Dating geological events is essential for putting quantitative constrain on the processes that have shaped the Earth on which we live today (e.g. volcanic events, mountain building processes and erosion, continental drift and plate tectonics, evolutionary trends of extinct/extant biota, absolute time calibration of fossil and climate records, etc.). The Ar/Ar lab at ISTO is dedicated to providing this information across a timescale spanning from about the age of the Earth (4.5 Ga) to volcanic events as young as the Holocene (< 10 ka) using the natural 40K → 40Ar radioactive decay in K-bearing minerals (the so called K-Ar ages). These can be conveniently measured as Ar/Ar isotopic ratios via the transformation of natural 39K into synthetic 39Ar by neutron irradiation. The lab features the latest technical developments for measuring such ratios at the highest temporal and spatial resolution using continuous (CO2-10.6 µm) and pulsed (UV-213 nm) laser sources capable to untangle the Ar/Ar isotopic composition of single, minute, grains via either stepwise heating or spatially-resolved in situ dating (down to 50 µm and smaller).
Présentation de la plateforme
The laboratory is actively involved in pushing the limits of the Ar/Ar dating technique either in terms of resolution and analytical precision, as well as in terms of range of applicability to natural systems (volcanism, magmatism, metamorphism and deformation, hydrothermalism, sedimentary record, etc.). While most Ar/Ar laboratories worldwide are specialized in either one or two such fields of application, we strive at ISTO to widen the scope of the technique by developing dedicated protocols to ensure the best precision for samples facing adverse or challenging experimental conditions specific to every particular setting (e.g. atmospheric contamination of ultra-young volcanic, ultra-low-blank spatially-resolved in situ dating of fabric-forming minerals, etc.).
Current projects are aimed at (1) Providing temporal constraints on active volcanic fields (southern Ethiopian and Pantellerian rifts, volcanic unrest at Tenerife, Mount Vesuvius and Phlegrean fields, Canaries archipelago and Italy), (2) Restoring the thermal-strain evolution of extensional detachment and exhumation of High-Pressure metamorphic units (Cyclades, Aegean Sea), (3) Constraining the thermal structure of the Scottish Caledonides, (4) Investigating deformation vs. Ar-in-mica relationships in crustal shear zones and exhumed Metamorphic Core Complexes (Tenda, Corsica, and Menderes, Turkey), (5) In situ K-Ar dating of low-T deformation fabric in exhumed crustal roots (Axial Zone, Pyrenees).
A strong and emerging component of the lab activity is also concerned with exploring the rare-gas isotope behavior in natural/synthetic minerals by means of P-T-fO2-controlled thermal-hydrothermal runs using state-of-the-art experimental facilities housed at ISTO (internally-heated pressure vessels, Griggs rig and Paterson apparatus, put a weblink here). Work currently underway is dedicated to calibrating the diffusion of Ar in micas to quantitatively constrain their thermochonometric potential as a function of composition and mineral structure. In parallel, deformation-induced experiments at relevant mid-crustal conditions and variable finite strain are being coupled to in situ Ar/Ar dating to explore the effects of ductile/brittle deformation on the Ar-isotope record of common rock-forming minerals (micas, K-feldspars), with the goal of clarifying closure vs. crystallization age interpretations.
Finally, we are currently expanding the lab capabilities for tracing heavy halogens via the determination of noble-gas isotopes produced by thermal-neutron capture on Cl, Br, and I. The goal is to provide a highly-sensitive geochemical tracing tool (at the ppb level) of fluid/solid interactions involving halogen-bearing fluids (metamorphic or supra-crustal aquifers and magmas) trapped in fluid/melt inclusions or recorded as matrix-dissolved volatiles. These can be selectively sampled in the fluid- or solid-phase state by step-heating, in situ laser ablation, or stepped vacuum crushing, opening the way for quantitative assessment of fluid/solid partition behavior and solute transport properties in two-phase systems involved in the degassing of magmas (e.g., volatile input of climate-forcing species into the atmosphere) and fluid/rock interactions (e.g., fluid/mass balance and fluid/deformation triggers of metamorphic reactions).
As an endeavor to improve the technique, the lab is extensively developing stand-alone hardware (and post-processing) control routines to by-pass technical limitations inherent to proprietary software (e.g., QTegra) and provide maximum flexibility in exploiting and improving every system component (mass-spectrometers, laser sources, vacuum generation, gas transfer and purification, etc.) .
- Stéphane Scaillet (Full-time (FT), Lab manager, CR CNRS)
- Florian Duval (FT, Research Engineer CNRS)
- Angelo Mottolese (FT, CDD Engineer Labex)
- Laurent Perdereau (10% part-time, Engineer CNRS)
PhD and MSc students
- Zhou Xin (PhD, ISTO-Nanjing Univ. internship)
- Eloïse Bessière (PhD, ISTO)
- Maxime Waldner (PhD, iSTeP, UPMC)
- Jehiel Nteme-Mukonzo (PhD ISTO)
- Zara Franceschini (PhD, ISTO-Florence Univ. internship)
- Alexane Legeay (PhD, Labex, ISTO)
Alumni and past collaborators
- Valentin Laurent (PhD, ISTO 2017)
- Alexandre Beaudoin (PhD, ISTO 2017)
- Nicolas Mora (MSc, ISTO 2017)
- Morgan Bezard (MSc, ISTO 2017)
- Ting Fang (MSc, ISTO-Nanjing Univ. internship)
- Quentin Thibault (Assistant Engineer)
- Vincent Roche (PhD, ISTO)
- Chaolei Yan (PhD, ISTO-Nanjing Univ. internship)
- Fang Fang (PhD, ISTO-Nanjing Univ. internship)
- Ella Jewison (PhD, iSTeP, UPMC)
- 3 last generation high resolution, high sensitivity spectrometers (Helix SFT, Thermo Fisher)
- 1 high-resolution, high-sensitivity mass-spectrometer (GV 5400; currently under complete UHV reprocessing and electronic upgrading)
- 3 fully automated ultra-low blank extraction lines (+ 1 under construction) each featuring fully automated (pneumatically actuated) transfer valves, two independent laser viewports (for either CO2 and/or UV laser analysis; all laser sources are independent and interchangeable between units), a computer-regulated cryogenic cold trap (-296 °C to room-T), two air-cooled GP50 SAES getter with automated T-setpoint, a 6 litter air tank fitted via a fully automated dual (0.1 and 1.0 cc) pipetting system.
- 2 UV-laser (213 nm) for high spatial resolution in situ ablation (CETAC LSX-213 G2 with coaxial viewing and zooming & delivery optics, adjustable spot geometry and size, and fully automated X-Y-Z and power/ratemeter control)
- 2 continuous-wave CO2 (10.6 µm) lasers sources (SYNRAD 20W with coaxial viewing and zooming & delivery optics, and fully automated X-Y-Z and power control)
- 1 LB-1 Barrier Frantz magnetic separator
- 1 high-precision (± 0.001 mg) microweighing balance
- 1 laminar flow extraction hood for sample preparation/cleaning, including drying oven & ultrasonic bath
- 1 fully equipped manipulation room for handling radioactive material
- 1 fully equipped room for radioactive material storage and disposal
- 2 high-resolution (x 20 to x 60) binocular microscopes for sample preparation & conditioning (single grain and thin section)
The lab also makes extensive use of high-resolution µ-scale imaging/probing techniques housed in the institute (SEM, EBSD, computerized µ-tomography, XR diffraction, put a weblink here) for the textural/compositional characterization of Ar/Ar samples prior to irradiation (especially for in situ ablation on thin sections). Custom-made hardware components are mostly developed and machined in our workshop /*put a weblink here*/.