KArMars: A Breadboard Model for In Situ Absolute Geochronology Based on the K–Ar Method Using UV‐Laser‐Induced Breakdown Spectroscopy and Quadrupole Mass Spectrometry

Abstract

We present a breadboard prototype to perform in situ dating applicable to planetary exploration. Based on the K-Ar dating method and using instruments inspired by flight-proven analytical components, 'KArMars' ablated a geological sample under high vacuum with a quadrupled ultraviolet (UV at 266 nm) Nd:YAG laser. During ablation, the K content of the target material was given by laserinduced breakdown spectroscopy and the released 40 Ar was measured with a quadrupole mass spectrometer. Because K was measured as a concentration and 40 Ar as a count of atoms, these values were converted using the ablated mass given by the product of the density and the ablated volume. The uncertainties of the age measurement were < 15%. The quality of the K-Ar measurements was enhanced by the advantages of UV laser ablation such as the minimisation of thermal effects on argon diffusion. This work demonstrates that a specialised instrument inspired by this set-up could provide in situ absolute geochronology with sufficient precision for scientific investigations, particularly where the crater density counting provides higher uncertainties on Mars.Keywords: geochronology, K-Ar, laser ablation, laser-induced breakdown spectroscopy, mass spectrometry, in situ analysis.Nous pr esentons un prototype d edi e a la datation in situ pour l'exploration plan etaire. Bas e sur la m ethode de datation K-Ar et inspir e par instruments d ej a spatialis es, « KArMars » ablate un echantillon de basalte plac e sous ultravide avec un laser Nd:YAG puls e quadrupl e a 266 nm. Lors de l'ablation, le plasma emet un spectre qui une fois analys e, fournit la concentration en K par la technique LIBS et l'ablation lib ere l' 40 Ar qui est quantifi e par un spectrom etre de masse quadrupolaire. Comme nous mesurons la concentration de K et le nombre d'atomes d' 40 Ar, ces valeurs doiventêtre converties par la masse ablat ee qui est le produit du volume ablat e par la densit e. Les principaux int erêts d'utiliser un laser UV concernent l'efficacit e d'ablation et la minimalisation des effets thermiques, ce qui limite la diffusion d'Argon. Les incertitudes sur la mesure de l'âge peuventêtre de 15%. Ce travail d emontre qu'un instrument spatial bas e sur ce principe pourrait fournir des datations absolues in situ et ce avec une pr ecision suffisante pour ajuster les courbes de densit e de crat eres qui, sur Mars, peuvent avoir des fortes incertitudes.Mots-clés : g eochronologie, K-Ar, ablation laser, LIBS, spectrom etrie de masse, analyse in situ.

Dating igneous rocks using the Potassium–Argon Laser Experiment (KArLE) instrument: A case study for ~380 Ma basaltic rocks

“…The larger mineral size of TL‐18 could account for the different ablation efficiency of different mineral species. The observed sample mass is comparable with those reported by Cohen et al (~50 μg) and between those reported by Devismes et al (~14 μg) and Cho et al (~100 μg). The volume differences with the latter two studies can be attributed to pulse numbers (i.e., 500 in this study vs 300), spot diameters (i.e., 400 μm in this study vs 250 μm), different laser wavelengths (i.e., 1064 nm in this study vs 266 nm), different laser pulse energies (i.e., 30 mJ in this study vs 100 mJ), or the relative hardness of the materials, where softer rocks develop deeper and larger‐diameter pits for the same laser energy deposited …”

“…Our isochrons show a large spot‐to‐spot variation of K contents (Figure ), reflecting the samples' heterogeneity on the scale of the laser spot (three‐dimensional parabolic pit with a ~400 μm opening at the sample surface and the depth of ~400 μm). For example, the range of K 2 O is larger than ×27 (<1000 ppm–2.7 wt%) for TL‐18 and larger than ×7 (<1000–7000 ppm) for TO‐35, whereas a fine‐grained basalt showed the variation of a factor of 8 (0.29–2.33 wt%) in a previous study . These lines of evidence suggest that a K abundance range larger than a factor of 4, which is used by Bogard as an example of desirable K range, is possible for basaltic samples.…”

“…To resolve these problems and expand the capability of in situ geochronology, several groups, including ours, have been developing K–Ar dating instruments based on a laser‐ablation approach . In this technique, laser pulses vaporize the sample surface, liberating K and Ar locally (hundreds of microns in diameter and depth).…”

“…For example, Cho et al used a high‐energy (100 mJ) laser, a cold trap cooled with liquid nitrogen, and a Ti–Zr getter heated to a high temperature (700–800°C). The same concern holds for an innovative LIBS–MS study using a quadrupled ultraviolet (UV) Nd:YAG laser; although a 170‐Ma basaltic rock was measured, the laser‐ablation process depends on the wavelength of laser emission, and only the infrared (IR) laser is flight qualified for LIBS at this time. Solé used the known terrestrial ratios of Ar isotopes to correct for trapped Ar in the samples studied, a technique that will not be applicable to the Moon or Mars.…”

In Situ Geochronology on Mars and the Development of Future Instrumentation

“…Several laboratories have developed breadboards that would provide multiple measurements on a single solid sample by measuring K and Ar using a combination of LIBS to measure parent K, and MS to measure daughter Ar (Cohen et al , 2014; Solé, 2014; Cho et al , 2016; Devismes et al , 2016; Cho and Cohen, 2018; Cattani et al , 2019). These LIBS-MS instruments completely release Ar from small pits by laser ablation (LA) rather than oven heating and admit the released gas to the MS.…”