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

Eucrites are the witnesses of an early magmatic activity in small planetary bodies. Tron meteorites also testify to an early differentiation in planet esimals. Are basaltic magmatism and iron segregation related to each other? Is there any link between eucrites and some groups of iron meteorites? The new Hf-182-W-182 chronometer is suitable for investigating this problem an d for constraining magmatic activity on the eucrite parent body. Here, we p resent a Hf-W study for eight eucrites and one angrite. Eucrites define a s traight line in the Hf-W isotopic diagram which is interpreted as an isochr on 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 fro m other chronometers. The very high Hf/W ratios for eucrites support the id ea that these rocks were derived from a reservoir from which iron was segre gated, simultaneously with or not much earlier than the time of basalt gene sis. On the other hand, our results suggest that this differentiation occur red much later than the iron differentiation generating all classes of know n iron meteorites. (C) 2000 Elsevier Science B.V. All rights reserved.