Whole-Rock 40Ar/39Ar Step-heating Analyses, Problems and Potential

Abstract:

Whole-rock ⁴⁰Ar/³⁹Ar step-heating analyses of extra-terrestrial materials are used to constrain the impact history of the inner solar system, the formation age of the Moon, and timing of paleomagnetic fields. Despite the importance of knowing the timing of these important events, the samples we have in hand are usually disturbed through mixing, (multiple?) impact events, and perhaps recoil loss. Extra-terrestrial ⁴⁰Ar/³⁹Ar data are typically interpreted through the assignment of essentially arbitrary plateau ages rather than through a robust physical model. Although the use of models capable of quantitatively assessing diffusive ⁴⁰Ar* loss in extra-terrestrial samples has been around for nearly 50 years, this early advance has been widely ignored. Here we present implications of applying a robust, multi-activation energy, multi-diffusion domain model to step-heated ⁴⁰Ar/³⁹Ar data, with temperature cycling. Our findings show that for even a single heating event, “plateau” ages are unlikely to record meaningful ages. Further, if the sample has experienced multiple heating events or contains inherited clasts, recovering a unique solution may be impossible. Indeed the most readily interpretable portion of the age spectrum is the early heating steps which represents a maximum age estimate of the last re-heating event. Our results challenge the chronologic validity of ⁴⁰Ar/³⁹Ar “plateau” ages and by extension the hypotheses that are based on this data (e.g., the Late Heavy Bombardment). Future work will require new analytical procedures, interpretative frameworks, and (potentially) the combination of multiple chronometers to derive a robust impact history for the early solar system.