Coupled Calibration and Application of the Extinct 107Pd-107Ag and 205Pb-205Tl Decay Systems

The earliest history of planetary bodies in the solar system encompasses the accretion of the objects and their differentiation into silicate mantles and metallic cores. Subsequently the newly formed bodies cooled and solidified. Recent developments in modelling and analytical techniques have significantly improved our understanding of the timing and duration of some of these processes. The timescales over which differentiated asteroids cooled and their metallic cores crystallized remain poorly constrained, however. In this research project, we will address this shortcoming by studying the only samples of planetary cores that are available for direct analyses - iron meteorites. The approach will be to measure the decay products of the short-lived palladium-silver (107Pd-107Ag) and lead-thallium (205Pb-205Tl) radionuclide systems in such meteorites. These two decay schemes have half-lives of only about 10 Myr, and they can thus provide very precise (+/- a few Myr) ages of 'metal crystallization'. These ages define the time at which the metal, which was originally present in liquid form, had cooled to form a solid iron-rich core. To obtain such ages, the research project involves the following investigations: 1) The Pd-Ag and Pb-Tl decay systems first need to be calibrated before they can provide absolute age information. To obtain such a calibration, we will analyze meteorite samples for which precise absolute ages are already available. Once completed, the calibration will be of long-lasting value because it provides the basic foundation for the use of the Pd-Ag and Pb-Tl dating systems as precise absolute 'clocks' of processes that took place in the early solar system. 2) With this calibration, we will analyze the decay products of the Pd-Ag and Pb-Tl clocks in iron meteorites. These analyses will provide the precise ages at which the metallic cores of the asteroids, from which the iron meteorites were derived, cooled and crystallized. 3) This information will then be combined with the results of previous studies, which dated the age of 'metal segregation'. This is the time at which an originally primitive asteroid was heated to melting temperatures, such that it differentiated into an outer mantle composed of silicates and a core composed mainly of liquid metallic iron. By comparing the 'metal segregation' with the 'metal crystallization' age, we can infer the rate at which the metal core of an asteroid cooled. As this 'cooling rate' is primarily a function of the size (with larger bodies cooling slower than smaller ones), we can use this information to estimate the diameter of the asteroidal precursor of an iron meteorite. This implies that our data will enable us, for the first time, to relate the duration of accretion to the size of a given asteroidal parent body. In this study, we will analyze various groups of iron meteorites, which are derived from distinct asteroids. The age information that we will obtain for these samples, together with previous results, will provide comprehensive chronological records for the parent asteroids of most iron meteorites. These records will span the period from accretion and concomitant core formation to the cooling and crystallization of the metallic cores. Taken together, this information will significantly expand our understanding of the chemical, physical and thermal evolution of asteroids in the early solar system.