Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules

Abstract

Age dating of meteorite inclusions suggests rapid formation of the building blocks of terrestrial planets within 1 My of the Sun.

Chondrules: Ubiquitous Chondritic Solids Tracking the Evolution of the Solar Protoplanetary Disk

“…Thus, the first chondrules formed contemporaneously with CAIs and the energy source responsible for the formation of chondrules lasted for at least 2.6 Myr following condensation of CAIs. A subsequent study of 17 individual chondrules from CR and ordinary chondrites has confirmed the oldest ages of chondrules and extends the age range of nebular chondrules formation to 3.6 Myr after CAIs (Bollard et al 2017). The timescales for the melting of disk solids inferred from the Pb-Pb dates is comparable to the 3 Myr median lifetime of disks around low-mass stars indicated by astronomical observations of young stellar objects within starforming regions (Evans et al 2009).…”

“…This implies that chondrule formation and asteroidal accretion are intrinsically linked processes. However, the protracted timescale for the formation of chondrules inferred from the assumption-free U-corrected Pb-Pb dating method (Connelly et al 2012;Bollard et al 2017) is inconsistent with this model. Indeed, the 3 Myr formation interval for chondrules from various chondrite groups indicates prolonged accretion timescales for chondritic parent bodies.…”

“…Given the mounting evidence for initial 26 Al heterogeneity, it appears unlikely that the Mg system can provide an accurate chronology of the early Solar System. In this respect, the apparent 1-2 Myr age gap inferred from the 26 Al-26 Mg system may simply reflect a reduced initial 26 Al abundance in chondrule precursors, which would bring the chondrule Mg age distribution in line with the U-corrected Pb-Pb dates (Bollard et al 2017). …”

“…Among the proposed mechanisms of chondrule formation are shock waves related to disk gravitational instability, eccentric planetesimals, and X-ray flares (Desch et al 2005;Morris et al 2012), magnetised turbulence in the disk (McNally et al 2013), collisions between chondritic or differentiated planetesimals (Asphaug et al 2011;Johnson et al 2015), and splashing of differentiated planetesimals (Asphaug et al 2011). Although none of the proposed mechanisms can be completely ruled out, the common presence of relict grains and Fe,Ni-metal in porphyritic chondrules as well as the large age range of chondrules from individual chondrites, including ages indistinguishable from CAIs (Connelly et al 2012;Bollard et al 2017), are inconsistent with formation of the majority of porphyritic chondrules by splashing of differentiated bodies.…”

Multi-element ion-exchange chromatography and high-precision MC-ICP-MS isotope analysis of Mg and Ti from sub-mm-sized meteorite inclusions

“…13,14 Given that Mg is a moderately volatile element and Ti a highly refractory one, potential differences in the degree of stable isotope fractionation may discern the condensation and/or evaporation history of CAIs. Also, the identication of planetary-scale mass-independent heterogeneity in both Mg (most likely associated with heterogeneities in the short-lived radioactive nuclide 26 Al [15][16][17] ) and Ti isotopes 10 calls for an evaluation of the large-scale mixing/unmixing processes of pre-solar components associated with the establishment of the protoplanetary disk and the formation of the rst planetesimals. The coupled isotope systematics of various elements (including Mg and Ti) from unique solar system components, including disk solids such as CAIs, AOAs (Amoeboid Olivine Aggregates) and chondrules from primitive meteorites, may aid in such a quest.…”