-Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean ¹⁴⁷Sm/¹⁴⁴Nd of 0.1960±0.0005, and a mean ¹⁴³Nd/¹⁴⁴Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm-Nd reference parameters, ¹⁴⁷Sm/¹⁴⁴Nd=0.1966 and ¹⁴³Nd/¹⁴⁴Nd=0.512638 (when fractionation-corrected to ¹⁴⁶Nd/¹⁴⁴Nd=0.7219).Lu-Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of ¹⁷⁶Lu/¹⁷⁷Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10⁻¹¹ year⁻¹ is used for the decay constant of ¹⁷⁶Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311-324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with ¹⁷⁶Lu/¹⁷⁷Hf less than 0.030 are employed, no combination of the main population of chondrite Lu-Hf data yields a regression with sufficiently low error to constrain the decay constant of ¹⁷⁶Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu-Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu-Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu-Hf characteristics results in a BSE Hf-Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε_Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu-Hf.     

87Rb–87Sr history of the Norton County enstatite achondrite

⁸⁷Rb⁸⁷Sr analysis of the Norton County achondrite has been achieved with special attention to the rubidium analysis. Enstatite crystals and polycrystalline material give an “age” of 4.48 ± 0.04 × 10⁹ years and an initial ratio ⁸⁷Sr/⁸⁶Sr_I= 0.7005 ± 0.0004 (λ = 1.39 × 10^?11yr^?1, maximum errors). The feldspar component of the meteorite contains about 70% of the strontium and 30% of the rubidium of the whole sample, and does not lie on the isochron. Its model age relative to the strontium initial ratio of Allende is 4.6 × 10⁹ years. The data are consistent with a complex history dealing with an incomplete isotopic reequilibration of the meteorite, 120 m.y. after its formation at 4.6 × 10⁹ years, with an initial ratio similar to that of Allende.     

210Pb in GEOSECS water profiles from the North Pacific

Ten GEOSECS profiles from the North Pacific have been analyzed for²¹⁰Pb. GEOSECS²²⁶Ra data on the same profiles are used to calculate²¹⁰Pb excess or deficiency relative to secular equilibrium. The resultant profiles are divisible into a thermocline zone (<2000m) showing an expected decrease with depth, a mid-water zone of about 2000 m showing small constant deficiencies with a zone of increasing deficiency to a bottom zone of about 1000 m having the highest deficiency virtually invariant with depth. The exponentially decreasing portion in the thermocline yields a “diffusion” coefficient of 3 cm²/s. The mid-water deficiencies yield ? model residence times of 400 years northeast of Hawaii decreasing to 100 years at the most marginal stations.     

Osmium isotopes as petrogenetic and geological tracers

Using terrestrial osmium-enriched samples of known ages, we have shown that¹⁸⁷Os/¹⁸⁶Os varies with time in result of the¹⁸⁷Re β^? radioactivity. Such a variation in the earth's mantle can be fitted by a straight line corresponding to¹⁸⁷Re/¹⁸⁶Os = 3.15 for the mantle, comparable to C1 carbonaceous chondrites. Using this result and the Re and Os contents of various rocks, several theoretical considerations and predictions can be deduced for the chemical evolution of the earth, such as a method for distinguishing between different processes of development of the continental crust. The special result of¹⁸⁷Os/¹⁸⁶Os from Bushveld is discussed with respect to the possible existence of an “enriched” subcontinental mantle.     

The behaviour of short-lived radiogenic lead isotopes (214Pb and212Pb) in groundwaters and laboratory leaching experiments

The concentrations of²¹⁴Pb (half-life=26.4minutes) and²²Rn (half-life=3.84days) have been measured in deep groundwaters of Gujarat, India. The results show that the abundance of²¹⁴Pb in the water is only ～25% of that expected from its production through the radioactive decay of dissolved²²²Rn. This deficiency if modelled in terms of a first-order removal, yields a residence time of ～10 minutes for²¹⁴Pb in these waters. The estimated residence time for²¹⁴Pb is the shortest observed for any nuclide in natural water systems and suggests that reactive nuclides lead like could be removed from aqueous phases to adjoining solid surfaces on extremely short time scales. Results of laboratory experiments using the²¹²Pb-²²⁴Ra pair are compatible with the observed fast removal of²¹⁴Pb from groundwaters.Re-evaluation of²³⁴Th residence times in these waters using a model with a recoil flux of²³⁴Th into aqueous phase, the same as that of²²²Rn, yields values in the range of 23 to<176 minutes, very similar to that of²¹⁴Pb. This “concordancy” in the residence times seems to suggest that the geochemical behaviour of²³⁴Th and²¹⁴Pb in these waters is quite similar.     

The effects of assimilation of country rocks by magmas on 18O/16O and 87Sr/86Sr systematics in igneous rocks

Examples of positive correlations between initial ⁸⁷Sr/⁸⁶Sr and δ¹⁸O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-¹⁸O, high-⁸⁷Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ¹⁸O-⁸⁷Sr/⁸⁶Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ¹⁸O-⁸⁷Sr/⁸⁶Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.