[PCG24-11] Development of a new method for in situ cosmic-ray exposure age measurements using laser ablation
Keywords:astrobiology, cosmic-ray exposure age, LIBS, QMS
On Mars, K–Ar and CRE ages of powdered rocks were measured by the NASA’s Curiosity rover using APXS to derive the elemental composition and thus production rates, quadrupole mass spectrometer, and onboard furnace to derive the concentrations of noble gases . Their method, however, had uncertainties in ages because (1) the production rates of cosmogenic nuclei were uncertain between the measured aliquots due to the heterogeneity of the samples and (2) the extraction of noble gases was incomplete due to insufficient heating temperature and duration . Drilling rocks and keeping the onboard furnace at 900℃ for 30 minutes were among the technical challenges for the rover operation.
To resolve these problems and measure the cosmic-ray exposure ages of rocks, we develop a new method that combines laser-induced breakdown spectroscopy (LIBS) and quadrupole mass spectroscopy (QMS). The LIBS-MS approach potentially yields the spot-by-spot CRE ages by measuring cosmic-ray production rates with LIBS and concentrations of noble gases with MS. In this study, we assess the feasibility of measuring very small amount of cosmogenic noble gases (~10-15 mol) in a meteorite, as well as to investigate the accuracy of the CRE ages obtained with this method. This study focuses on the noble gas analysis with QMS because LIBS measurements for major target elements are more straightforward.
The CRE ages were measured in the following procedures: 5000 laser pulses (266 nm, 20 mJ, spot size 350 μm) were applied to a slab sample of the Camel Donga meteorite. Camel Donga is a eucrite that exhibits CRE ages of 11.9 ± 1.3, 27.3 ± 1.4, and 28.8 ± 1.9 Ma for 3He, 21Ne, and 38Ar, respectively . The gases extracted by the laser were processed with a Ti-Zr getter and a charcoal trap before being introduced to the QMS. The abundances of noble gases, 3He, 21Ne and 38Ar, were determined after correcting procedural blank and isobaric interferences. The volume of laser-ablation spots was measured with a microscope and was converted to ablated mass with the bulk density of the sample of 3.16 g/cm3 . The CRE ages were calculated from the concentrations of the cosmogenic noble gases (3He, 21Ne and 38Ar) and published production rates .
In our preliminary CRE age measurements, noble gases of ~10-16 mol were detected although the signal-to-blank ratios were small (<5). The ablated mass was 5e-5 g. Isotopic ratios of noble gases, e.g., 38Ar/36Ar = 1.13 ± 0.07, were more consistent with their cosmogenic origin (1.55) rather than the trap of the terrestrial air. By assuming the typical shielding factor (22Ne/21Ne = 1.14), we obtained the CRE ages of 148 ± 8 Ma for 3He, 21.8 ± 7.0 Ma for 21Ne, and 60.4 ± 91.7 Ma for 38Ar. The CRE ages for 21Ne and 38Ar are consistent with those reported in the previous study, suggesting the validity of this approach. The much longer CRE age obtained with 3He is most likely because of the isobaric interference by HD+ released from the meteorite. To obtain accurate 3He-ages, HD+ should be removed more efficiently by adding a second getter near the QMS. We found that the reliable age for the Camel Donga meteorite is the 21Ne-age under the assumption that the shielding indicator 22Ne/21Ne is 1.14, but otherwise the 38Ar-age. Our noble gas analyses demonstrate that even very small amounts of cosmogenic noble gases can be measured with this method, suggesting the capability of in situ CRE age measurement based on laser ablation.
 Cohen B. A., et al (2019) Astrobiology, 19, 10
 Farley, K. A., et al. (2014) Science, 343, 1247166
 Miura Y. N., et al (1998) G. C. A. 62, 2369-2387
 Palme H., et al (1988) Meteoritics 23 49-57
 Vasconcelos P. M., et al (2016) J. Geophys. Res. Planets, 121, 2176-2192