RB-SR AGE OF THE SHERGOTTY ACHONDRITE AND IMPLICATIONS FOR METAMORPHIC RESETTING OF ISOCHRON AGES

An internal Rb-Sr isochron for the Shergotty achondrite yields the extremely young age of 165 ± 11 Myr and high initial 87Sr/86Sr = 0.72260 ± 0.00012. This age is interpreted as resulting from resetting of the Rb-Sr system by the shock event in which plagioclase was converted to maskelynite. A model quantifying the 'degree of resetting' of internal isochron ages is introduced in order to discuss the Rb-Sr age in the context of discordant ages obtained by the K-Ar and Sm-Nd methods. It is possible to consider the different apparent ages within a single conceptual framework based on the diffusive transport of elements out of and between mineral grains. Some generalizations about the relative resistance of the various types of ages to resetting are made from a semi-empirical treatment of the factors governing the activation energies of diffusion for Ar, Sr and Nd. Constraints are put on the burial depth and cooling time required to reset the radiometric ages measured for Shergotty. The scenario most consistent with all constraints, including in particular the relative diffusion rates of Ar, Sr and Nd, is: 1. (1) Shergotty crystallized from a melt about 650 Myr ago. 2. (ii) the Shergotty parent object was involved in an energetic collision about 165 Myr ago. Our sample was shock heated to ~300-400°C after which it remained buried under hundreds of meters of overburden as it cooled for ~ 104 yr. 3. (iii) Shergotty was exposed to cosmic rays as a small object ~2 Myr ago. © 1979.