Utilizing diffusion kinetics to learn more from 40Ar/39Ar geochronology

Abstract:

In ⁴⁰Ar/³⁹Ar geochronology, we typically assume that minerals become closed to Ar diffusion on geologically instantaneous timescales and subsequently remain closed systems. In contrast, for ⁴⁰Ar/³⁹Ar thermochronology we consider geologic problems where thermally-activated diffusion of radiogenic Ar has occurred, and we use Ar diffusion kinetics to constrain a rock’s thermal history over geologic timescales. In this talk, I will highlight several examples where Ar diffusion kinetics become relevant for problems conventionally in the realm of ⁴⁰Ar/³⁹Ar geochronology. In the first example, I will discuss ⁴⁰Ar/³⁹Ar data from sanidine grains in a sequence of Miocene ignimbrites, and will demonstrate that the age dispersion characterizing these ⁴⁰Ar/³⁹Ar datasets can be interpreted in the context of sanidine Ar diffusion kinetics to constrain the ignimbrites’ magma storage temperatures and residence times. In the second example, I will demonstrate how ⁴⁰Ar/³⁹Ar ages from iddingsite in the Martian meteorite Lafayette–which are interpreted to date when there was liquid water near the surface of Mars–are robust despite later heating associated with impact ejection, space transit, and atmospheric entry. Finally in the third example, I will describe a new numerical modeling method for interpreting ⁴⁰Ar/³⁹Ar data collected from mineral mixtures that utilizes differences in minerals’ Ar diffusion kinetics. This method can be used to deconvolve the ⁴⁰Ar/³⁹Ar ages of mixture components from fine-grained sediments as well as complex planetary materials. These three examples highlight how coupling knowledge of Ar diffusion kinetics to ⁴⁰Ar/³⁹Ar geochronology data can provide a richer understanding of diverse geologic problems.