The noble gas inventory in metal samples and troilite inclusions from IIIAB iron meteorites: Reinvestigating the live 129I-129Xe dating method

1,2Thomas Smith,1Ingo Leya
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1Physics Institute, University of Bern, Sidlerstrasse 5, Bern, CH-3012 Switzerland
2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng Western Road, Chaoyang District, Box 9825, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

The live 129I-129Xe dating technique, which internally corrects for shielding, is particularly well suited for large iron meteorites, for which shielding corrections might be difficult to obtain. In addition, the half-life of 129I of 16 Ma would allow the study of the important question of the constancy—or variability—of the galactic cosmic rays over a time scale not covered by other cosmogenic nuclides. Here, we present the results of a noble gas study of metal samples and adjacent troilite inclusions from the four IIIAB iron meteorites Cape York (including Agpalilik), Casas Grandes, Trenton, and Grant. The major result is that we can directly determine 129Xenc concentrations caused by (live) 129I decay for Cape York, Casas Grandes, Trenton, and Grant. The 129Xenc concentrations can be used, if combined to 129I activity concentrations (not measured by us), to calculate cosmic ray exposure (CRE) ages using the (live) 129I-129Xe chronometer. For the light noble gases measured in metal and troilite samples, the new data confirm the earlier estimates of a production rate ratio 21Necos(troilite)/21Necos(metal) in the range 3.08–3.53. Surprisingly, 38Arcos in troilite from Agpalilik and Casas Grandes is only, respectively, ~36% and ~44% relative to that in the respective metal phases. Considering that 38Arcos is only produced from iron but not from sulfur, the 38Arcos concentration measured in troilite is expected to be ~64% relative to that in adjacent metal, that is, some 38Arcos is missing. Considering krypton in troilite, only 80Kr, 82Kr, and 83Kr are higher than the blank, likely indicating a spallogenic contribution. On average, ~4.2% of measured 80Kr, ~2.5% of 82Kr, and ~11.6% of 83Kr are spallogenic.


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