Light noble gases and cosmogenic radionuclides in Estherville, Budulan, and other mesosiderites: Implications for exposure histories and production rates

• DOI: 10.1111/j.1945-5100.2000.tb01486.x
• Publication Date: 2010-02-04T11:09:42Z
• Authors: A. ALBRECHT; C. SCHNABEL; S. VOGT; S. XUE; G. F. HERZOG; F. BEGEMANN; H. W. WEBER; R. MIDDLETON; D. FINK; J. KLEIN
• Keywords: 13. Climate action; 01 natural sciences; 0105 earth and related environmental sciences

Abstract—We report measurements of26AI,10Be,41Ca, and36Cl 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 are26Al/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 find26Al/10Be = 0.72 ± 0.01 and36Cl/10Be = 4.5 ± 0.2. The average41Ca/36Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The41Ca 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 of26Al and10Be (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 the36Cl and41Ca activities of the metal phases, matrix effects are unimportant for10Be and minor for26Al.Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of36Cl/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 measured21Ne 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 the21Ne 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 the21Ne production from Mg. As10Be production is relatively insensitive to this matrix effect,10Be/21Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard22Ne/21Ne based “shielding” corrections give fairly reliable21Ne production rates in the mesosiderite silicates.