In situ60Fe-60Ni Systematics of Chondrules from Unequilibrated Ordinary Chondrites

1,2Myriam Telus, 2Gary R. Huss, 2Kazuhide Nagashima, 2Ryan C. Ogliore, 3Shogo Tachibana
Geochimica et Cosmochmica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.013]
1Geology and Geophysics, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
2Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
3Department of Natural History Sciences, Hokkaido University, N10 W8, Sapporo 060-0810, Japan
Copyright Elsevier

The initial 60Fe/56Fe ratio of chondrules from unequilibrated ordinary chondrites (UOCs) can potentially help constrain the stellar source of short-lived radionuclides and develop the 60Fe-60Ni (t1/2=2.6 Ma) system for early solar system chronology. However, progress with the 60Fe-60Ni system has been hindered by discrepancies between initial ratios inferred from bulk and in situ Fe-Ni analyses. Telus et al. (2016) show that discrepancies between these different techniques stem from late-stage open-system Fe-Ni mobilization. Here, we report in situ analyses of the Fe-Ni isotopic composition of ferromagnesian silicates in chondrules from UOCs using the ion microprobe. Of the 24 chondrules analyzed for this study, a few chondrules have clearly resolved excesses in 60Ni of up to 70‰; however, the correlations with the Fe/Ni ratios are weak. Although complications from Fe-Ni redistribution make it difficult to interpret the data, we show that the initial 60Fe/56Fe ratio for UOC chondrules is between 5×10-8 and 3.0×10-7. This is consistent with a late supernova source for 60Fe, but self-enrichment of the molecular cloud is another possible mechanism for incorporating 60Fe in the solar system. Discrepancies between bulk and in situ analyses remain, but likely stem from late-stage open-system Fe-Ni mobilization.

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