Zirconolite: A new U-Pb chronometer for mafic igneous rocks

Precise dates for mallic igneous events are essential for tectonic reconstructions and understanding mantle dynamics, mass extinctions, and paleoclimate. Zirconollite (CaZrTi2O7) is a uranium-bearing accessory mineral, found in a wide range of terrestrial and lunar rocks, that has been largely overlooked as a chronometer. In situ U-Pb geochronology of zirconolite from three dolerite intrusions in Western Australia demonstrates that it yields emplacement ages that are more precise than those obtained from coexisting zircon and baddeleyite. Dikes in the Stirling Range Formation give a zirconollite Pb-207/Pb-206 age of 1218 +/- 3 Ma, indistinguishable from the less precise dates obtained from zircon (1215 +/- 10 Ma) and baddeleyite (1217 +/- 39 Ma) and coincident with dike emplacement in the adjacent craton margin and peak metamorphism in the Albany-Fraser orogen. Zirconolite from the 755 Ma Mundine Well dike swarm yields a 207Pb/206Pb age of 754 +/- 5 Ma. Sills intruding the Proterozoic Manganese Group contain zirconolite crystals that give a Pb-207/Pb-206 age of 523 +/- 14 Ma. Despite high U contents (550 ppm to 14,000 ppm) and greenschist facies metamorphism, zirconolite in these samples is apparently unaffected by loss of radiogenic Ph. Because of its remarkable properties for U-Pb geochronology, it may soon become the dominant tool for dating malic igneous rocks as young as 500 Ma by ion microprobe, and thus will prove especially valuable in reconstructing Precambrian geologic history.