Differentiation of asteroids in the early solar system from Hf-W systematics

Abstract

The first major differentiation in primitive planetesimals is the formation of metal- or metal-sulfide cores. To constrain the time of asteroidal differentiation, meteoritic metals and silicates coexisting with metals were analyzed for Wisotopes and Hf and W concentrations. The study focused on silicate inclusions in non-magmatic IAB iron meteorites and the corresponding metal phases as well as on winonaites and acapulcoites were analyzed. IAB iron meteorites formed by crystallization of metal ponds under low pressure conditions at or near the surface of their asteroidal parent body. The presence of abundant silicate inclusions reflects either incomplete metal-silicate separation or mixing of metal and silicates by impacts. Hafnium-W measurements were performed on magnetic and non-magnetic separates from seven IAB iron meteorites to constrain the exact timing of metal separation, silicate differentiation and metamorphism. IAB metals have a deficit in 182W of - 3.1 ± 0.2 e-units (relative to terrestrial standard materials). The silicate fractions in IAB iron meteorites have eW values ranging from -3 to +35, indicating that the exchange of W between metals and silicates ceased within the lifetime of 182Hf. Based on a combined IAB silicate isochron defined by three different bulk inclusions, silicate differentiation in the mantle-like reservoir of the IAB parent body must have occurred 2.9 ± 2.2 Myr after the formation of Ca,Al-rich inclusions (CAIs), the oldest objects known from the solar nebula. This event therefore occurred contemporaneously with the main metal segregation event on the IAB parent body at 3.5 ± 2.3 Myr after CAI formation, which is obtained from the W isotope composition of the metal phase (assuming evolution in a chondritic Hf/W ratio before metal separation). Hence, metal segregation and silicate differentiation occurred early enough, so that 26Al was responsible for parent body heating causing differentiation. On the other hand, internal isochrones for silicates of the two IAB-iron meteorites El Taco and Lueders yield ages 11.3 ± 2.3 Myr and ~12 Myr after CAI formation, reflecting impact induced heating and redistribution of radiogenic W. Additional later metal-silicate equilibration on the IAB parent body is also found in Mundrabilla (-2.6 ± 0.5 e- units). Therefore, a prolonged epoch of most likely impact-controlled W reequilibration on the IAB parent body can be inferred. In contrast to the combined IAB silicate isochron, most of the analyzed Winonaites define a combined isochron with an age that postdates CAI formation by 14.5 ± 2.8 Myr. This age is apparently too young for heating and melting of the parent body by an internal heat-source and indistinguishable within uncertainty from the internal El Taco and Lueders isochrones. Assuming a common origin for IABs and winonaites these ages could be related to a single impact-controlled W re-equilibration on the Winonaite/IAB parent body. A combined isochron defined by different Acapulcoite separates yields an age of 4.6 ± 1.3 Myr after CAI formation. This is within the range of Hf-W ages of ordinary chondrites, although Acapulcoites were heated to significantly higher temperatures than ordinary chondrites, implying a higher level of 26Al. Other chronometers indicate fast cooling of acapulcoites, which is difficult to reconcile with similar cooling rates as those of ordinary chondrites. A comparison with Hf-W literature data shows that asteroidal differentiation on the IAB parent body occurred after the segregation of most magmatic iron meteorites and contemporaneously with the accretion of chondrite parent bodies, lending further support for a revised solar system chronology where chondrites cannot be the precursor of most differentiated asteroids. Chondritic meteorites with higher equilibration temperatures, such as winonaites and acapulcoites have even younger ages. They are certainly no precursor material for iron meteorites and their comparatively younger ages probably reflect an impact origin after accretion of chondrite parent bodies.