Hf—W chronology of the accretion and early evolution of asteroidsand terrestrial planets

摘要

The ~182Hf-182W W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accre-tion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events inthe earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion anddifferentiation of the parent bodies of some magmatic iron meteorites within less than -1 Myr. Chondrules from H chondritesformed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their ~26A1_~26Mg Mgages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon —3 Myr after CAI formation and may have continued until -10 Myr. A similar timescale is obtained for the high-temperature meta-morphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that thedifferent thermal histories of meteorite parent bodies primarily reflect their initial abundance of ~26A1, which is determined bytheir accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear tohave induced large-scale melting. For instance, Hf—W ages for eucrite metals postdate CAI formation by -20 Myr andmay reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf—W systematicsof some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remainspoorly constrained.