Cho Yuichiro, Horiuchi Misa, Shibasaki Kazuo, Kameda Shingo, Sugita Seiji
1 NASA Marshall Space Flight Center, Huntsville, AL, USA.
2 Department of Physics, Rikkyo University, Tokyo, Japan.
Appl Spectrosc. 2017 Aug;71(8):1969-1981. doi: 10.1177/0003702817701941. Epub 2017 Apr 27.
In situ radiogenic isotope measurements to obtain the absolute age of geologic events on planets are of great scientific value. In particular, K-Ar isochrons are useful because of their relatively high technical readiness and high accuracy. Because this isochron method involves spot-by-spot K measurements using laser-induced breakdown spectroscopy (LIBS) and simultaneous Ar measurements with mass spectrometry, LIBS measurements are conducted under a high vacuum condition in which emission intensity decreases significantly. Furthermore, using a laser power used in previous planetary missions is preferable to examine the technical feasibility of this approach. However, there have been few LIBS measurements for K under such conditions. In this study, we measured K contents in rock samples using 30 mJ and 15 mJ energy lasers under a vacuum condition (10Pa) to assess the feasibility of in situ K-Ar dating with lasers comparable to those used in NASA's Curiosity and Mars 2020 missions. We obtained various calibration curves for K using internal normalization with the oxygen line at 777 nm and continuum emission from the laser-induced plasma. Experimental results indicate that when KO < 1.1 wt%, a calibration curve using the intensity of the K emission line at 769 nm normalized with that of the oxygen line yields the best results for the 30 mJ laser energy, with a detection limit of 88 ppm and 20% of error at 2400 ppm of KO. Futhermore, the calibration curve based on the K 769 nm line intensity normalized with continuum emission yielded the best result for the 15 mJ laser, giving a detection limit of 140 ppm and 20% error at 3400 ppm KO. Error assessments using obtained calibration models indicate that a 4 Ga rock with 3000 ppm KO would be measured with 8% (30 mJ) and 10% (15 mJ) of precision in age when combined with mass spectrometry of Ar with 10% of uncertainty. These results strongly suggest that high precision in situ isochron K-Ar dating is feasible with a laser used in previous and upcoming Mars rover missions.
通过原位放射性同位素测量来获取行星地质事件的绝对年龄具有重大科学价值。特别是钾 - 氩等时线很有用，因为其技术成熟度相对较高且精度高。由于这种等时线方法涉及使用激光诱导击穿光谱法（LIBS）逐点测量钾以及用质谱法同时测量氩，LIBS测量是在高真空条件下进行的，在这种条件下发射强度会显著降低。此外，使用先前行星任务中所使用的激光功率来检验这种方法的技术可行性是比较合适的。然而，在这种条件下针对钾的LIBS测量很少。在本研究中，我们在真空条件（10Pa）下使用30 mJ和15 mJ能量的激光测量了岩石样品中的钾含量，以评估使用与美国国家航空航天局“好奇号”和“火星2020”任务中所使用的类似激光进行原位钾 - 氩测年的可行性。我们使用777 nm处的氧线进行内部归一化以及激光诱导等离子体的连续发射，获得了各种钾的校准曲线。实验结果表明，当氧化钾（KO）<1.1 wt%时，对于30 mJ激光能量，使用769 nm处钾发射线强度与氧线强度归一化得到的校准曲线效果最佳，检测限为88 ppm，在氧化钾含量为2400 ppm时误差为20%。此外，基于钾769 nm线强度与连续发射归一化得到的校准曲线对于15 mJ激光效果最佳，检测限为140 ppm，在氧化钾含量为3400 ppm时误差为20%。使用所获得的校准模型进行误差评估表明，对于含有3000 ppm氧化钾的40亿年岩石，当与氩的质谱测量结合且氩测量不确定度为10%时，年龄测量的精度在8%（30 mJ）和10%（15 mJ）。这些结果有力地表明，使用先前和即将到来的火星漫游车任务中所使用的激光进行高精度原位等时线钾 - 氩测年是可行的。
