J. Klein

Cosmic ray produced 10Be and 26Al in Antarctic rocks: exposure and erosion history

We have measured cosmic ray produced (t1/2 = 1.5 million years) 10Be and (t1/2 = 0.705 million years) 26Al in purified quartz fractions of selected rock samples from Antarctic mountains. From these data we calculate (1) mean erosion rates, for the limiting case of steady-state surface exposure to cosmic rays, and (2) minimum exposure ages, for the limiting case of no erosion. Calculated mean erosion rates are very low, on the order of a few times 10−5 cm/yr; we believe the sampling to be sufficient to generalize this result to exposed bedrock in Antarctica. In favorable cases it is possible to distinguish between the limiting cases: steady-state erosion seems a better description in such cases. Most samples, including some taken a few meters above the present ice level, seem to have been exposed for millions of years, without major episodes of burial or abrasion by ice.

In situ10Be-26Al exposure ages at Meteor Crater, Arizona

A new method of dating the surface exposure of rocks from in situ production of 10Be and 26Al has been applied to determine the age of Meteor Crater, Arizona. A lower bound on the crater age of 49,200 ± 1,700 years has been obtained by this method.

Cosmogenic10Be,26Al, and3He in olivine from Maui lavas

We report the presence of cosmogenic10Be and26Al in olivines from the Maui Haleakala basalts in which cosmogenic3He and21Ne were discovered. Based on production-rate calibrations in quartz crystals from late Pleistocene granite exposures in the Sierra Nevada, the Haleakala crater erosion rates derived from10Be and26Al concentrations agree within 10% and give a mean rate of 12 m/Myr, about 40% greater than the rate of 8.5 m/Myr previously derived from3He measurements. These results establish the feasibility of the simultaneous use of radioactive and stable cosmogenic nuclides for studies of exposure-ages and erosion-rates in basaltic and andesitic terrains.

26Al and10Be production in iron meteorites

Abstract We have measured by accelerator mass spectrometry the 26 Al contents of 20 and the 10 Be contents of 14 iron meteorites. The 26 Al contents are typically 30% or more lower than values obtained by counting techniques; the 10 Be contents are 10–15% lower. The production rates ( P ) of these nuclides decrease by more than a factor of two as the 4 He/ 21 Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimate P 26 (Fe) as 3–4 dpm/kg and P 10 (Fe) as 4–5 dpm/kg. The average P/ 10 P 26 activity ratio is close to 1.5. Exposure ages calculated from 21 Ne/ 26 Al ratios cannot be calibrated so as to agree with both 40 KK/ ages and ages based on the shorter-lived nuclides 39 Ar and 36 Cl. If agreement with the latter is forced, then the disagreement with 40 KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10 7 a.

Measurement of scintillation efficiency for nuclear recoils in liquid argon

The scintillation light yield of liquid argon from nuclear recoils relative to electronic recoils has been measured as a function of recoil energy from 10 keVr up to 250 keVr. The scintillation efficiency, defined as the ratio of the nuclear recoil scintillation response to the electronic recoil response, is 0.25 \pm 0.01 + 0.01(correlated) above 20 keVr.

Detection of erosion events using10Be profiles: example of the impact of agriculture on soil erosion in the Chesapeake Bay area (U.S.A.)

10Be concentration, total carbon and grain-size were measured in cores collected in undisturbed estuarine sediments of three tributaries of the Chesapeake Bay. These cores were previously studied by Davis [1] and Brush [2,3] for pollen content, age and sedimentation rate. In this work, we compare the results obtained for these various analyses. In the cores, we observed two increases in10Be concentration concomitant with two major changes in the pollen composition of the sediments. These two pollen changes each correspond to well-dated agricultural horizons reflecting different stages in the introduction of European farming techniques [2]. In the Chesapeake Bay area, the agricultural development, associated with forest clearing, appears to have triggered the erosion, transport, and sedimentation into the river mouths of large quantities of10Be-rich soils. This phenomenon explains the observed rise in the sedimentation rate associated with increases in agricultural land-use.

Depth profile of41Ca in an Apollo 15 drill core and the low-energy neutron flux in the Moon

Abstract Systematic measurements of the concentrations of cosmogenic 41 Ca (half-life = 1.04 × 10 5 yr) in the Apollo 15 long core 15001–15006 were performed by accelerator mass spectroscopy. Earlier measurements of cosmogenic 10 Be, 14 C, 26 Al, 36 Cl, and 53 Mn in the same core have provided confirmation and improvement of theoretical models for predicting production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides, 41 Ca in the lunar surface is produced mainly by thermal neutron capture reactions on 40 Ca. The maximum productions of 41 Ca, about 1 dpm/g Ca, was observed at a depth in the Moon of about 150 g/cm 2 . For depths below about 300 g/cm 2 , 41 Ca production falls off exponentially with an e-folding length of 175 g/cm 2 . Neutron production in the Moon was modeled with the Los Alamos High Energy Transport Code System, and yields of nuclei produced by low-energy thermal and epithermal neutrons were calculated with the Monte Carlo N-Particle code. The new theoretical calculations using these codes are in good agreement with our measured 41 Ca concentrations as well as with 60 Co and direct neutron fluence measurements in the Moon.

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.