Palladium isotopic heterogeneity in the solar system

Introduction: Isotopic anomalies have been identified in a number of refractory or moderately refractory elements in bulk meteorites (Ti, Cr, Ni, Zr, Mo, Ru, Pd, Ba, Nd, and Sm), reflecting primordial heterogeneities that point towards variations in space and/or time of the isotopic composition of these elements within the proto-solar nebula [1-6]. Refractory elements like Mo and Ru show large correlated nucleosynthetic anomalies in contrast to more volatile elements, e.g., Cd and Te. This suggests that volatility of elements plays an important role in determining the magnitude of nucleosynthetic anomalies. Palladium, a less refractory element than Mo and Ru, has six naturally occurring stable isotopes—Pd, Pd, Pd, Pd, Pd, Pd. Pd is a p-process isotope, Pd is the s-only isotope, Pd, Pd, Pd are produced by both sand r-processes, and Pd is the r-process only isotope. Correlated εPd deficits and εPd excesses from IVB irons [2] and other iron groups have been reported [7]. A recent Pd isotope study of chondrites and irons claimed that standard nucleosynthetic processes do not adequately describe the anomalies [7]. This finding is not consistent with previous work from our laboratory [2, 8]. A significant factor could be the galactic cosmic ray (GCR) induced neutron-capture effects on Rh that decays to yield Pd, which must be corrected to obtain Pd nucleosynthetic anomalies [2,9]. In this study, we report the Pd isotope composition of five different groups of iron meteoritesIAB (n=1, Campo del Cielo), IIAB (n=18), IID (n=1, NEA 002), IVA (n=1, Muonionalusta), and IVB (n=12)combined with the ungrouped meteoritesChinga and Gebel Kamil. Platinum isotopic composition of the same aliquots were measured to serve as an in-situ neutron dosimeter [10,11], and W and Re isotopes are reported in a companion abstract for some of these irons [12].