Early solar system chronology from short-lived chronometers

Abstract

Age constraints on early solar system processes and events can be derived from meteorites and their components using different radioisotope systems. Due to the short time interval from the first formation of solids in the solar nebula to the accretion and differentiation of planetesimals and some planets, a high temporal resolution of the chronometers is essential and can be obtained in most cases only with short-lived isotope systems, particularly the decay schemes ²⁶Al-²⁶Mg, ¹⁸²Hf-¹⁸²W and ⁵³Mn-⁵³Cr. These chronometers provide highly resolved time constrains for the formation of the first solids (Ca-Al-rich inclusions or CAIs), chondrules, planetary cores, for the accretion and differentiation of planetesimals and hydrous/thermal alteration. Formation of Ca-Al-rich inclusions was restricted to the inner solar system and to a short time interval of ≪1 Ma, and marks the “beginning of the solar system”. It was immediately followed by planetesimal formation. The oldest planetesimals accreted within a few 10⁵ a after the formation of CAIs. The accretion of early formed planetesimals and their subsequent differentiation into a metallic core and a silicate mantle was a continuous process that occurred at different times in different locations of the solar nebula and extended over a time interval of at least ~4 Ma. During this time interval the accretion process may have changed from planetesimal formation via streaming instability to pebble accretion. The earliest formed bodies that still needed to settle into stable orbits could have created bow shocks in the adjacent regions still composed of dust and gas which resulted in the formation of silicate chondrules in a narrow time interval from 1.8 to 3 Ma. The chondrule forming interval was immediately followed by the accretion of the chondrite parent bodies, which did not differentiate due to their late accretion when most of the heat producing ²⁶Al had already decayed. Thus, the chondrite parent bodies are a second generation of planetesimals, but chemically they are the most primitive material preserved from the early solar system. Aqueous alteration of volatile rich planetesimals peaked at ca. 3.5 Ma and coincided with metamorphism recorded in ordinary chondrite parent bodies. The compilation of ages from different meteorites and their components demonstrates that similar accretion and differentiation processes do not follow an identical time line from dust to planetesimal formation and they do not correlate with the location in the disk. The accretion of matter into planetesimals was a local phenomenon with stochastic spatial distribution. The spatial distribution of accretion processes operating in the early solar system appears to be similar to those in some directly observable nascent exo-planetary systems.