Mobility of iron and nickel at low temperatures: Implications for ⁶⁰Fe-⁶⁰Ni systematics of chondrules from unequilibrated ordinary chondrites

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• 作者：Myriam Telus^a ; ^b ; ^{mtelus@carnegiescience.edu" class="auth_mail" title="E-mail the corresponding author} ; Gary R. Huss^b ; Ryan C. Ogliore^b ; ¹ ; Kazuhide Nagashima^b ; Daryl L. Howard^c ; Matthew G. Newville^d ; Andrew G. Tomkins^e
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：1 April 2016
• 年：2016
• 卷：178
• 期：Complete
• 页码：87-105
• 全文大小：6933 K

文摘

The Fe and Ni isotopic composition of ferromagnesian silicates in chondrules from unequilibrated ordinary chondrites (UOCs) have been used to estimate the initial abundance of the short-lived radionuclide, ⁶⁰Fe, in the early Solar System. However, these estimates vary widely, and there are systematic discrepancies in initial ⁶⁰Fe/⁵⁶Fe ratios inferred from in situ and bulk analyses of chondrules. A possible explanation is that the Fe–Ni isotope system in UOC chondrules has not remained closed (a necessary condition for isotopic dating), and Fe and Ni have been redistributed since the chondrules formed. In order to evaluate this, we collected high-spatial-resolution X-ray fluorescence (XRF) maps of UOC chondrules to better understand the distribution and mobility of Fe and Ni at the low metamorphic temperatures of these chondrites. We used synchrotron X-ray-fluorescence microscopy to map the distribution of Fe, Ni and other elements in portions of 71 chondrules from 8 UOCs (types 3.00–3.2). The synchrotron XRF maps show clear enrichment of Fe and/or Ni in fractures ranging down to micrometer scale in chondrules from all UOCs analyzed for this study regardless of petrologic type and regardless of whether fall or find, indicating that there was significant exchange of Fe and Ni between chondrules and matrix and that the Fe–Ni system was not closed. Sixty percent of chondrules in Semarkona (LL3.00) have Fe and Ni enrichment along fractures, while 80–100% of chondrules analyzed from the other UOCs show these enrichments. Mobilization was likely a result of fluid transport of Fe and Ni during aqueous alteration on the parent body and/or during terrestrial weathering. In situ and bulk Fe–Ni analyses that incorporate extraneous Fe and Ni from chondrule fractures will result in lowering the inferred initial ⁶⁰Fe/⁵⁶Fe ratios.