The Early Solar System Abundance of Iron-60: New Constraints from Chondritic Silicates

The abundance of iron-60 in the early solar system is important for planetary evolution models, and has been hotly debated. To put further constraints on the initial Fe-60/Fe-56 ratio of the solar system, here we present new iron-nickel isotope data, measured in situ by NanoSIMS, for 14 silicate chondrules from three carbonaceous and three unequilibrated ordinary chondrites. NanoSIMS measurements were performed at high spatial resolution (200-300 nm primary beam diameter), to avoid inclusion of unwanted phases in the analysis volume. The average initial Fe-60/Fe-56 ratios that can be estimated from our pooled chondrule data are 2.1 (+/- 1.3) x 10(-7) and 0.8 (+/- 1.0) x 10(-7) for carbonaceous and ordinary chondrites, respectively (1 sigma uncertainties). The estimated average initial Fe-60/Fe-56 ratio of all analyzed chondrules is 1.0 (+/- 0.7) x 10(-7). These results are inconsistent with initial Fe-60/Fe-56 ratios >2.4 x 10(-7) (2 sigma upper limit of our entire data set) reported in the literature for some chondrule silicates based on in situ isotope data, and agree better with our previously published in situ data on chondritic troilites (0.10 +/- 0.15 x 10(-7)), as well as with Fe-60/Fe-56 ratios estimated from isotope data of bulk meteorites and chondrules (0.10-0.75 x 10(-7)). Our isotope data hint at a possible difference between the initial Fe-60/Fe-56 ratios of the early solar system's two major isotope reservoirs, with the carbonaceous chondritic reservoir having higher iron-60 abundance than the non-carbonaceous reservoir. Nevertheless, in light of similar hints in the literature, this possibility deserves further investigation.