Disturbance of isotope systematics during experimental shock and thermal metamorphism of a lunar basalt with implications for Martian meteorite chronology

摘要

To better determine the effects of impact-related processes on radiometric chronometers in meteorites, we undertook an isotopic study of experimentally shocked and heated samples of lunar basalt 10017. Shock experiments at 55GPa were completed on one subsample, and a second subsample was heated in an evacuated quartz tube at 1000°C for 170h. A third subsample was maintained as a control. Samarium-neodymium, Rb-Sr, ~(238)U-~(206)Pb, and ~(206)Pb-~(207)Pb isotopic analyses were completed on mineral fractions (leached and unleached), leached whole rocks, and complementary acid leachates. Disturbance in the shocked and heated samples was evaluated through comparison of their isochron diagrams with those of the control sample. The Sm-Nd isotope system was the least disturbed, the Rb-Sr isotope system was more disturbed, and the ~(238)U-~(206)Pb and ~(206)Pb-~(207)Pb isotope systems were the most disturbed by shock and annealing. Samples that experienced extended heating demonstrated greater isotopic disturbances than shocked samples. In some cases, the true crystallization age was preserved, and in others, age information was degraded or destroyed. In no case did the experiments generate isochrons that maintained linearity while being rotated or completely reset. Although our results show that neither experimental shock nor thermal metamorphism alone can account for the discordant ages represented by different isotope systems in some Martian meteorites, we postulate that shock metamorphism may render a meteorite more susceptible than its unshocked counterpart to subsequent disturbance during extended impact-related heating or aqueous alteration. The combination of these processes may result in the disparate chronometric information preserved in some meteorites.