Volatile accretion and impact histories of early-formed planetesimals

Description: Large, possibly Mars-sized planetary bodies, formed extremely early, some as little as 1 to 2 Ma after the earliest dated solids in the Solar System [1]. The angrite group of achondritic meteorites are without doubt the most enigmatic of these early-formed, differentiated materials [2]. Contradictory evidence means that we currently have little understanding as to the size of their parent asteroid, or the role of impact processes during their formation. The apparent lack of widespread shock features in most angrites has led some to argue that they originated on small asteroids and so escaped the early catastrophic bombardment that affected larger bodies [3]. On the other hand, metamorphic textures found in coarser-grained angrites have been interpreted in favour of a planet-sized parent body [4]. The recent discovery of deformation features in olivine xenocrysts from a number of finer-grained "quenched" angrites has raised the possibility that they represent impact-melt rocks [5]. Preliminary oxygen isotope data collected at the Open University indicates an isotopic disequilibrium between matrix and xenocrystic olivines, in some quenched angrites, evidence that can be interpreted in favour of an impact origin for these samples.

Another puzzling attribute of angrite meteorites is their extreme alkali-depleted signature contrasting with their relatively enriched signatures for highly-volatile elements such as H and C [7-8]. This has led to the suggestion for mixing of extremely volatile-depleted material, located well inside the snow line, with volatile-rich material derived from outside the snow line [7]. However, these deductions are based upon a relatively small dataset, derived from two angrites [7-8] and, therefore, additional work is required in order to fully evaluate the formation and evolution of early-formed planetesimals in the Solar System.

This PhD project will investigate the origin of angrites, primarily making use of stable isotope analysis techniques (laser fluorination, NanoSIMS), supplemented by microbeam analysis (SEM, EPMA). There also may be an opportunity to work alongside our established collaborators in Japan, possibly including some study at the National Institute of Polar Research (NIPR) as part of this project. Oxygen isotope analysis has suggested a possible link between angrites and the anomalous achondrite Ibitira [6]. The nature of this relationship, if any, will be investigated as part of this study, together with an assessment of the role of impact processes in the origin of anomalous achondrites in general.