In search of extant Tc in the early solar system:: 98Ru and 99Ru abundances in iron meteorites and chondrites

The short-lived Tc-98-Ru-98 (Half-life between 4 and similar to10 Ma) and Tc-99-Ru-99 (Half-life 0.21 Ma) decay systems have the potential to provide important information on the relative chronology of processes that affected metal phases in the early solar system. The proof of extant Tc-99 in the solar system would also tightly constrain the time interval between production of the nuclide and injection of s-process isotopes into the protosolar cloud. High-precision Ru isotopic data for group IIAB (Negrillos, Bennett County, Coahuila, Filomena, Old Woman) and IIIAB (Casas Grandes, Costilla Peak) iron meteorites, and the chondrites Allende (CV3) and Allegan (H5) indicate that the isotopic compositions of Ru-98 and Ru-99 in these meteorites overlap with the terrestrial values at the +/- 0.8 and +/- 0.3epsilon levels (parts in 10,000), respectively. Previous reports of positive deviations of Ru-98 in Negrillos and Casas Grandes likely reflect inaccurate measurements. Evaluation of processes that may fractionate Tc/Ru in solar system objects suggests that this ratio likely varied by only a factor of 2 or less. Using this constraint and solar system initial abundances of Tc-99 derived from astrophysical models, evolution models for the Tc-99-Ru-99 system predict deviations in Ru-99 of > 0.3epsilon only, if these fractionation events occurred during collapse of the molecular cloud (< 10 ka). Thus, positive identification of enriched Ru-99 may require better than 0.1epsilon unit resolution. Absence of resolvable Ru-98 deviations indicates Tc-98/Ru-96(circle doti) < 2 X 10(-5). (C) 2002 Elsevier Science B.V. All rights reserved.