Pb/Pb geochronology, petrography and chemistry of Zr-rich accessory minerals (zirconolite, tranquillityite and baddeleyite) in mare basalt 10047

In situ U-Pb isotopic measurements were carried out by ion microprobe on the Zr-rich accessory minerals zirconolite [CaZrTi2O7], tranquillityite [Fe-8(2+)(ZrY)(2)Ti3Si3O24] and baddeleyite [ZrO2] in low-K, high-Ti mare basalt 10047 collected during the Apollo I I mission. The analysed minerals are concentrated in pockets of late-stage mesostasis that comprises an inter-growth of silica, barian K-feldspar and Si-Al-K glass, from a phaneritic, subophitic, basalt comprising mainly pyroxene, plagioclase, ilmenite, cristobalite and troilite. Most Zr-rich minerals are unaltered, however, some tranquillityite is replaced by a complex intergrowth of zirconolite, baddeleyite, ilmenite and fayalite, suggesting that the mineral became unstable during crystallization. Several baddeleyite crystals have also undergone alteration to secondary zircon. Zirconolite was analysed in thin section 10047,11 and tranquillityite and baddeleyite in 10047,227, using a similar to 6 mu m primary ion beam. Both zirconolite and tranquillityite have significant U and low initial Pb contents, and are highly suitable for Pb/Pb dating. Fifteen analyses of zirconolite give a Pb-207/Pb-206 age of 3708 +/- 7 Ma (Pb-207/Pb-206:Pb-204/Pb-206 isochron; 95% confidence, including renormalisation of ratios) and twenty-five analyses of tranquillityite give 3710 +/- 6 Ma. The Pb-207/Pb-206 dates are consistent with each other and refine results from an earlier study. Baddeleyite data were less precise, mainly due to lower secondary ionisation efficiency. Our results show that zirconolite and tranquillityite can provide precise isotopic dates and, given their presence in other samples. they represent important U-Pb chronometers for refilling lunar geology. (C) 2008 Elsevier Ltd. All rights reserved.