Fe IN EUCRITE NWA 4523: EVIDENCES FOR SECONDARY REDISTRIBUTION OF Ni AND FOR SECONDARY APPARENT HIGH Fe/Fe RATIOS IN TROILITE

Introduction: Fe (T1/2= 1.49 My) is a key a short-lived radioactive nuclide because it is the only one for which an origin from irradiation by cosmic rays is impossible. Fe is a neutron-rich nuclei and its production rate during irradiation processes around the early Sun (because of the low cross sections of appropriate reactions and of the low abundance of targets) is at least two orders of magnitude too low to explain the observed abundance [1]. The problem is however that, though several evidences do exist for the presence of lived Fe in the early solar system, the inferred intial Fe/Fe ratios span a large range due either to an heterogeneity in the distribution of Fe or to perturbations of the Fe-Ni system in various meteoritic components, or to both. The presence of Fe has been discovered from the existence of Ni excesses in CAIs [2] and in eucrites [3]. However, because of the presence of nucleosynthetic Ni isotope anomalies in CAIs [2, 4] no initial Fe/Fe ratio can be determined precisely: an upper limit of 1.6±0.5x10 has been proposed by [2] and a value of 4.7±2.9x10 has been tentatively proposed by [4] from the analysis of CAIs with no Ni nucleosynthetic anomaly. Clear Fe isochrons have been obtained for troilite and magnetite in the matrix of Semarkona [5] and for pyroxene-rich chondrules from Semarkona and Bishunpur [6]. The problem is that the different Fe/Fe ratios are difficult to reconcile. Pyroxene-rich chondrules show a Fe/Fe ratio of 2.2-3.7x10 while the Fe/Fe ratio in the troilite in the matrix of Semarkona is of 0.9±0.2x10, i.e. higher by a factor of ≈ 3 to 4 compared to chondrules. Because high-precision Ni isotopic analyses of metal in chondrites and iron meteorites [7] limit the possible Fe heterogeneity in the inner solar system to less than 10%, the differences observed between the different components of Semarkona may be a sign of previously unrecognized secondary perturbations of the Fe-Ni system. Thus the initial Fe/Fe of the solar system remains at present quite unconstrained. Better constrains on the initial Fe/Fe of the Solar system are required in the use of Fe as a smoking gun for injection of supernova products in the early solar system. Recent measurement of the γ-ray lines of Fe in the interstellar medium gives a value of 0.148±0.06 for the galactic Fe/Al ratio [8] which, using the present day average galactic Al/Al ratio of 8.6x10 [9] would imply a galactic background for Fe/Fe perhaps as high as ≈1.4x10. This is not one order of magnitude lower than initial Fe/Fe inferred from meteorites. In order to bring additional constraints on the initial Fe/Fe ratio and on the possible perturbations of the Fe-Ni system, we looked for Fe in sulfides oxides and silicates from the eucrite NWA 4523.