Timing of global crustal metamorphism on Vesta as revealed by high-precision U–Pb dating and trace element chemistry of eucrite zircon

Abstract

Non-cumulate eucrites represent basaltic crust that experienced a complex thermal history involving multistage metamorphism and metasomatism, probably on asteroid Vesta. To better constrain the thermal history of these rocks and their parent body, we have integrated high-precision U–Pb age and trace element data for zircon grains with sizes up to 80 μm in the eucrite Agoult. All analyzed zircon grains yielded concordant U–Pb dates that correspond to the precise 207Pb/206Pb age of 4554.5±2.0 Ma. The Ti contents in these zircon grains indicate their crystallization at subsolidus temperatures of ca. 900 °C, which are similar to the inferred conditions of pyroxene exsolution in most basaltic eucrites that occurred during protracted thermal metamorphism. The zircon crystallization temperatures, together with the presence of baddeleyite needles and variable Zr concentration in Agoult ilmenite grains, indicate metamorphic origin of the Agoult zircon through Zr release from ilmenite followed by reaction with silica. We therefore consider the zircon 207Pb/206Pb age as the timing of the widespread thermal metamorphism in Vesta's crust. The metamorphic age is coincident with the oldest Mn–Cr date for cumulate eucrites, supporting the view that the thermal metamorphism is a result of burial of basaltic crust and subsequent heating from the hot interior rather than collision of asteroids. The zircon rare earth element patterns with restricted Ce positive anomalies suggest that the metamorphism occurred at an oxygen fugacity below the iron–wüstite buffer, implying the absence of oxidizing agents such as aqueous fluid within the crust at that time.