Isotope systematics and shock-wave metamorphism: I. U-Pb in zircon, titanite and monazite, shocked experimentally up to 59 GPa

Abstract

This study reports the first U-Pb isotope analyses on experimentally shocked zircon, titanite, and monazite extracted from Proterozoic granitoid rocks. In all three types of minerals, shock-waves produce drastic changes in the crystal lattices, causing strong lowering of birefringence, turbidization, and decolorization of the individual grains. Moreover, X-ray patterns indicate transition of the crystals into polycrystalline aggregates of <10 −5 mm block-size. Precisely dated grains with concordant or nearly concordant ages were embedded in KBr and shocked at 35, 47.5 and 59 GPa. U-Pb isotope analyses on these grains show that shock metamorphism does not fractionate Pb isotopes within the analytical precision of ± 0.1%. As far as chemical fractionation is concerned, there is no difference in degree of concordancy between shocked and unshocked monazite, and small degrees (< 2%) of relative U Pb fractionation in shocked zircon and titanite are due to time-integrated Pb-loss and not to the shock experiment. In consequence, the data document that shock-wave metamorphism alone does not measurably effect the U-Pb chronometer, questioning the view that lower intercept ages of discordant U-Pb data reflect shock-induced re-equilibration of the chronometer in moderately to highly shocked, rapidly cooling rocks.