Browsing by Author "Herzog, GF"

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    41Ca, 26Al, and 10Be in lunar basalt 74275 and 10Be in the double drive tube 74002/74001
    (Elsevier, 1998-07) Fink, D; Klein, J; Middleton, R; Vogt, S; Herzog, GF; Reedy, RC
    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. © 1998 Elsevier Science Ltd.
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    Accelerator mass spectrometry ultrasensitive analysis for global science
    (CRC Press, 1998-03-25) Tuniz, C; Kutschera, W; Fink, D; Herzog, GF; Bird, JR
    This extensive undertaking, Accelerator Mass Spectrometry, conducts an elaborate and comprehensive summary of one of the foremost catalysts of progress in scientific research. Accelerator mass spectrometry (AMS), an innovative analytical technique, measures rare atoms at unprecedented levels of sensitivity, revolutionizing the science of radiocarbon dating and accessing new natural radioisotopes as environmental tracers and chronometers. This book demonstrates how AMS is applied in the studies of extraterrestrial materials, the earth sciences, the future of the global environment, and the history of mankind. This compendium also highlights the significant impact of AMS on several fields of scientific investigation, spurring remarkable studies in global climate change, ancient artifacts, pollution, nuclear safeguards, geochronology, and materials characterization. The myriad of sample types and variety of applications in this examination include: Meteorites from Mars Ancient air trapped in Antarctic ice The Shroud of Turin The dating of human bones The colonization of the Americas and Australia Ancient rock art The crown of Charlemagne Cancerogenic effects of cooked meat The consequences of the Chernobyl accident The role of aluminum in Alzheimer's Disease This unique edition has compiled the diverse set of scientific literature into a single volume, suitable as a text or resource on the major AMS-related outcomes, issues, and methods.
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    Complex exposure histories for meteorites with “short” exposure ages
    (Wiley, 1997-05) Herzog, GF; Vogt, S; Albrecht, A; Xue, S; Fink, D; Klein, J; Middleton, R; Weber, H; Schultz, L
    We report measurements of 26Al and 10Be activities in nine ordinary chondrites and of the light noble gas concentrations and 36Cl and 41Ca activities in subsets of those meteorites. All but Murray have low 21Ne concentrations (<1.0 × 10−8cm3STP/g) and have previously been used to estimate 21Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21Ne production rates that are consistent with the standard L-chondrite value of 0.33 × 10−8cm3STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower 3He/21Ne ratios than expected based on their 22Ne/21Ne ratios. A general association between low 21Ne contents and 3He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low 21Ne contents, one-stage exposure histories, and losses of cosmogenic 3He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia. © 1999-2021 John Wiley & Sons, Inc.
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    Cosmic-ray exposure history of the Norton County enstatite achondrite
    (Wiley, 2011-01-28) Herzog, GF; Albrecht, A; Ma, P; Fink, D; Klein, J; Middleton, R; Bogard, DD; Nyquist, LE; Shih, CY; Garrison, DH; Reese, Y; Masarik, J; Reedy, RC; Rugel, G; Faestermann, T; Korschinek, G
    We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N-Particle codes. Assuming a one-stage irradiation of a meteoroid with a pre-atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (<6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one-stage cosmic-ray exposure (CRE) age of 115 Ma. Argon-36 released at intermediate temperatures, 36Arn, is attributed to production by thermal neutrons. From the values of 36Arn, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 1016 neutrons cm−2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2–1.4 × 1016 neutrons cm−2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto-Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma. © The Meteoritical Society, 2011
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    Exposure history of the Torino meteorite
    (Wiley, 1996-03) Wieler, R; Graf, T; Signer, P; Vogt, S; Herzog, GF; Tuniz, C; Fink, D; Fifield, LK; Klein, J; Middleton, R; Jull, AJT; Pellas, P; Masarik, J; Dreibus, G
    We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ∼20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ∼65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. © 1999-2021 John Wiley & Sons, Inc.
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    Light noble gases and cosmogenic radionuclides in Estherville, Budulan and other mesosiderites: Implications for exposure histories and production rates
    (Wiley, 2000-08) Albrecht, A; Schnabel, C; Vogt, S; Xue, S; Herzog, GF; Begemann, F; Weber, H; Middleton, R; Fink, D; Klein, J
    We report measurements of 26AI, 10Be, 41Ca, and 36Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are 26Al/10Be = 0.77 ± 0.02; 36Cl/10Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26Al/10Be = 0.72 ± 0.01 and 36Cl/10Be = 4.5 ± 0.2. The average 41Ca/36Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The 41Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of 26Al and 10Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the 36Cl and 41Ca activities of the metal phases, matrix effects are unimportant for 10Be and minor for 26Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36Cl/36Ar exposure ages for the metal phases agree well with published values. Neon-21 production rates for mesosiderite silicates calculated from these ages and from measured 21Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the 21Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low-energy secondary particles and hence the 21Ne production from Mg. As 10Be production is relatively insensitive to this matrix effect, 10Be/21Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard 22Ne/21Ne based “shielding” corrections give fairly reliable 21Ne production rates in the mesosiderite silicates. © 1999-2021 John Wiley & Sons, Inc.
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    Measurements of 59Ni in meteorites by accelerator mass spectrometry
    (Elsevier, 1993-10-01) Paul, M; Fifield, LK; Fink, D; Albrecht, A; Allan, GL; Herzog, GF; Tuniz, C
    Isotopic abundances of the radionuclide 59Ni (T12 = 76000 yr) were measured by accelerator mass spectrometry with the 14UD Pelletron tandem accelerator at the Australian National University and a detection system solely based on a multianode ionization chamber. The sensitivity limit in the measurement of 59Ni isotopic abundances is 5 × 10−13, as determined by residual interferences from isobaric 59Co and isotopic 58Ni ions. Cosmogenic 59Ni abundances 59NiNi = (8–20) × 10−12 were measured in four samples prepared from the metal phase of two meteorites (mesosiderites). The ratio of the 59Ni abundances to those measured for 41Ca in the silicate phase of the same samples, is in fair agreement with the ratio of the production rates via thermal-neutron capture on 58Ni and 40Ca. © 1993 Published by Elsevier B.V.
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    Neutron-capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: evidence for high fluences of thermalized neutrons
    (Wiley, 1995-05-25) Bogard, DD; Nyquist, LE; Bansal, BM; Garrison, DH; Wiesmann, H; Herzog, GF; Albrecht, A; Vogt, S; Klein, J
    We have measured significant concentrations of 36Cl, 41Ca, 36Ar from decay of 36Cl, and 150Sm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 36Cl and 41Ca, corrected for a high-energy spallogenic component and a terrestrial age of ∼50 ka, give average neutron-capture production rates of 208 atoms/min/g-Cl and 1525 atoms/min/kg-Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm2/s and 4.3 n/cm2/s, respectively. If sustained for the ∼65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ∼1.3×1016 and 0.8 × 1016 n/cm2 for 36Cl and 41Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 36Ar/38Ar ratios as large as ∼9. The correlation of high 36Ar/38Ar with high Cl/Ca phases in neutron-irradiated Chico indicates that the excess 36Ar above that expected from spallation is due to decay of neutron-produced 36Cl. Excess 36Ar in Chico requires a thermal neutron fluence of 0.9–1.7×1016 n/cm2. Decreases in 149Sm/152Sm due to neutron-capture by 149Sm correlate with increases in 150Sm/152Sm for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in 149Sm/152Sm shown by Chico corresponds to a neutron fluence of 1.23×1016 n/cm2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 36Ar identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of ∼0.2–0.6×1016 n/cm2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8×1016 n/cm2 and 0.25×1016 n/cm2, respectively. Neutron fluences of ∼1016 n/cm2 in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron-capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron-capture has the potential for identifying large meteorites with complex exposure histories. © 2021 American Geophysical Union