The oldest zircons in the solar system

Here we report the occurrence, chemistry, and U-Th-Pb isotopic systematics of three meteoritic zircon assemblages, two from the Vaca Muerta mesosiderite and one from the Simmern H5 chondrite. One of the Vaca Muerta zircons occurs in the mesosiderite proper, the other in a eucritic clast associated with chromite, ilmenite, and tridymite, whereas the Simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar. Like terrestrial zircons, the meteoritic zircons are enriched in the heavy rare-earth elements, but unlike terrestrial zircons they do not show a positive Ce anomaly. This feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment: under oxidising conditions C e 4+ (which can substitute for Zr 4+ in the zircon structure) is stabilized whereas under relatively reducing conditions Ce 3+ is stable. The zircon from Simmern has depletions in the relative abundances of Tm and Yb, a characteristic of volatility fractionation of the REE. The U concentrations of the two Vaca Muerta zircons are quite different at 1 and 50 ppm, respectively. Both zircons have extremely low initial lead and give radiogenic ages that are the same, albeit with relatively large errors on VM-1 zircon because of its much lower U concentration and hence radiogenic Pb concentration. The mean of the two analyses made on VM-2 zircon is concordant with a 2°7pb/2°6pb age of 4563 _+ 15 Ma (2~r). The Simmern zircon has an exceptionally low U concentration of around 180 ppb and only a poorly constrained 2°7pb/2°6Pb age of 4100 _+ 700 Ma could be obtained.