Developing the 60Fe-60Ni System for Early Solar System Chronology

Abstract

This dissertation focuses on in situ Fe and Ni isotope analyses of chondrules from unequilibrated ordinary chondrites (UOCs) using the ion microprobe in order to constrain the initial 60Fe/56Fe ratio of UOC chondrules for early solar system chronology. Most of the chondrules analyzed for this dissertation do not have resolved excesses in 60Ni. A few chondrules have clear excesses in 60Ni (up to ~30‰) that can only be explained by the decay of 60Fe. However, the isochrons are clearly disturbed as shown by the weak correlation between the excesses in 60Ni and the Fe/Ni ratios. This, along with the discrepancies between the initial ratios inferred from bulk and in situ analyses, indicates that the Fe-Ni isotopic system in UOCs was disturbed. Synchrotron X-ray fluorescence maps of Fe and Ni and other trace elements in UOC chondrules confirm this. We found Fe and Ni enrichment along chondrule fractures, indicating extensive open system Fe-Ni redistribution occurred between chondrules and the surrounding matrix. These complications make the Fe-Ni isotope data difficult to interpret. Nevertheless, our data indicate that the initial 60Fe/56Fe ratio of UOC chondrules is between 5×10-8 and 2.6×10-7. Our ion microprobe measurements consist of counting Fe and Ni ions from a chondrule and calculating isotope ratios from those counts. However, ratios calculated this way are systematically higher than the true ratio in the sample. The bias increases proportionally with decreasing count rates of the normalizing isotope and can produce linear correlations similar to those of an isochron. This dissertation provides a detailed discussion of the influence of ratio bias on isochrons and it includes re-calculated ratios for several in situ studies, including most of the previously published in situ Fe-Ni data. Additionally, a study of the influence of ratio bias on in situ 26Al-26Mg (t1/2=0.7 Myr) systematics of plagioclase from H4 chondrites is included in this dissertation. We find that ratio bias is not significant for these analyses. We argue that the 26Al-26Mg ages for these chondrites date impact excavation and cooling at the surface of the H chondrite parent body, not cooling at depth as the onion shell model predicts.