Palladium isotopic evidence for nucleosynthetic and cosmogenic isotope anomalies in IVB iron meteorites

The origin of ubiquitous nucleosynthetic isotope anomalies in meteorites may represent spatial and/or temporal heterogeneity in the sources that supplied material to the nascent solar nebula, or enhancement by chemical processing. For elements beyond the Fe peak, deficits in s-process isotopes have been reported in some (e.g., Mo, Ru, W) but not all refractory elements studied (e.g., Os) that, among the iron meteorites, are most pronounced in IVB iron meteorites. Palladium is a non-refractory element in the same mass region as Mo and Ru. In this study, we report the first precise Pd isotopic abundances from IVB irons to test the mechanisms proposed for the origin of isotope anomalies. First, this study determined the existence of a cosmogenic neutron dosimeter from the reaction ¹⁰³Rh(n, β⁻)¹⁰⁴Pd in the form of excess ¹⁰⁴Pd, correlated with excess ¹⁹²Pt, in IVB irons. Second, all IVB irons show a deficit of the s-process only isotope ¹⁰⁴Pd ($\epsilon$ ¹⁰⁴Pd = −0.48 ± 0.24), an excess of the r-only isotope ¹¹⁰Pd ($\epsilon$¹¹⁰Pd = +0.46 ± 0.12), and no resolvable anomaly in the p-process ¹⁰²Pd ($\epsilon$ ¹⁰²Pd = +1 ± 1). The magnitude of the Pd isotope anomaly is about half that predicted from a uniform depletion of the s-process yields from the correlated isotope anomalies of refractory Mo and Ru. The discrepancy is best understood as the result of nebular processing of the less refractory Pd, implying that all the observed nucleosynthetic anomalies in meteorites are likely to be isotopic relicts. The Mo–Ru–Pd isotope systematics do not support enhanced rates of the ²²Ne(α,n)²⁵Mg neutron source for the solar system s-process.