Elucidating planet formation using chondrule oxygen fugacity

Background

Recently, there has been a dramatic shift in understanding of planet formation and the circumstellar disk, following incredible images from the ALMA telescope. They have revolutionised understanding of planetary formation and caused a suite of new theories. For example, Johansen et al. 2014 suggested that regions with increased dust and gas pressure (pressure maxima) must have been present in the protoplanetary disk to form the first planetary bodies.

The origin and nature of pressure maxima is poorly understood, despite their potential importance in planetary formation. The snow line, where water transitions from solid to vapour because of disk temperature, has been suggested as a possible cause of pressure maxima. If this is true, then planetary formation is a natural result of disk evolution.

Aims

Condensation and vaporisation of water at the snow line affect oxygen fugacity, through altering H2:H2O in the gas. fO2 records from throughout the protoplanetary disk therefore could enable us to investigate the nature and origin of pressure maxima, as well as their consequences, with depth and clarity unprecedented in planetary science research.

Chondrites are made of mm sized solids from the disk, and therefore carry fO2 records dating from the early Solar System. This is demonstrated by Sutton et al. (2017), who showed that carbonaceous chondrites (CC) recorded a more positive fO2 than non-carbonaceous (NC). This is due to the parent body oxygen fugacity, as CCs incorporated more solid water than NC bodies.

In order to discover the fO2 of the actual disk, we need to record data from individual chondrules, which range from 100-1000um. Their size means that high precision, high-resolution techniques are required to recover values, this has never been done before in a systematic and focussed way.

In the project, I hope to use the variable partitioning of trace elements among metal, silicates and sulphides as a proxy for fO2 values in individual chondrules. This will allow me to understand the environments in which chondrules formed, giving new understanding about the snow line and its consequences in our Solar System, and the associated pressure maxima. As such, this project could unlock several key new insights into planet building processes.

Research Methodology

The redox state of elements including Fe, Cr, W, Ti, and V varies with fO2. The thermodynamic stability of these elements in molten metal, silicates and sulphides is controlled by their redox states, and therefore fO2. As a result, we can create a proxy for oxygen fugacity in individual chondrules by looking at relative concentrations of these elements within metal, silicate and sulphide.

In this project, values will be estimated through a comparison of concentration inside a chondrule to the value measured in artificial samples created under a range of controlled fO2 conditions.

Concentrations would be measured using high-resolution techniques, including laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron probe micro-analysis (EMPA), and micro X-ray fluorescence (uXRF) at Diamond Light Source.

Artificial samples will be created in the Department of Earth Sciences using specialised furnaces. The proxies discussed here have been developed and used in Oxford for decades, applied to terrestrial and martian rocks, but never before on other extra-terrestrial material, highlighting the potential of this project to unlock new information on the process of planet building.