Paleomagnetism of samples from the axial zone of the Afar depression

We have studied SmNd systematics in pyroxene and phosphate mineral separates of Angra dos Reis. A pyroxene-phosphate internal isochron age ofT₂ = 4.55 ± 0.04AE is obtained, in excellent agreement with reported Pb-Pb ages.¹⁴²Nd/¹⁴⁴Nd ratios in pyroxene samples are systematically larger than those in phosphates by 6 parts in 10⁵. This variation is tentatively assigned to a radiogenic contribution from extinct¹⁴⁶Sm. Fission xenon components in pyroxene and phosphate separates are characterized by discrete ratios of fission/spallation and evidence is presented for a third ratio in celsian. It is shown that this characteristic is due to a close association of²⁴⁴Pu with the light REE. Computed ratios²⁴⁴Pu/Nd are the same in pyroxene and phosphate separates, but²⁴⁴Pu/²³⁸U and²⁴⁴Pu/²³²Th ratios are not. Taking the fission xenon retention age to be 4.55 AE, we obtain an abundance ratio²⁴⁴Pu/Nd= 1.5 × 10^?4 (or an atomic ratio²⁴⁴Pu/¹⁵⁰Nd= 1.6 × 10^?3) at this time and in the region of the solar system where the Angra dos Reis parent body formed. The exposure age of Angra dos Reis, as obtained by the⁸¹Kr-⁸³Kr method is55.5 ± 1.2m.y. Neutron capture during the 55.5-m.y. exposure to cosmic rays increased the ratio¹⁵⁰Sm/¹⁴⁹Sm in Angra dos Reis by 6 parts in 10⁴.     

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.     

Formation chronology for C2 meteorites

The times at which phyllosilicate matrix and euhedral olivines became associated have been determined for five C2 meteorites. The ages, calculated from fission track densities on crystal surfaces, are based on an initial²⁴⁴Pu/²³⁸U ratio in the matrix material of 0.0154 at 4.6 b.y., and range from 4.22 b.y. for Nogoya to 4.42 b.y. for Murray. Unless the initial²⁴⁴Pu/²³⁸U ratio was less than 0.004, the meteorites cannot have existed in their present form for 4.6 × 10⁹ yr. The measured ages place limits on the time when pre-compaction effects such as micrometeorite craters and solar flare tracks were produced, and they may approximately date the formation of the olivine crystals themselves.     

Nitrogen isotopes in lunar highlands breccias

Some breccias from the lunar highlands have probably trapped solar wind gases at a very early epoch in the history of the moon, as implied by their high contents of parentless fissiogenic xenon and sometimes, of parentless radiogenic¹²⁹Xe. Four samples of this type, on which noble gas data already exist, have been selected for analysis of nitrogen contents and isotopic composition, by using step-wise heating techniques: 14047, 14055, 14307, 60255. Since uncertainties in the evolution of the solar wind¹⁵N/¹⁴N ratio with time are due in part to uncertainties in the measurement of the epoch of exposure, those samples provided the opportunity to measure the isotopic composition of nitrogen which has been trapped in the remote past, avoiding the problems inherent in the use of spallogenic nuclides. Results show that, in the samples studied from the Apollo 14 landing site, nitrogen is not particularly light, and has not been acquired, as a whole, in very ancient times. The conflicting presence of both parentless xenon and nitrogen of relatively “recent” isotopic signature can be explained if the hypothetical light nitrogen is diluted by more abundant, heavier nitrogen. Accordingly, the very ancient soil components which are implied in these objects by the presence of excess fission xenon have been re-exposed at a much later epoch, or mixed with some younger soil components, before the compaction event. The present data do not question the hypothesis of a secular isotopic variation of lunar trapped nitrogen, but cannot prove that very light nitrogen was trapped together with parentless fission xenon in the soil components of the highlands soil breccias. The very unusual release pattern of nitrogen in breccia 60255 can result from nitrogen isotopic homogenization with gas loss.     

In-situ measurement of the rate of235U fission induced by lunar neutrons

The depth profile of the neutron-induced fission rate of²³⁵U was directly measured to a depth of 350 g/cm² by the Apollo 17 Lunar Neutron Probe Experiment. The fission rate rises sharply from the surface to a broad maximum from 110 to 160 g/cm² and drops off at greater depths. The shape of theoretical depth profile of Lingenfelter et al. fits the measured capture rates well at all depths. The absolute magnitude of the experimental fission rates are (11±17)% lower than those calculated theoretically. The excellent agreement between theory and experiment implies that conclusions drawn previously by interpreting lunar sample data with the theoretical capture rates will not require revision. In particular lunar surface processes, rather than uncertainties in the capture rates, are required to explain the relatively low neutron fluences observed for surface soil samples compared to the fluences expected for a uniformly mixed regolith.     

Fission track age and cooling rate of the Marjalahti pallasite

Nuclear tracks were studied in olivine and merrillite (phosphate previously called whitlockite) from the Marjalahti pallasite. The merrillite contains an important fission contribution due mainly to the spontaneous decay of now extinct²⁴⁴Pu. The U contents of 29 merrillite grains range from 60 to 140 ppb (median value: 85 ppb). Assuming a reasonable fractionation temperature of ～ 1750 K for the pre-pallasitic material, a lower limit of ～ 5 K/Myr is obtained for the cooling rate, in strong contrast with the previous metallographic result (～ 0.5 K/Myr). This disagreement, together with those observed in the case of mesosiderites, strengthens the need for a revision of the metallographic method of retracing the cooling histories of meteorites, as suggested by Narayan and Goldstein [31].     

The registration-temperature dependence of heavy-ion track-etch rates and annealing sensitivity in crystals: implications for cosmic ray identification and fission track dating of meteorites

Etch rates and etchable lengths of cosmic ray tracks in meteoritic crystals have been used by several workers to derive the charge spectrum of ancient cosmic rays. This is done by comparing the fossil cosmic ray track record with fresh accelerator-produced calibration tracks. These calibration tracks are generally produced at room temperature, while meteorites spend a high proportion of their lifetimes orbiting at large distances from the Sun ( 3–5 AU) and are, consequently, at much lower temperatures (typically 100–150 K) during most of their cosmic ray exposure ages. We have irradiated crystals of apatite, olivine, enstatite and diopside held at 77, 293, 473 and 573 K, with 2 MeV/nucleon⁸¹Br ions, and then etched them. We find that their track etching properties are dependent upon the temperature of the mineral during registration. The track etch velocity generally increases with registration temperature up to 300 or 500 K (the upper limit depending upon the type of crystal). Our results also indicate that the annealing sensitivity of fission tracks in fluorapatite may be influenced by the registration temperature. This temperature dependence has important implications not only for cosmic ray particle identification but also for fission track dating of meteorites in view of the fact that the meteorite parent bodies were at elevated temperatures at the beginning af their life when²⁴⁴Pu fission tracks were being generated abundantly.     

Isotopic and chemical investigations on Angra dos Reis

We report on extensive isotopic studies of Pb, Sr and Xe and on chemical abundance measurements of K, Rb, Sr, Ba, Nd, Sm, U and Th for total meteorite and mineral separates of the Angra dos Reis achondrite. U-Pb, Th-Pb and Pb-Pb ages are concordant at 4.54 AE for the total meteorite and for high-purity whitlockite in Angra dos Reis. This establishes Angra dos Reis as an early planetary differentiate which has not been disturbed for these systems since 4.54 AE ago. Measured⁸⁷Sr/⁸⁶Sr in pyroxene and whitlockite for Angra dos Reis (ADOR) are distinctly below BABI by two parts in 10⁴ and only one part in 10⁴ above the lowest⁸⁷Sr/⁸⁶Sr (ALL) measured in an Allende inclusion. The difference in ADOR-ALL corresponds to an interval of condensation in the solar nebula of ～3 m.y. If²⁶Al was the heat source for the magmatism on the parent planets of Angra dos Reis and the basaltic achondrites (BABI) then the relatively large difference in⁸⁷Sr/⁸⁶Sr, BABI - ALL, must be the result of planetary evolution rather than condensation over ～10 m.y. Xe isotopic measurements confirm the presence of large amounts of²⁴⁴Pu-produced fission Xe and show that²⁴⁴Pu was enriched in the whitlockite relative to the pyroxene by a factor of ～18. We present chemical element enrichment factors between the whitlockite and the fassaitic pyroxene in Angra dos Reis. The enrichment factors demonstrate close analogy between the rare earth elements and their actinide analogs. The enrichment factor for Pu is intermediate to the enrichment factors of Nd and Sm.     

Genesis and Equilibrium of Natural Lithospheric Radioxenon and its Influence on Subsurface Noble Gas Samples for CTBT On-site Inspections

During on-site inspections to verify the comprehensive nuclear-test-ban treaty (CTBT), soil gas samples may be taken and analysed for their content of the xenon isotopes ^131mXe, ¹³³Xe, ^133mXe and ¹³⁵Xe in order to identify a suspected underground nuclear test. These samples might contain natural radioxenon which is present as a trace gas in the ground. This work analyses the different production mechanisms of natural lithospheric radioxenon to assess theoretically the background concentration under different sampling conditions. The results imply that the equilibrium concentrations of the examined xenon isotopes can be measured in certain rock types using actual CTBTO on-site inspection equipment. Radioxenon production is dominated by spontaneous fission of ²³⁸U, resulting in a reactor-like xenon isotopic signature rather than an explosion-like signature.     

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.     

Depositional history of artificial radionuclides in the Ross Ice Shelf,Antarctica

The annual fluxes of artificial radionuclides (²³⁸Pu,^239+240Pu,²⁴¹Am,¹³⁷Cs,⁹⁰Sr and³H) from the atmosphere to the Ross Ice Shelf in Antarctica were determined from measurements in strata dated by²¹⁰Pb. Recognizable sources include the U.S. tests (Mike-Ivy and Castle Hill) in the early 1950s, the U.S.S.R. tests of the early 1960s, the SNAP-9A burnup of 1964 and the French and Chinese tests in the late 1960s and 1970s. There are several problems still awaiting resolution: the differences in atmospheric chemistries of fission products and of transuranics produced in weapons tests and the anomalous fluxes of²³⁸Pu to the ice shelf which do not appear to reflect a one-year stratospheric residence. There is no evidence for a smearing of the fallout record as a consequence of diffusion of these radionuclides in the glacial column.     

The Oklo natural reactor: Cumulative fission yields and retentivity of the symmetric mass region fission products

Solid source mass spectrometry has been used to determine the relative cumulative fission yields of five elements in three samples of uranium ore from reactor zones in the Oklo mine site. Eighteen fission chains covering the mass range from105 ≤ A ≤ 130 have been measured for Pd, Ag, Cd, Sn and Te. These measurements have enabled a number of nuclear parameters to be calculated including the relative proportions of²³⁵U,²³⁸U and²³⁹Pu involved in the fission process.The concentration of the five elements in the Oklo samples have also been measured using the stable isotope dilution technique. These values have then been compared to the estimates of the amount of these elements produced by fission under the conditions that are appropriate to the three samples. This procedure enables the retentivity of the elements in the reactor zones to be evaluated. Our work confirms the fact that Pd and Te are retained almost in their entirety in the samples, whereas the other three elements have been partially lost from the reactor site. Almost all the Cd fission products have been lost, and more than 50% of the Ag and Sn fission-produced material has been removed.     

Angra dos Reis: Plutonium distribution and cooling history

The preatmospheric mass of Angra dos Reis is estimated to be about 80 kg, based on cosmic ray track densities of 1?3 × 10⁶ cm^?2 in feldspars, olivines, and pyroxenes. The²⁴⁴Pu-fission track densities in pyroxenes and adjacent olivine-feldspar track detectors, as well as in whitlockites and adjacent pyroxene-olivine detectors are about 20 and 280 × 10⁶ cm^?2 respectively. These values correspond exactly to the²⁴⁴Pu-fission xenon contents in the two minerals. This suggests an almost instantaneous cooling (>70 K per m.y.) of the Angra dos Reis material 4.55 b.y. ago. This fast cooling is the reason for a quantitative⁴He retention since that time. If the⁴He excess found in Angra dos Reis is a real effect, the source of this excess remains unknown.     

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%.     

Chemistry and structure of lunar and synthetic armalcolite

A study of the chemical trends displayed by lunar armalcolites has been made in conjunction with single-crystal X-ray structure refinements of lunar and synthetic armalcolite in order to assess the possible importance of Ti³⁺ in lunar armalcolite and to characterize the effects of cation substitutions on the structure. The apparent cation deficiencies found in lunar armalcolites analyzed with the electron microprobe most likely reflect the presence of Ti³⁺, although the existence of vacancies cannot be ruled out. Structure refinements of an Apollo 17 armalcolite are consistent with either interpretation. These results support experimental evidence suggesting the presence of Ti³⁺ in armalcolite and indicate that virtually all lunar armalcolites probably contain ～4–11 mol.% Ti₂³⁺Ti⁴⁺O₅ component in solid solution. The cation distribution in lunar armalcolite is essentially completely ordered. However, synthetic crystals quenched from near 1200°C have been found to retain significant cation disorder.     

Estimation of Explosion Energy Yield at Chernobyl NPP Accident

The value of the ¹³³Xe/^133mXe isometric activity ratio for the stationary regime of reactor work is about 35, and that for an instant fission (explosion) is about 11, which allowed estimation of the nuclear component of the instant (explosion) energy release during the NPP accident. Atmospheric xenon samples were taken at the trajectory of accident product transfers (in the Cherepovetz area); these samples were measured by a gamma spectrometer, and the ¹³³Xe/^133mXe ratio was determined as an average value of 22.4. For estimations a mathematic model was elaborated considering both the value of instant released energy and the schedule of reactor power change before the accident, as well as different fractionation conditions on the isobaric chain. Comparison of estimated results with the experimental data showed the value of the instant specific energy release in the Chernobyl NPP accident to be 2·10⁵–2·10⁶ J/Wt or 6·10¹⁴–6·10¹⁵ J (100–1,000 kt). This result is matched up to a total reactor power of 3,200 MWt. However this estimate is not comparable with the actual explosion scale estimated as 10t TNT. This suggests a local character of the instant nuclear energy release and makes it possible to estimate the mass of fuel involved in this explosion process to be from 0.01 to 0.1% of total quantity.     

Fallout240Pu and239Pu in Atlantic marine samples

Mass spectrometric analyses of low levels of global fallout plutonium separated from Atlantic marine samples have differentiated fallout²³⁹Pu and²⁴⁰Pu in aquatic samples for the first time. The results show no single characteristic²⁴⁰Pu/²³⁹Pu ratio in marine samples; the observed range is from 0.11 to 0.24 on an atom basis. There are indications that differences exist in the chemical or physical form of plutonium from atmospheric fallout in Atlantic surface water and that selective concentration in surface organisms is occurring. No single²⁴⁰Pu/²³⁹Pu value is found in pelagic sediments collected from different depths and locations. Discounting sources other than fallout, our results show that the plutonium deposited at any given time since atmospheric testing began may have carried a unique²⁴⁰Pu/²³⁹Pu tag. This label may be extremely useful to trace fallout plutonium through biogeochemical cycles.     

Pu and137Cs in coastal sediments

Analyses are presented of¹³⁷Cs,²³⁸Pu, and^239,240Pu, in relation to depth in sediment, in 21 gravity cores. These cores span the ranges of times 1964–1975, and of water depths 12–2000 m; they come from three distinct sedimentation areas off the northeast coast of the United States. Although the ranges of total sediment inventories of^239,240Pu and of¹³⁷Cs from the various areas hardly overlap, the range of ratios of the inventories of these two nuclides is probably the same in all the areas. In the shallow-water cores the^239,240Pu/¹³⁷Cs ratio regularly diminishes with depth in the core, and a tendency is seen for curves of this function to have similar slopes in each area; ratios of²³⁸Pu/^239,240Pu show no change with depth in these shallow-water cores. In the deeper-water cores, the^239,240Pu/¹³⁷Cs ratio shows no systematic change with depth, but sometimes the²³⁸Pu/^239,240Pu ratio shows a minimum at the sediment surface, and is much higher deeper in the cores. We believe that these phenomena can be explained in terms of a complicated bioturbational process moving the nuclides, together, down into the sediments, of chemical resolubilization, at depth, of plutonium only, and of its subsequent upward translocation in the interstitial solution. Some re-immobilization of plutonium near the sediment surface is implied, and a mechanism is suggested for this, based on displacement of plutonium from organic complexes by the increasing concentrations, in upper layers of the sediment, of re-oxidized dissolved iron.     

Pu in coastal marine environments

Analysis of water samples from the New York Bight area and Narragansett Bay reveals that a small fraction of the total Pu (probably Pu (III + IV) species) is continuously removed to the sediments at a rate similar to that of the particle-reactive isotope²²⁸Th. A more “soluble” Pu species appears to be released at times from the sediments to the water column in these nearshore regions. Sediments in shallow areas of the New York Bight south of Rhode Island and Narragansett Bay have high Pu inventories and relatively deep penetration of this element, although the net sediment accumulation rate is generally low (<0.03 g/cm² yr). The high Pu inventories can be explained if both sediment resuspension and sediment mixing are assumed to be the major controlling factors for the effective transfer of Pu from the water column to the sediments. By simultaneous modelling of the depth distribution of three tracers which operate on vastly different time scales:²³⁴Th (half-life 24 days),²¹⁰Pb (half-life 22 years) and^239,240Pu (introduced into the environment during the past 30 years), bioturbation rates ranging from 4 to 32 cm²/yr in the surface mixed layer (5–10 cm thick) and from 0.3 to 2.5 cm²/yr in the layer below (up to 40 cm thick) and net sediment accumulation rates of approximately zero to 0.14 g/cm² yr were calculated for these areas.