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

We report measurements of 26A1, loBe, 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 (atodatom) for metal phase samples from Estherville and Budulan are 26AVlOBe = 0.77 f 0.02; 36CYlOBe = 5.3 f 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26AVloBe = 0.72 f 0.01 and 36CVlOBe = 4.5 f 0.2. The average 41Ca/36Cl production rate ratio is 1.10 f 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 1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of 26A1 and loge (dpmkg 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 loge and minor for 26A1. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36C1/36Ar exposure ages for the metal phases agree well with published values. Neon-2 1 production rates for mesosiderite silicates calculated from these ages and from measured 2lNe 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 2lNe 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 2lNe production from Mg. As lOBe production is relatively insensitive to this matrix effect, 10BePlNe ages give erroneously low production rates and high exposure ages. By coincidence, standard 22NePlNe based "shielding" corrections give fairly reliable 2lNe production rates in the mesosiderite silicates.