Uranium-Lead Dating of Zagami Phosphates by NanoSIMS

Introduction: Shergottites are members of the SNC meteorites, which are thought to be impact ejecta from the planet Mars. They are basaltic achondrites possibly crystallized from a magma in Mars’ crust and show geochemical variation from enriched to depleted incompatible element concentrations [1]. It is well documented that the enriched and intermediate types indicate radiometric age of 165-200 Ma, while the age of depleted type varies from 327 to 575 Ma [2]. Shergottites are grouped into three petrological types; olivine-phyric, basaltic and lherzolitic rocks [3]. Zagami and Shergotty are textually and mineralogically similar and belonging to enriched basaltic type with complex generation history [4]. Early attempt of U-Pb dating of these meteorites by chemical separation and TIMS instrument showed that leached residues and whole rocks of Shergotty yielded an isochron U-Pb age of 200±4 Ma except for a few leaches susceptible to laboratory contamination [5]. On the other hand, leachate, residue and whole rock of Zagami suggested the U-Pb age of 230±5 Ma. In 2001, ion microprobe U-Th-Pb dating of Shergotty phosphates was conducted by SHRIMP instrument and provided a formation age of 204±68 Ma based on ten apatite and three merrillite analyses [6], which is consistent with TIMS age of ~200 Ma. On the other hand, SHRIMP U-Pb dating of Zagami phosphates showed ~360 Ma [7], significantly older than TIMS age of ~230 Ma. In 2005, U-Pb dating of Zagami by TIMS method suggested much younger age of 156±6 Ma based on selected residue fractions [8], while whole rock and leaches showed more radiogenic signature. In 2008, Pb-Pb dating of Zagami gave an ancient age of 4048±17 Ma by MC-ICP-MS instrument after chemical separation [9]. The most recent results of U-Pb age of Zagami phosphates was 153±81 Ma measured by Cameca IMS-1280 instrument [10]. The U-Pb ages of Zagami are, thus, significantly complex and not fully consistent in literatures. We present here U-Pb dating of Zagami phosphates by NanoSIMS.