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 [CaZrTi 2O 7], tranquillityite [Fe 82+(ZrY) 2Ti 3Si 3O 24] and baddeleyite [ZrO 2] in low-K, high-Ti mare basalt 10047 collected during the Apollo 11 mission. The analysed minerals are concentrated in pockets of late-stage mesostasis that comprises an intergrowth 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 ∼6 μm 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 207Pb/ 206Pb age of 3708 ± 7 Ma ( 207Pb/ 206Pb: 204Pb/ 206Pb isochron; 95% confidence, including renormalisation of ratios) and twenty-five analyses of tranquillityite give 3710 ± 6 Ma. The 207Pb/ 206Pb 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 refining lunar geology.