The W isotope composition of eucrite metals: constraints on the timing and cause of the thermal metamorphism of basaltic eucrites

Basaltic eucrites formed as lava flows at or near the surface of asteroid Vesta. After crystallization, most eucrites were affected by thermal metamorphism, the timing and cause of which are enigmatic. We present Hf-W data for magnetic (largely metal) and non-magnetic fractions (mostly silicates and oxides) from six basaltic eucrites that allow the first direct dating of this thermal metamorphism. All metals have high W contents and show excesses in the abundance of 182W (between 2 and 16 ɛW) relative to the initial ɛW of the whole-rock isochron (ɛW∼−0.5), indicating a late re-mobilization of radiogenic W from silicates into metal. The Hf-W ages of metal formation/re-equilibration in five out of six eucrites agree and give a weighted average of 16±2 Myr after mantle-crust differentiation in Vesta, corresponding to an 'absolute' age of 4547±2 Myr when using the H chondrite Ste. Marguerite as an absolute time marker. Hf-W isotopic closure in metal from Stannern occurred 0.5-3.6 Myr after mantle-crust differentiation, which is significantly earlier than in the other eucrites. The different whole-rock and non-magnetic fractions from Bouvante, Ibitira and Juvinas plot along the eucrite whole-rock isochron but are not collinear with their respective metal fractions. Thus, metal and silicates in eucrites have not been in complete W isotopic equilibrium, indicating that the Hf-W metal ages correspond to a short re-heating rather than a magmatic event. Based primarily on age comparison, the temperature required for W diffusion from silicates into metal is estimated to be at least ∼600 °C, similar to the peak metamorphic temperatures deduced from diffusion profiles in eucritic pyroxenes. The Hf-W ages of metal formation/re-equilibration, therefore, provide the first direct age constraints on the thermal metamorphism that affected almost all eucrites. The ∼16 Myr time gap between mantle-crust differentiation and metal formation/re-equilibration indicates that the thermal metamorphism cannot be related to differentiation of a magma ocean into layers or rapid burial of early crust. Impacts instead must have provided the energy for re-heating of early-erupted basalts. Given that five out of six eucrites studied here exhibit identical Hf-W ages, the thermal metamorphism may have been caused by a large impact on Vesta. The Hf-W systematics in Stannern were not affected by this event, indicating that Stannern is derived from a different part of Vesta's crust than the other examined eucrites. Ar-Ar ages for unbrecciated and brecciated eucrites [Bogard and Garrison, Meteorit. Planet. Sci. 38 (2003) 669-710] are ∼50 Myr and more than ∼450 Myr younger than the Hf-W metal ages, reflecting later impact-induced heating events that caused resetting of Ar-Ar ages but did not affect the Hf-W systematics.