Rare gases and Ar-Ar geochronology

The measurement of time is essential in Earth Sciences in order to quantify the geological processes at the origin of our current planet (e.g., age of volcanic crises, rate of mountain range exhumation, absolute age of paleontological stratotypes, etc.). The platform “Rare gases and Ar/Ar geochronology” of ISTO is intended to provide high resolution temporal constraints on these phenomena based on the K-Ar chronometer whose range of application extends from a few thousand years (Holocene volcanism, < 10 ka) to the age of the Earth (4.5 billion years: Precambrian meteorites). The platform incorporates the latest technological advances allowing the measurement of argon isotopes at the single crystal scale by the use of continuous and pulsed lasers able to decipher the intracrystalline spatial distribution of isotopes by differential thermal analysis (“step-heating”) and in situ (point ablation at high spatial resolution, < 50 µm).

The Ar-Ar laboratory is developing a dual applied and experimental approach. The first one is based on the dating of natural samples (metamorphic minerals, volcanic and magmatic minerals) in order to constrain the chronology of the geological processes presiding over their formation and their evolution over time (age of eruptions, crystallization, subsolidus deformation, crustal cooling, etc.). The studied targets cover the complete range of endogenous and superficial geological phenomena and superficial geological phenomena (active volcanism, magmatism, metamorphism of present and ancient orogens, sedimentary archives).

The experimental axis is intended to constrain the physical parameters (diffusion, crystal structure, recrystallization) controlling the kinetics of Ar and the behavior in closed or open system of the Ar-Ar chronometer in natural environments. This approach is based on the specific expertise of ISTO in the field of petrology experimental petrology (Pressure-Temperature-Controlled Fluid autoclaves) in order to quantify precisely the blocking temperatures of the Ar-Ar chronometers for their numerical use in thermochronology numerical use in thermochronology (reconstruction of temperature-time paths of exhumed crustal units).

In parallel, the Ar-Ar laboratory develops geochemical tracing tools through in situ (laser ablation) and gas phase (“step-heating”, “in vacuo crushing”) of halogens involved in hydrothermal, magmatic and volcanic transfers through the measurement of fluorine, chlorine, bromine and iodine via their derivatives (Ne, Ar, Kr, Xe) activated by neutron irradiation. This approach is specifically developed at ISTO in order to support the study of the mechanisms and balances of in the crust (accretion prisms, metamorphic aquifers), as well as to allow the experimental quantification of the capacity of trapping, transfer and degassing degassing of magmas in volatile elements with high environmental impact.