Unlike most chemical elements that are linked predominantly by chemical bonds within the mineral lattice, argon is a chemically inert gas. Diffusion of argon within or oustside a mineral is therefore controlled by several parameters among which temperature plays a major yet non-exclusive role. By generating defects in a broad sense, deformation may also induce a drastic decrease of the lattice capacity to retain in-grown radiogenic argon and therefore influence geochronological ages (i.e. the 40Ar/39Ar ratio). Accordingly, recent field-based studies proposed a correlation between 40Ar/39Ar ages and strain gradient. However, loss of argon from existing micas (i.e clasts) is often accompanied by the crystallization of syn-kinematic micas (i.e. blasts) or by grain-size reduction (i.e clasts). This also results in age younging and an apparent correlation between ages and finite strain. This project aims at investigating experimentally the potential effect of deformation on radiogenic argon retentivity for both white micas and biotite.
To explore the behaviour of the 40Ar/39Ar (K/Ar) system during deformation, we will conduct series of experiments using two-mica bearing natural samples cores in a solid-medium high-pressure deformation apparatus (Griggs press) in dynamic conditions for fixed temperature/fluid conditions. The effect of temperature will be then constrained by clone experiments in static conditions. Both conventional IR laser step-heating experiments will be accompanied by intensive in-situ UV laser datings that offer the best opportunity for evaluating intragrain 40Ar/39Ar age variations at high spatial resolution.
This project relates to the first order question of accurate measurements of time of the TelluS CT2 (SysTer, processus et couplages) call. As it participates to the understanding of the geological meaning of 40Ar/39Ar ages, we request funding from the CNRS-INSU for this project that may bring pioneer results on the potential of the 40Ar/39Ar method as a geochronological tool to directly date increments of deformation. It is envisioned as a first exploratory step, which will lead to the preparation of a larger project, to be submitted to the ANR for the next call.