Using phosphate minerals to investigate the geological history of carbonaceous asteroids

Summary

Carbonaceous asteroids offer a way to see back through time to the formation of the Solar System ~4.6 billion years ago.

This project will investigate the geological history of these dark, primitive bodies through laboratory analyses of phosphate minerals in carbonaceous chondrite meteorites and samples of asteroids Ryugu and Bennu, recently returned to Earth by space missions.

The mineralogy, textural settings, and chemical and isotopic composition of phosphates will be used to constrain environmental conditions (e.g., water/rock ratios, fluid compositions, peak temperatures) on carbonaceous asteroids, thereby providing new insights into the nature of the Solar System’s earliest volatile reservoirs.

Project Description

Asteroids are the debris left over from planet formation. Their physical and chemical properties can be used to investigate the processes and events that have shaped our Solar System over the last ~4.6 billion years.

Dark carbonaceous (C-type) asteroids are thought to be rich in volatiles such as carbon and water, having accreted in the cold outer regions of the Solar System. C-type asteroids represent some of the most primitive extraterrestrial materials and likely played an important role in the delivery of water and pre-biotic organic molecules to early Earth [e.g., 1]. However, important questions remain regarding the geological history of C-type asteroids, including the extent to which they were altered by water-rock reactions and/or thermal metamorphism.

Fragments of C-type asteroids naturally arrive on Earth as carbonaceous chondrite meteorites, while two recent space missions, JAXA’s Hayabusa2 [e.g., 2] and NASA’s OSIRIS-REx [e.g., 3], returned samples collected from the surface and sub-surface of the near-Earth C-type asteroids Ryugu and Bennu, respectively. Laboratory analysis of the chemical composition, mineralogy, and petrography of these extraterrestrial materials is a crucial step towards understanding the formation of C-type asteroids and the nature of volatile reservoirs in the Solar System.

This project aims to constrain the evolution of C-type asteroids by investigating phosphate minerals in carbonaceous meteorites and pristine samples returned from asteroids Ryugu and Bennu. Phosphate mineral grains with different chemical compositions and morphologies form in diverse environments, making them potentially excellent tracers for reconstructing past aqueous and thermal alteration processes on C-type asteroids [4]. The goals of this project include:-

1. Identify and characterize phosphate minerals in carbonaceous chondrites, Ryugu, and Bennu.

Phosphates in carbonaceous chondrites are typically rare (<1 vol.%) and have small grain sizes (<50 μm). This project will employ new optical and electron microscopy techniques capable of characterizing minerals at the micron-scale to locate phosphates in samples of carbonaceous chondrites, Ryugu, and Bennu that record a range of aqueous and/or thermal alteration. Scanning electron microscopy (SEM) and electron probe micro analysis (EPMA) will be used to image the phosphate grains and determine their major and minor element compositions. The mineralogy and textures of the grains will then be used to constrain asteroid conditions such as water/rock ratio and peak metamorphic temperatures.

2. Determine the trace element concentration of phosphate minerals.

Phosphates are likely a significant host of trace elements, such as the Rare Earth Elements (REEs), in samples of carbonaceous asteroids. Trace element compositions of the phosphates will be analyzed in-situ using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Abundances will be used to track episodes of alteration and the chemical evolution of fluids within asteroids, and to better understand the influence of phosphate minerals on the bulk composition of carbonaceous bodies. Furthermore, the application of U-Pb dating to constrain the formation time of phosphates in carbonaceous asteroids will be explored.  

3. Investigate the isotopic composition of volatiles in phosphate minerals.

Some phosphate minerals contain a high concentration of elements such as hydrogen, chlorine, and fluorine. The abundance and isotopic composition of these elements (e.g., hydrogen (δD) and chlorine (δ³⁷Cl) isotopes) reflect both their initial composition plus any fractionation that occurred during parent body alteration. This project will use secondary ion mass spectrometry (SIMS) to characterize volatile elements in phosphate grains. 

Suggested Skills and Background

This project will provide training in the preparation and laboratory analysis of extraterrestrial materials, in particular techniques for the characterization of minerals at high spatial resolution (e.g., SEM, EMPA, LA-ICP-MS).

The student will become a member of the Planetary Materials Group at the Natural History Museum (NHM), London and the Isotopes and Planetary Science Group at the University of Manchester, taking advantage of access to a world-class meteorite collection, our involvement in the Hayabusa2 and OSIRIS-REx missions, and a diverse suite of analytical facilities. The project would suit an enthusiastic individual with a keen interest in planetary science and a strong background in geosciences.

The student will be registered at the University of Manchester.

For informal enquiries or further information, please contact Ashley King

Application Process

Deadline: Sunday 4^th February 2024.

Please upload the following documents

• Curriculum vitae
• Covering letter outlining your interest in the PhD project, relevant skills training, experience and qualifications, and a statement of how this PhD project fits your career development plans.
• Transcripts of undergraduate and Masters’ degree results.
• Two academic references, including (if applicable) Masters’ project supervisor.

We strongly recommend contacting potential supervisors in advance so that you can ask any questions and also meet the supervisory team before you apply. This project is eligible for funding from the Science and Technology Facilities Council, which provide information for students: https://www.ukri.org/what-we-do/developing-people-and-skills/stfc/training/studentship-information-for-students/

This is a competitive application process. All applications will be reviewed by the project supervisory team and an academic panel. Shortlisted applicants will be invited for an interview, which usually lasts 30-60 minutes.

As part of the process, shortlisted applicants will also be offered the opportunity to visit the Natural History Museum to meet the wider research group and tour relevant facilities. Shortlisted applicants will usually find out the outcome a few days after all interviews have been held.