Iron 58 Deficits in Iron Meteorites


Mar 13, 2023 ,

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iron 58 is the stable isotope of Fe and a popular tracer in studies of iron metabolism. It is a good choice for this application, since it can be measured directly with instrumental neutron activation analysis (INAA) and does not require the use of radioactive tracers.

Observed e56Fe deficits in IID and IVB irons have been debated, and a range of possible causes has been proposed, including galactic cosmic-ray (GCR) effects and the preservation of a stellar component. Here we present results from a multi-sample study of 13 iron meteorites belonging to magmatic groups IIID and IVB, primarily from the Cape of Good Hope region (Walker 2012).

The d56Fe values in the samples range from -1.48 +- 0.07%0 to -1.33 +- 0.10%0 and are in excellent agreement with the theoretical d57Fe value of 1.47 x d56Fe. The d56Fe measurements are also comparable to the measurements in the nine international biological standards and USGS rock standard BHVO-2.

We find that a nucleosynthetic variation can explain the observed e56Fe deficits, without collateral effects on the eiNi or e58Fe values. However, the e60Ni deficits are best explained by radiogenic effects from the former presence of live 60Fe in the early solar system. Three matching mixtures provide a good match to the eiNi and e58Fe values in IID and IVB irons, but only material from an SN II provides the necessary amount of live 60Fe. This indicates that live 60Fe was present in the early solar system but did not evolve into the oxidized FeIII state in terrestrial chondrites, as expected from GCR effects and the preservation of a stellar component.