He,Ne and Ar in chondritic NiFe as irradiation hardness sensors |
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| Authors: | L Nyquist H Funk L Schultz P Signer |
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| Institution: | Swiss Federal Institute of Technology, CH-8006 Zürich, Sonneggstr. 5, Switzerland |
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| Abstract: | Noble gas analyses of the Ni-Fe of 9 L, 5 H and 2 LL chondrites quantitatively support previous suggestions of radiogenic 4He recoil and 3He deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic 4He and in some cases for recoil loss of 38Ar and gain of 21Ne.The ratio of spallogenic 21Ne and 38Ar in the metal phase is found to correlate strongly with 3He/21Ne and 22Ne/21Ne in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. ‘Hardness indices’ n = 1.9–2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic 21Ne/38Ar ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic 3He/21Ne and 4Ne/21Ne in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses. 3He and 21Ne production rates calculated from previously determined 36Ar/38Ar exposure ages for four L chondrites indicate that they are probably not single-valued functions of the 3He/21Ne ratio. The ratio of 3He in bulk samples to 38Ar in metal samples of the same meteorite is constant (= 20 ± 3) whereas the ratio of 21Ne in the bulk to 38Ar in the metal varies by as much as a factor of two in correlation with 3He/21Ne. |
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