182Hf–182W systematics in eucrites: the puzzle of iron segregation in the early solar system

Abstract

Eucrites are the witnesses of an early magmatic activity in small planetary bodies. Iron meteorites also testify to an early differentiation in planetesimals. Are basaltic magmatism and iron segregation related to each other? Is there any link between eucrites and some groups of iron meteorites? The new 182Hf–182W chronometer is suitable for investigating this problem and for constraining magmatic activity on the eucrite parent body. Here, we present a Hf–W study for eight eucrites and one angrite. Eucrites define a straight line in the Hf–W isotopic diagram which is interpreted as an isochron corresponding to an age of 11.1±1.1 Myr relative to the formation of primitive ordinary chondrites. This time scale is consistent with data from other chronometers. The very high Hf/W ratios for eucrites support the idea that these rocks were derived from a reservoir from which iron was segregated, simultaneously with or not much earlier than the time of basalt genesis. On the other hand, our results suggest that this differentiation occurred much later than the iron differentiation generating all classes of known iron meteorites.