Dating a Martian meteorite with 20 Myr precision using a prototype in-situ dating instrument
We are continuing the development of a compact resonance ionization mass spectrometer for geochronology of planetary specimens, ultimately for in-situ use on the Moon or Mars. We previously demonstrated 87Rb–87Sr isochrons for the Martian meteorite Zagami and the Duluth Gabbro with a best precision for Zagami of 100 Myr, using 10-ns, 250-μJ laser ablation pulses of 213-nm radiation. In that work, we suggested that our precision had been limited by instabilities in the nanosecond ablation process, which strongly and variably fractionated Rb from Sr. To overcome this limitation, we have replaced the ablation laser with one generating 150-fs, 300-μJ pulses. Furthermore, by modifying the geometry of the sample to reduce plasma penetration into the mass spectrometer, we have improved our instrumental sensitivity, which in turn improves the dating precision we are able to achieve. Our age determination of 150 ± 20 Ma is 5 times more precise than our best previous reported age, and is no longer limited by the precision we can achieve on the Rb/Sr ratios of individual spots. Instead, we are now limited by the precision we can achieve in the 87Sr/86Sr ratios of the relatively few spots with the highest Rb/Sr ratios. We discuss prospects for even further improvements in the dating precision achievable with resonance ionization mass spectrometry.