Sequencing planetary accretion using chronology of ungrouped achondrites

Abstract

Determining how our Solar System formed is an important step towards understanding our origins. Key to this is sequencing the events which transformed a cloud of interstellar gas into the Solar System we see today. Currently, the U-Pb system is the only absolute isotopic chronometer able to produce the resolution needed for high precision dating of the early Solar System. Additionally, only well preserved and ancient achondrites provide the material which record early planetary formation processes. At present, much of the sequencing of early Solar System planetary formation has revolved around a small group of well-preserved achondrites called angrites. While angrites have proved invaluable for sequencing early planetary formation, they also all originate from the same parent body. With the recent discovery of several more well-preserved achondrites which are not from the angrite group, we are presented with an opportunity to expand our understanding of early planetary formation. In this study, four well-preserved unique ungrouped achondrites are analysed and sequenced in order to understand more about early planetary formation within the early Solar System. In addition to this, analysis techniques were modified and monitored in order to improve the accuracy and precision of the calculated ages across all laboratories. The two most significant analysis findings were that firstly, HF as a leaching acid induces Pb isotopic fractionation, while secondly; care must be taken when monitoring TIMS SEM detector drift as it could cause Pb-Pb age discrepancies of up to 1 Ma. The U-Pb systematics of four different achondrites (NWA 7325, Asuka 881394, NWA 6704 and NWA 10132) from three different planetary bodies are analysed within this study. In addition, NWA 7325 underwent Al-Mg analysis in order to further understand its formation history. The chronological analysis of these meteorites resulted in ages of: NWA 7325 = Pb-Pb age of 4563.4 ± 2.6 Ma, Al-Mg age of 4563.09 ± 0.26 Ma; Asuka 881394 = Pb-Pb age of 4564.95 ± 0.35 Ma; NWA 6704 = Pb-Pb age of 4562.55 ± 0.49 Ma; and NWA 10132 = Pb-Pb age of 4563.46 ± 0.67 Ma. In addition to age data, the U-Pb systematics of each meteorite shows evidence for varied and complex formation histories. A combination of the age data from this study and previously published age data show an overall timescale of planetary formation within the Solar System. The earliest planetesimal crustal formation began around 4565 Ma, roughly 2-3 Myr after the formation of CAIs and simultaneous with chondrule formation. Following on from this, a multitude of planetesimals differentiated and crystallised crustal material between 4564 Ma and 4562 Ma. By 4562 Ma, large scale planetesimal collisions were occurring, with many early planetesimals accreted into larger bodies. Some early planetesimals survived this period however, with the HED parent body possibly enduring until today (4 Vesta). Nevertheless, large scale planetesimal impacts, which represent the final stage of planetary formation, were underway within 5 Myr of the formation of the first solids and within ~3 Myr of the crystallisation of the first crustal material. Show more