S. Vogt

Exposure histories of the lunar meteorites: MAC88104, MAC88105, Y791197, and Y86032

Four lunar meteorites, MacAlpine Hills (MAC) 88104, MacAlpine Hills 88105, Yamato (Y) 791197, and Yamato 86032 were analyzed for the cosmogenic radionuclides 10Be, 26Al, 36Cl, and 41Ca. From these and published data, histories of exposure to cosmic rays were modelled in terms of two-stage irradiations each with a long first stage on the Moon lasting a time T2π > 5 Ma at a burial depth d2π[gcm2] followed by a second stage in space, i.e., the transit time between the Moon and the Earth, lasting a time T4π [Ma] in a body of typical meteoroidal size. The terrestrial age Tt [Ma]gives the time elapsed between meteorite fall and recovery in Antarctica. The following sets of parameters were obtained: MAC88104/5, 390 ≤ d2π ≤ 500, 0.04 ≤ T4π ≤ 0.11, 0.10 ≤ Tt ≤ 0.19; Y791197, d2π 1000, T4π = 10 ± 2, 0.08 < Tt < 0.12. From the number and exposure histories of lunar meteorites we infer a production rate on the order of 5 Ma−1 and an arrival rate worldwide of about 3 × 106 meteorites Ma−1. These results suggest that each impact event large enough to produce lunar meteorites sends a large number of them to the Earth.

41Ca in iron falls, Grant and Estherville: production rates and related exposure age calculations

Recent technical developments in accelerator mass spectrometry have enabled us to measure routinely and with a precision of 5–7% the41Ca (1/2 = 104 ky) contents of extraterrestrial samples weighing approximately 100 mg. In essence, these advances have elevated41Ca to the role of a new and potent cosmogenic radioisotope with wide-ranging applications. We present here the results from the first phase of our41Ca cosmogenic studies program, aimed at establishing baseline concentrations and trends in selected meteorites and the use of41Ca in estimating exposure ages and pre-atmospheric meteorite radii. The average41Ca saturation activity recorded in four small iron falls is 24 ± 1 dpm/kg. This result, together with measurements at the center and surface of the large iron Grant, indicates that production of41Ca from spallation on iron is weakly dependent on shielding to depths as large as 250 g/cm2. We estimate the41K—41Ca exposure age of Grant to be 330 ± 50 My, and an upper limit to its terrestrial age of 43 ± 15 ky. A comparison of the41Ca contents of stony and metallic material separated from the mesosiderite Estherville identifies low-energy neutron capture on native Ca as a second important channel of production. We find that the41Ca signal in the stone phase from three meteorites correlates with their size, and that the inferred low-energy neutron fluxes vary by a factor of at least 20.

41CA, 26AL, AND 10BE IN LUNAR BASALT 74275 AND 10BE IN THE DOUBLE DRIVE TUBE 74002/74001

We report depth profiles of the cosmogenic radionuclides 10Be, 26Al, and 41Ca in the titanium-rich lunar basalt 74275. The 10Be profile is flat: 10Be activities are confined to a narrow range between 9.6 and 11.2 dpm/kg but are nonetheless consistent with a small contribution of about 1–2 dpm/kg from solar cosmic rays. The 26Al profile shows the steep decrease with increasing depth that is characteristic of nuclides whose production is dominated by solar cosmic rays. 41Ca activities decrease from about 22 dpm/kg at the surface to a minimum of ∼9 dpm/kg at a depth of 4.7 g/cm2 and then increase to ∼11 dpm/kg at a depth of 15.8 g/cm2. The sharp decrease near the surface identifies for the first time production of 41Ca by solar cosmic rays. We also report 10Be measurements for six samples from lunar core 74002/1. The 10Be activities range from approximately 8 to 14 dpm/kg. We model the production of 10Be, 26Al, and 41Ca in lunar rock 74275 by including published data that indicate a long exposure to galactic cosmic rays at a depth of 140 g/cm2 followed by one at the surface lasting 2.8 Ma. Cosmogenic radionuclide production by galactic cosmic rays, and, in the case of 41Ca, by thermal neutrons is estimated from published measurements and semi-empirical calculations. Our model includes a new calculation of production rates due to solar cosmic rays and incorporates recently published cross section measurements. Although many parameterizations of the flux of solar energetic particles give acceptable fits to the experimental data for 74275, we prefer a best fit obtained for 10Be and 26Al alone, which incorporates an erosion rate of ∼2 mm/Ma, a rigidity of 100 MV, and a 4π flux of protons with energies greater than 10 MeV of 89 cm−2 s−1. For 41Ca alone, the corresponding values are 2 mm/Ma, 80 MV, and 198 cm−2 s−1. The differences between the two sets of parameters may reflect uncertainties in the calculations of 41Ca production or a secular change in the solar cosmic ray flux. Calculations for a slab and for a hemispherical knob with a radius of 23 cm yield similar results.

Production of 41Ca and K, Sc and V short-lived isotopes by the irradiation of Ti with 35 to 150 MeV protons: applications to solar cosmic ray studies

In order to accurately model 41Ca production from solar cosmic ray (SCR) proton interactions on titanium at the lunar surface, we have measured the natTi(p,3pχn)41Ca excitation function for Ep = 35−150 MeV. The irradiated Ti foils were initially analyzed for their induced γ-activity to identify short-lived products and subsequently measured for their 41Ca content by accelerator mass spectrometry. Excitation curves, over the above energy range, were also obtained for 42,43K, 44g,44m,46,47,48Sc and 48V results for 41Ca are compared to both semi-empirical and model-based nuclear spallation calculations. We also used it to determine the magnitude and energy spectrum of the SCR flux from the measured Ca production profile in the Ti-rich lunar basalt, 74275. The results for K, Sc and V radionuclides were compared to previous measurements and agree favourably at low energies (E < 50 MeV) but are larger by ∼10–30% for higher energies.

Low 10Be and 26Al contents of ureilites: Production at meteoroid surfaces

Abstract We report the 10Be contents of 22 ureilites and the 26Al contents of 18 ureilites. When corrected for exposure age, where known, and for composition, the results give production rates, P, that are on average 20% lower than values typical of other meteorite classes. This depression of production rates, the weak correlation of both 26 and P10 with 22 Ne 21 Ne , and the generally high 3 He 21 Ne and 22 Ne 21 Ne ratios in ureilites all suggest that many of the samples occupied near-surface locations in their parent meteoroids. A reduced flux of cosmic rays does not by itself explain the elevated 22 Ne 21 Ne ratios and appears inconsistent with a proposed source for ureilites in the inner asteroid belt. Cosmogenic radionuclide data do not support the tentative pairing of ALHA 78019 with ALHA 78262 proposed on the basis of mineralogical similarities. The high 26Al content of LEW 85440 may be the result of irradiation by solar cosmic rays.