Precise Pb baddeleyite geochronology by the thermal extraction-thermal ionization mass spectrometry method

Abstract

The ability to easily obtain precise (~0.1%) age determinations on mafic rocks can facilitate identification of contemporaneous magmatic events across continents and hence open new avenues of research investigations related to large scale mantle processes. We present high precision Pb age determinations on baddeleyite (ZrO2), a common accessory mineral in mafic rocks using the recently developed thermal extraction-thermal ionization mass spectrometer (TE-TIMS) procedure for dating of zircon. In order to constrain the conditions for thermal extraction of Pb from baddeleyite, a sequence of SEM back-scattered electron images were acquired along with energy-dispersive X-ray spectroscopy spot analysis of baddeleyite grains embedded in silica beads at progressively increasing temperature between 1200 and 1590°C. Replicate analyses of twelve preheated (at 1250°C) and silica embedded baddeleyite grains from the Paleoproterozoic Phalaborwa carbonatite using this procedure have yielded a 207Pb–206Pb weighted mean age of 2060.3±0.4Ma, consistent with previously reported 207Pb–206Pb ages for this carbonatite occurrence determined by the conventional U–Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) technique. Fourteen baddeleyite fractions from a large Paleoproterozoic N–S striking mafic dyke swarm in the Dharwar craton yielded a 207Pb–206Pb weighted mean age of 2215.9±0.3Ma, also consistent with previously determined conventional ages. All data were corrected for mass fractionation of 0.18%/amu as determined previously on zircon. Both data sets scatter outside measurement errors, implying an external 2 sigma error of 0.055% in the case of the carbonatite and 0.043% for the mafic dyke, which we suggest represents the reproducibility of the fractionation correction. Our results demonstrate that thermal extraction 207Pb–206Pb ages are comparable with the conventional U–Pb ages in both accuracy and precision. This procedure does not need ultra-clean laboratory conditions (<1pg procedure blanks), a prerequisite for conventional high precision U–Pb geochronology, and hence can be easily practised in most TIMS isotope geochemistry laboratories.