Comparison of Re-Os Systematics Between the Nonmagmatic and Magmatic Iron Meteories

Abstract:

Re-Os system is a valuable tool for understanding the origin and crystallization sequence of asteroidal cores, and the evolution of HSE in other planetary cores. In the past several decades, numerous Re-Os data of magmatic iron meteorites have been published, but few for nonmagmatic iron meteorites. The nonmagmatic iron meteorites experienced incomplete melt, therefore they cannot represent the asteroidal core samples. It is ambiguous question for the origin and crystallization process of nonmagmatic iron meteorites. In this study, 10 iron meteorites have been analyzed by the high-precision NTIMS (TRITON) Faraday cup static measurement through using isotopic dilution method. Among 10 iron meteorites studied here, 3 samples of group ⅢAB and one sample of group ⅢE belong to magmatic iron meteorites. Another 3 samples of groupⅠAB and 3 samples of group ⅢCD are nonmagmatic iron meteorites. The precision is ≤ 20 ppm 2σ for ¹⁸⁷Os/¹⁸⁸Os and blank of Re and Os is < 80 pg.

In the ¹⁸⁷Os/¹⁸⁸Os, ¹⁸⁷Re/¹⁸⁸Os isochron diagram, the 3 ⅢAB and a ⅢE samples define a straight best-fit line corresponding to a slope of 0.07917±0.00062 and an intercept with the vertical axis of 0.09575±0.00038. The rang of ¹⁸⁷Re/¹⁸⁸Os ratios is from 0.460391 to 0.691148. Application of a λ for ¹⁸⁷Re=1.666×10^－11 yr^－1 yields an age of 4590±37 Ma. In error rang, this age is consistent with reported Re-Os isochron age of 4558±12 Ma. The large uncertainty likely result from small number data.

3 samples of groupⅠAB in this study produce a slope of 0.07904±0.00011, and initial ¹⁸⁷Os/¹⁸⁸Os of 0.09663±0.00005, corresponding to T=4582.6±6.4Ma, which is older than the published data for theⅠA, 4529±23Ma, and 4537±21 Ma. Another 3 samples of group ⅢCD also defined a slop of 0.07315±0.00073, and initial ¹⁸⁷Os/¹⁸⁸Os of 0.09873±0.00038, the age calculation is 4253±44 Ma. ¹⁸⁷Os/¹⁸⁸Os ratios vary rather narrow (the biggest difference is 0.087) in the 6 nonmagmatic iron meteorites, which lead to increase the uncertainty of the ages. But one thing can be known that the crystallization of ⅠAB is earlier than ⅢCD. The distribution of Re and Os concentration also shows same compositional feature. More Re-Os data of nonmagmatic iron are required in order to better understand their precision isochron and evolution history in future study.