Alexander P. Meshik

Verification and interpretation of the I-Xe chronometer

Abstract Comparisons of I-Xe and Pb-Pb relative ages are made for phosphate and feldspar separates from 12 different meteorites. In all cases where I-Xe ages can be measured in phosphate, I-Xe and Pb-Pb chronometers agree within experimental uncertainty. No discordant phosphate samples are observed. With the exception of Allegan, I-Xe ages for feldspar separates generally agree with Pb-Pb ages for the corresponding phosphate. The general concordancy observed between I-Xe and Pb-Pb chronometers suggests that the I-Xe system is a reliable and interpretable chronometer when applied to single minerals systems. Allegan feldspar differs from the other feldspar separates, predating its phosphate by ∼17 Ma, with an I-Xe age indistinguishable from those of Orgueil and Murchison magnetites, among the oldest observed.

The Genesis solar-wind collector materials

Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector arrays and elsewhere targeted for the analyses of specific solar-wind components.Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community throughout the 21st Century.

The I-Xe record of alteration in the Allende CV chondrite

Abstract Complex I-Xe and mineralogical studies have been performed on four heavily-altered Allende fine-grained spinel-rich Ca, Al-rich inclusions (CAIs) and four Allende dark inclusions (DIs) showing various degrees of iron-alkali metasomatic alteration. The CAIs are largely composed of Fe-rich spinel, Al-diopside, and secondary nepheline and sodalite. The DIs consist of chondrules and Allende-like matrix composed of lath-shaped fayalitic olivine, nepheline, sodalite, and Ca, Fe-rich pyroxene ± andradite ± FeNi-sulfide nodules. Chondrule phenocrysts are extensively or completely replaced by fayalitic olivine, nepheline, and sodalite; metal nodules are replaced by FeNi-sulfides, andradite and Ca, Fe-rich pyroxenes. The chondrules and matrices are crosscut by Ca, Fe-rich pyroxene ± FeNi-sulfide ± fayalitic olivine veins. DIs are surrounded by continuous Ca-rich rims composed of andradite, wollastonite, kirschsteinite, and Ca, Fe-rich pyroxenes, whereas the outer portions of the inclusions are depleted in Ca. Three CAIs yield well-defined I-Xe isochrons with ages 3.1 ± 0.2, 3.0 ± 0.2 and 3.7 ± 0.2 Ma younger than the Shallowater internal standard (4566 ± 2 Ma). Similar release profiles suggest the same iodine carrier (most probably sodalite) for all four CAIs. The Allende DIs yield I-Xe ages from 0.8 ± 0.3 to 1.9 ± 0.2 Ma older than Shallowater. Based on the petrographic observations, we infer that the DIs experienced at least two-stage alteration. During an early stage of the alteration, which took place in an asteroidal setting, but not in the current location of the DIs, chondrule silicates were replaced by secondary fayalitic olivine, nepheline, and sodalite. Calcium lost from the chondrules was redeposited as Ca, Fe-rich pyroxene veins and Ca, Fe-rich pyroxene ± andradite nodules in the matrix. The second stage of alteration resulted in mobilization of Ca from the DIs and its re-deposition as Ca-rich rims composed of Ca, Fe-rich pyroxenes, andradite, and wollastonite, around the DIs. We interpret I-Xe ages of the DIs as time of their alteration prior incorporation into Allende. The younger I-Xe ages of the fine-grained spinel-rich CAIs may reflect hydrothermal alteration of the Allende host, which could have occurred contemporaneously with the second stage of alteration of the Allende DIs. The lack of evidence for the disturbance of I-Xe system in the Allende DIs may suggest that fluid responsible for the alteration of the Allende CAIs was in equilibrium with the I- and Xe-bearing phases of the DIs.

Reexamination of anomalous I-Xe ages: Orgueil and Murchison magnetites and Allegan feldspar

Abstract The extremely old I-Xe ages of Orgueil and Murchison magnetites, difficult to integrate into conventional meteorite evolutionary models, are not confirmed. New measurements indicate that Orgueil magnetite is 3 Ma younger than the Shallowater reference sample rather than 7 Ma older as previously reported by Lewis and Anders (1975) . If these differences are due to difficulties with the irradiation monitors in the earlier studies, there are wider implications because the same KI monitor was used to establish the I-Xe age of Murchison magnetite, which is used, in turn, as a reference sample in many subsequent I-Xe studies. Confirmation of these new results and a careful review of existing I-Xe age data are therefore in order. The old I-Xe age of Allegan feldspar, reported by Brazzle et al. (1999) may also be an artifact of shock disturbance.

Evaporative fractionation of zinc during the first nuclear detonation

Glass formed in the first nuclear detonation shows zinc loss by evaporation, indicating similar volatile loss during lunar formation. Volatile element and compound abundances vary widely in planets and were set during the earliest stages of solar system evolution. Experiments or natural analogs approximating these early conditions are limited. Using silicate glass formed from arkosic sands during the first nuclear detonation at the Trinity test site, New Mexico, we show that the isotopes of zinc were fractionated during evaporation. The green silicate glasses, termed “trinitite,” show +0.5 ± 0.1‰/atomic mass unit isotopic fractionation from ~200 m to within 10 m of ground zero of the detonation, corresponding to an α fractionation factor between 0.999 and 0.9995. These results confirm that Zn isotopic fractionation occurs through evaporation processes at high temperatures. Evidence for similar fractionations in lunar samples consequently implies a volatile-depleted bulk Moon, with evaporation occurring during a giant impact or in a magma ocean.

I–Xe systematics of the impact plume produced chondrules from the CB carbonaceous chondrites: Implications for the half-life value of 129I and absolute age normalization of 129I–129Xe chronometer

It is inferred that magnesian non-porphyritic chondrules in the CB (Bencubbin-type) carbonaceous chondrites formed in an impact generated plume of gas and melt at 4562.49 ± 0.21 Ma (Bollard et al., 2015) and could be suitable for the absolute age normalization of relative chronometers. Here xenon isotopic compositions of neutron irradiated chondrules from the CB chondrites Gujba and Hammadah al Hamra (HH) 237 have been analyzed in an attempt to determine closure time of their I-Xe isotope systematics. One of the HH 237 chondrules, #1, yielded a well-defined I-Xe isochron that corresponds to a closure time of 0.29 ± 0.16 Ma after the Shallowater aubrite standard. Release profiles and diffusion properties of radiogenic 129*Xe and 128*Xe, extracted from this chondrule by step-wise pyrolysis, indicate presence of two iodine host phases with distinct activation energies of 73 and 120 kcal/mol. In spite of the activation energy differences, the I-Xe isotope systematics of these two phases closed simultaneously, suggesting rapid heating and cooling (possibly quenching) of the CB chondrules. The release profiles of U-fission Xe and I-derived Xe correlate in the high temperature host phase supporting simultaneous closure of 129I-129Xe and 207Pb-206Pb systematics. The absolute I-Xe age of Shallowater standard is derived from the observed correlation between I-Xe and Pb-Pb ages in a number of samples. It is re-evaluated here using Pb-Pb ages adjusted for an updated 238U/235U ratio of 137.794 and meteorite specific U-isotope ratios. With the addition of the new data for HH 237 chondrule #1, the re-evaluated absolute I-Xe age of Shallowater is 4562.4 ± 0.2 Ma. The absolute I-Xe age of the HH 237 chondrule #1 is 4562.1 ± 0.3 Ma, in good agreement with U-corrected Pb-Pb ages of the Gujba chondrules (Bollard et al., 2015) and HH 237 silicates (Krot et al., 2005). All I-Xe data used here, and in previous estimates of the absolute age of Shallowater, are calculated using 15.7 ± 0.6 Ma value for 129I half-life. The slopes of I-Xe - Pb-Pb correlation lines plotted for different sets of samples for Shallowater normalization are always ≤1. Assuming uranium half-life values are correct; this restricts the half-life of 129I to ≤15.7 Ma.

The I-Xe chronometer

Abstract129Xe, from the decay of the now-extinct 16.7 Ma129I, accumulates in iodine-bearing sites and since most iodine host phases are secondary, the I-Xe system is typically a chronometer for post-formational processes. The validity of the I-Xe chronometer is confirmed by comparison with Pb-Pb ages on phosphate and feldspar separates from twelve meteorites. Phosphate separates are found to be concordant with Pb-Pb for all six samples in which useful I-Xe data were obtained. Feldspar is a better iodine host than apatite in H chondrites, typically providing good I-Xe isochrons. These too are concordant with the Pb-Pb ages of the corresponding phosphates for five out of six feldspar separates. The exception is Allegan whose feldspar yields one of the oldest I-Xe ages observed, similar to those for CI and CM magnetites. We attribute this to a more primary mineralization, predating the secondary phosphate from which the comparison Pb-Pb age was obtained. Absolute I-Xe ages, found using the reported Pb-Pb age of Acapulco phosphate provide an absolute I-Xe age of 4.566 ± 0.002 Ga for both Shallowater and Bjurböle irradiation standards. This allows relative I-Xe ages to be interpreted in the context of absolute ages.

Chemically fractionated fission-xenon in meteorites and on the earth

Abstract 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 132 Xe, 129 Xe, and to a lesser extent in 131 Xe. Similar large excesses of 132 Xe as well as of 131 Xe, 134 Xe, and 129 Xe 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 134 Xe, 132 Xe, 131 Xe, and 129 Xe. 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 238 U, 235 U, and/ or 244 Pu) 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 129 Xe in mantle rocks and natural gases are necessarily connected with the decay of primordial 129 I.

New evidence for chemical fractionation of radioactive xenon precursors in fission chains

Mass-spectrometric analyses of Xe released from acid-treated U ore reveal that apparent Xe fission yields significantly deviate from the normal values. The anomalous Xe structure is attributed to chemically fractionated fission (CFF), previously observed only in materials experienced neutron bursts. The least retentive CFF-Xe isotopes, 136Xe and 134Xe, typically escape in 2:1 proportion. Xe retained in the sample is complimentarily depleted in these isotopes. This nucleochemical process allows understanding of unexplained Xe isotopic structures in several geophysical environments, which include well gasses, ancient anorthosite, some mantle rocks, as well as terrestrial atmosphere. CFF is likely responsible for the isotopic difference in Xe in the Earths and Martian atmospheres and it is capable of explaining the relationship between two major solar system Xe carriers: the Sun and phase-Q, found in meteorites.

Discovery of fissionogenic Cs and Ba capture five years after Oklo reactor shutdown

Significance The Oklo natural nuclear reactors provide a wealth of information regarding the migration and retention of fission products in nuclear wastes. Radioactive volatile and gaseous elements easily escape from reactor fuel into the environment without proper containment. Cesium, in particular, represents a significant environmental and health hazard. Here, we used an isotope imaging system to identify the location of sequestered fissionogenic Cs and Ba in Oklo. Cesium and Ba were captured in Ru metal/sulfide aggregates shortly after reactor criticality ceased. These elements were otherwise nearly completely lost from the reactor. We have further discovered the most depleted natural U on Earth, indicating that these fission products were retained in the most active region of the reactor. Understanding the release and sequestration of specific radioactive signatures into the environment is of extreme importance for long-term nuclear waste storage and reactor accident mitigation. Recent accidents at the Fukushima and Chernobyl nuclear reactors released radioactive 137Cs and 134Cs into the environment, the former of which is still live today. We have studied the migration of fission products in the Oklo natural nuclear reactor using an isotope imaging capability, the NAval Ultra-Trace Isotope Laboratory’s Universal Spectrometer (NAUTILUS) at the US Naval Research Laboratory. In Oklo reactor zone (RZ) 13, we have identified the most depleted natural U of any known material with a 235U/238U ratio of 0.3655 ± 0.0007% (2σ). This sample contains the most extreme natural burnup in 149Sm, 151Eu, 155Gd, and 157Gd, which demonstrates that it was sourced from the most active Oklo reactor region. We have discovered that fissionogenic Cs and Ba were captured by Ru metal/sulfide aggregates shortly following reactor shutdown. Isochrons from the Ru aggregates place their closure time at 4.98 ± 0.56 y after the end of criticality. Most fissionogenic 135Ba and 137Ba in the Ru migrated and was incorporated as Cs over this period. Excesses in 134Ba in the Ru point to the burnup of 133Cs. Cesium and Ba were retained in the Ru despite local volcanic activity since the reactor shutdown and the high level of activity during reactor operation.