4:30 PM - 4:45 PM
[PPS06-23] Uranium-Lead dating and Rare Earth Element analysis of phosphates in Enriched Shergottites using NanoSIMS
Keywords:Martian meteorite, uranium-lead dating, rare earth element
Both 238U-206Pb and 207Pb-206Pb ages of phosphates in the two samples were analyzed using NanoSIMS 50 installed at Atmosphere and Ocean Research Institute, the University of Tokyo. First, 238U-206Pb dating was conducted. We also conducted 207Pb-206Pb dating to obtain “3-D Total U/Pb age (3-D age)”.
In addition to 238U-206Pb ages of the phosphates, we have also analyzed their REE concentrations. NanoSIMS analytical technique of REE in phosphates by NanoSIMS has been established in this study to obtain chondrite normalized REE patterns.
A conventional method of REE analysis was the energy filtering method, which needs much effort for accurate measurement because interference effects of light REE oxides and/or fluorides on heavy REE isotopes are not negligibly small in the case of phosphates. To get over this problem, we applied the high-mass-resolution method. The entrance and emission slits of NanoSIMS were narrowed to achieve the mass resolution power (MRP) of approximately 10,000 at the peak height of 10 %. This MRP value is high enough to separate the peak of light REE oxide from that of heavy REE adequately.
There are two important advantages with this method; (i) the intensities of secondary ions of REEs are kept relatively high, and (ii) we do not need to compromise the product rates of REE oxides. First, terrestrial apatite standards, PRAP (Sano et al., 2006) and Morocco, were analyzed to confirm the accuracy of our NanoSIMS measurements. The REE concentrations of the two standards were previously confirmed by conventional ICP-MS analyses. After that, this new method was applied to the phosphates in Zagami and RBT04261.
Calculated 3-D ages of Zagami and RBT04261 were respectively 113±70 Ma and 230±50 Ma (all errors are 2σ). A previous study of SIMS (Cameca IMS 1280) reported the 238U-206Pb age of Zagami phosphates as 153±81 Ma (Zhou et al., 2013). For RBT04261 phosphates, no studies have previously reported their 238U-206Pb ages. On the other hand, its baddeleyite has the 238U-206Pb age of 235±37 Ma (Niihara, 2011). The 3-D age of phosphate in Zagami obtained in this study is consistent with the literature. Also, we obtained the crystallization age of phosphate in RBT04261 which is consistent with that of baddeleyite. We discuss these crystallization ages and REE of phosphates in this presentation.