B. Lavielle

The 36Cl–36Ar–40K–41K records and cosmic ray production rates in iron meteorites

This is the first contribution towards a reevaluation of exposure histories of iron meteorites and of the constancy of the cosmic ray flux over the last billion years, as recorded in these fossil detectors. We have performed new 36Cl, 26Al, 10Be, and noble gas measurements, including determination of the shielding parameter, S = 4He/21Ne, in samples with published K data. The K isotopic data, coupled to 36Ar and 36Cl concentrations permit selection of meteorites which have only experienced simple (constant geometry) irradiation histories. These objects can be used for the calibration of shielding-dependent production rates within these metallic detectors. In order to carry out production rate calibrations based on 40K–41K data, we assume constancy of the cosmic flux during the interval 150 to 700 My ago. We note that meteorites with very old potassium ages cannot be included in this calibration, as these meteorites require distinct parameter sets. A calibration data set representing a total of 13 meteorites was used to compute long-term (0.5 Gy) average production rates. These average production rates of 36Cl from this particular calibration set are significantly (28%) lower than those determined for the recent (≤10 My) cosmic ray flux. We also document here the quality of the resulting potassium production rate parameter M0(S) with a calculated isochron for irons of group IVA.

Isotopic signatures and origin of nitrogen in IIE and IVA iron meteorites

Nitrogen concentrations and isotopic signatures have been determined in groups IIE and IVA iron meteorites. Contrary to assumptions made in the literature, the present data show that spallation components significantly modify the N signatures of the metal. All 15N data are corrected for cosmic-ray produced spallation components using 21Ne concentrations measured in aliquots. A production-rate ratio 21Ne/15N = 0.80 is obtained, which can reliably be used for this correction, since it is not sensitive to shielding differences. The trapped N signatures in group IVA irons fall into two subgroups IVA (−26) with δ15N = −26 ± 2 ‰ and IVA (−6) with δ15N = −6 ± 1.4‰, respectively. Only the latter is close to values reported for metal of L-chondrites. Group IIE irons also define two distinct subgroups IIE (Y) with δ15N = −7.5 ± 1.5‰ and IIE (O) with δ15N = −2.3 ± 1‰. Therefore, the earlier proposed subdivision of IIE irons into “young” and “old” subgroups is substantiated by the different trapped N signatures. With regard to a possible relationship with H-chondrites as suggested by oxygen isotopes, only the young IIE subgroup overlaps the range of signatures reported in H-chondrite metal. Seymchan has a distinctly lighter signature (δ15N = −54‰), consistent with its reclassification as an ungrouped iron. We observe no correlation of either nitrogen concentrations or isotopic signatures with abundances of Ga, Ge, Ir or Ni. The distinct N components in the so-called magmatic group IVA constrain the thermal history of the parent body, as these signatures need to be reconciled with the magmatic history. Implications for the origin of nitrogen components are discussed. Some of the spread in N isotopic data in the literature apparently is due to inclusions.

Ultratrace analysis of krypton isotopes by resonant ionization spectroscopy-time of flight mass spectrometry (RIS-TOF)

A new RIS-TOF instrument, called FAKIR (Facility for Analyzing Krypton Isotope Ratios), has been developed at CENBG in order to measure Kr isotope ratios with an extremely high sensitivity. The instrument uses a single color Kr ionization scheme with tunable coherent UV photons near 216.6 nm. A two-photon resonance excitation allows it to reach the 5p[5/2]2 excitation level followed by a single-photon ionization. Krypton ions are accelerated towards an electron multiplier. The instrument includes a cryogenic concentrator that increases the efficiency of the ion source and a new selecting system allowing the deflection of the abundant isotopes before they impact the detector. This device eliminates the blinding effect on the detector that alters the detection of the less abundant Kr isotopes. The current sensitivity of the instrument of ∼6700 atoms has been demonstrated by extracting the gas from 37 mg of the Boguslavka iron meteorite. The associated errors on the 81Kr isotope ratio measurements did not exceed ∼12%. A good agreement is observed with conventional mass spectrometry analysis, which requires several grams of material, by measuring the cosmic ray exposure age of the Boguslavka meteorite.

Vertical distribution of helium and 40Ar/36Ar in porewaters of the Eastern Paris Basin (Bure/Haute-Marne): constraints on transport processes through the sedimentary sequence

Abstract As part of its ongoing project on repositories for high-activity, long-lived radioactive waste, a 2000 m deep borehole was drilled by the French Nuclear Waste Agency (ANDRA) in the layered structure of alternating aquifers and aquitards of the Eastern Paris Basin. Among the information retrieved from this borehole, the vertical distribution of chloride in porewaters showed that, in addition to vertical diffusion, lateral advection in the aquifers plays a major part in transporting chlorine away from the study area. Helium concentrations were also measured in porewaters along the borehole. Because the helium input function is different from that of chlorine, it represents an excellent alternative tracer to further constrain transport characteristics. We applied an advection–diffusion model to the helium profiles with the appropriate source term for 4He based on U–Th measured concentrations of uranium and thorium. 40Ar/36Ar data, which were available along the whole sequence, were also simulated. The modelled and measured 4He profiles were in good agreement, indicating that the transport parameters used for the chlorine simulations were robust. 40Ar/36Ar simulations also gave coherent results and confirmed that most of the radiogenic 40Ar remained trapped in the rocks (primarily in clays and feldspars).

Development toward a double focusing isotopic separator for noble gas isotope enrichment

A double focusing sector field mass filter used in Nier-Johnson geometry has been built in order to perform Kr isotope enrichment for 81 Kr and 85 Kr isotopes. The principle consists in implanting Kr+ ions accelerated at 7 keV in Al foils after separation using the magnetic sector. A specific ion source has been designed capable of generating high Kr+ ion beams (>0.5 μA) to transfer into the collecting Al foils in 3 to 5 h significant fractions of large Kr samples (1015 to 1016 atoms) initially introduced in the instrument. Implanted Kr isotopes can be further selectively released from the Al foil by surface ablation using an infrared laser beam. Implantation yields and enrichment factors are measured using a conventional mass spectrometer. Copyright

History of the solar environment

Galactic cosmic rays (GCR) provide information on the solar neighborhood during the suns motion in the galaxy. There is now considerable evidence for GCR acceleration by shock waves of supernova in active star-forming regions (OB associations) in the galactic spiral arms. During times of passage into star-forming regions increases in the GCR-flux are expected. Recent data from the Spitzer Space Telescope (SST) are shedding light on the structure of the Milky Way and of its star-forming-regions in spiral arms. Records of flux variations may be found in solar system detectors, and iron meteorites with GCR-exposure times of several hundred million years have long been considered to be potential detectors (Voshage, 1962). Variable concentration ratios of GCR-produced stable and radioactive nuclides, with varying half-lives and therefore integration times, were reported by Lavielle et al . (1999), indicating a recent 38% GCR-flux increase. Potential flux recorders consisting of different pairs of nuclides can measure average fluxes over different time scales (Lavielle et al ., 2007; Mathew and Marti, 2008). Specific characteristics of two pairs of recorders ( 81 Kr-Kr and 129 I- 129 Xe) are the properties of self-correction for GCR-shielding (flux variability within meteorites of varying sizes). The 81 Kr-Kr method (Marti, 1967) is based on Kr isotope ratios, while stable 129 Xe is the decay product of the radionuclide 129 I, which is produced by secondary neutron reactions on Te in troilites of iron meteorites. The two chronometers provide records of the average GCR flux over 1 and 100 million year time scales, respectively.