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Light noble gases and cosmogenic radionuclides in Estherville, Budulan, and other mesosiderites: Implications for exposure histories and production rates
Author(s) -
ALBRECHT A.,
SCHNABEL C.,
VOGT S.,
XUE S.,
HERZOG G. F.,
BEGEMANN F.,
WEBER H. W.,
MIDDLETON R.,
FINK D.,
KLEIN J.
Publication year - 2000
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2000.tb01486.x
Subject(s) - meteorite , silicate , chemistry , chondrite , metal , radiochemistry , presolar grains , iron meteorite , analytical chemistry (journal) , mineralogy , environmental chemistry , astrobiology , physics , organic chemistry
— We report measurements of 26 AI, 10 Be, 41 Ca, and 36 Cl 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 26 Al/ 10 Be = 0.77 ± 0.02; 36 Cl/ 10 Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26 Al/ 10 Be = 0.72 ± 0.01 and 36 Cl/ 10 Be = 4.5 ± 0.2. The average 41 Ca/ 36 Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The 41 Ca 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 26 Al and 10 Be (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 36 Cl and 41 Ca activities of the metal phases, matrix effects are unimportant for 10 Be and minor for 26 Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36 Cl/ 36 Ar 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 21 Ne 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 21 Ne 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 21 Ne production from Mg. As 10 Be production is relatively insensitive to this matrix effect, 10 Be/ 21 Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard 22 Ne/ 21 Ne based “shielding” corrections give fairly reliable 21 Ne production rates in the mesosiderite silicates.