On the aggregation, differentiation and devolatilisation of volatile-rich asteroids
Creators: name: Langendam, Andrew Danial
Publisher: name: Monash University
Year: 2017
Subjects: Uncategorized

description: The development of chemically distinct parent bodies from the protoplanetary disk is controlled by the composition of their formation regions. After accretion the chemistry of planetesimals can be greatly affected by the presence of phases that can alter their oxidation state. The protoplanetary disk was stratified with respect to the volatility of elements such that in the hotter inner solar system, planets and planetesimals are composed of refractory elements, whereas those that formed further out are progressively richer in volatile elements. At about 2.6-2.8 AU, temperatures in the early solar nebula were cool enough the H₂O ice was stable, and this is known as the snowline transition. Planetesimals that formed within or beyond the snowline transition accreted with higher proportions of H₂O ice, which was later capable of driving oxidation of the silicate mineralogy through serpentinisation reactions. Thus, the planets and planetesimals that formed further from the Sun are more oxidised and more volatile rich. This thesis investigates the effect of redox processes during the aggregation, differentiation and early cooling of planetesimals. The aggregation of planetesimals within the snowline transition has been investigated by assessing the amalgamation history of the R chondrites breccias, a group of highly oxidised and brecciated chondrites that have undergone metamorphism. Chemical differences in orthopyroxene between clasts within these breccias shows that several bodies formed in close proximity to one another, but with different volatile contents. Some clasts show that they had been altered by serpentinisation-deserpentinisation reactions, whereas other clasts were equilibrated to the metal-olivine-pyroxene buffer. Since there was no distinct trend in petrographic type between these populations, this oxidation is likely to have occurred before the disruptive impact and reamalgamation of the R chondrite parent body. The R chondrite parent body is thus interpreted to be composed of an amalgamation of three or more distinct volatile-rich asteroids. Melting within planetesimals that formed near the snowline transition was investigated by performing experiments on the Karoonda CK chondrite. These experiments showed that a body that has been oxidised through reaction with water ice will produce a dense Fe-Ni-S-O melt at temperatures above 950°C. This melt wets silicate minerals, allowing it to develop an interconnected network and migrate rapidly towards a core via percolative flow. This wetting behaviour is only inhibited once large proportions of silicate melt has developed (above 1100°C). Hence, oxidised bodies are able to form cores at lower temperatures and more efficiently than their inner solar system counterparts, as metal melts in the refractory bodies are unable to wet silicates, and require high silicate melt fractions to migrate effectively. Although the primitive achondrites known as ureilites are not highly oxidised, they do contain a significant fraction of carbon, implying that their parent body accreted with a significant proportion of volatiles Ureilites display an unusual smelting texture developed at olivine grain boundaries, which has previously been attributed to a reduction reaction between olivine and graphite. However, fracture-based smelting was observed in many ureilites, along with typical of fluid-rock interaction. Redox modelling of different possible smelting reactions for known ureilite chemistries showed that graphite-based smelting cannot generate the observed compositions in smelted regions, but that a CH₄-H₂S-rich fluid could do so easily. This fluid could well have originated as a fluid exsolved from the crystallising core of the ureilite parent body. By understanding the aggregation, differentiation and devolatilisation processes of volatile-rich planetesimals, the ability for similar extrasolar planets to form conditions capable of supporting life for geologically significant times can be assessed. It is found that they are perhaps more favourable for life than the refractory inner solar system planets. Rock-ice bodies need to be treated differently when assessing their internal structure and evolution than those that formed in the relatively ice free inner solar system.