Browsing by Author "Weber, H"

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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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    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.