2024-OU5 The strains of dating: Rb/Sr to the rescue?

Project highlights

Fieldwork in the North and South Armorican Shear Zone, Brittany (France)
Training in cutting-edge geochronology techniques and technique development
Developing novel “age-to-stage” geochemical tools

Overview

Ductile shear zones accommodate vast amounts of strain in the crust and are a focus for fluids, magmas and elemental transfer. The rates and timescales of shear zone deformation, and the timing of associated fluid, magma flow or mineralisation events, are critical for constraining large-scale tectonic processes, the rheological behaviour of the lithosphere and the formation of economically important resources through geological time [e.g. 1]. As micas deform and recrystallise during shearing, the geochronological information that they record may be modified or completely re-set.

Following recent analytical advances, micas can now be dated in-situ by Rb-Sr LA-ICP-QQQ-MS analysis [e.g. 2], with trace element datasets gathered from the same ablation spot. Micas incorporate trace elements that allow processes such as deformation, fluid infiltration and/or changes in pressure and temperature to be ‘fingerprinted’, and when linked to their geochronolgoical record, provide rich data about their history. However, there are still challenges to be overcome with the precision and accuracy of in-situ Rb-Sr geochronology techniques such as finding and validating community agreed reference materials, determining crystallographic effects on data quality, and choice of analytical parameters and their influence on fractionation and matrix effects.

The aim of this project is to develop a new framework for confidently linking mica age to geological process within shear zones. Part of this project will involve helping to develop Rb-Sr LA-ICP-QQQ-MS geochronology at the Open University (OU), and developing a framework for linking geochronological data to other petrographic and geochemical datasets collected at the University of Portsmouth (UoP).

This project will make use of an outstanding natural laboratory for shear zone evolution, the South Armorican Shear Zones in Brittany, France [3,4], Figure 1. This shear zone and its northern counterpart separate two major terranes in the Armorican Massif and was active during the Variscan orogeny, about 300-400 Ma ago. The structures are well exposed, and cut through a number of different granite plutons, with variable degrees of deformation and fluid infiltration that could affect how, when and why micas in the shear zone record (differences in) geological time. Furthermore, parts of the shear zones and local granites are mineralised [5], making this region an important case study for investigating the role and timing of element-laden fluids through the shear zone with respect to the timing of shearing.

Figure 1: (a) Geological map of the Armorican domain in Brittany, France, with location of South Armorican Shear Zone and sample location (white star). (b) Sample of orthogneiss with S–C fabric. Figure and caption from [3].

CENTA Flagship

This is a CENTA Flagship Project

Host

The Open University

Theme

Dynamic Earth

Supervisors

Project investigator

Dr Barbara Kunz (OU) [email protected]

Co-investigators

Prof. Clare Warren (OU) [email protected]
Dr Tom Argles (OU) [email protected]
Prof. Craig Storey (Portsmouth) [email protected]
Dr Nick Roberts (BGS) [email protected]

How to apply

Each host has a slightly different application process.
Find out how to apply for this studentship.
All applications must include the CENTA application form.
Choose your application route

Methodology

Fieldwork to collect carefully-characterised samples will be carried out in Brittany, France, concentrating on locations that show different levels of protolith shearing.

Oriented thin sections and polished thick sections will be made at the OU for further analysis. Careful petrographic observation and major element geochemical analyses at the OU’s Electron Microscopy Suite will identify different mica generations. Electron backscatter diffraction (EBSD) analysis at the UoP will support the characterisation of deformation histories. LA-LIBS analysis (UoP) will provide further characterisation of light mica components such as Li, OH and F. Rb/Sr geochronology and petrochronology (collecting other trace elements simultaneously) analytical procedures will be developed in conjunction with the supervision and laboratory teams at the OU and BGS. The ages of different mica generations and chemical zones will be compared with the petrological and geochemical datasets to develop a new framework for age interpretation.

Training and skills

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

The successful candidate will receive specific scientific training in safe fieldwork planning, first aid, rock preparation, lab safety, data collection using a variety of cutting-edge geochemical instruments, data processing and interpretation using a variety of chemical and statistical plotting methods.

The School of Environment, Earth and Ecosystem Sciences at the OU has a thriving postgraduate community. Additionally, our students can gain excellent skills in science communication by contributing to outreach activities at local schools, science festivals or to CENTA or OU social media communications. The OU is a centre of excellence for public engagement with research.
The student will also be enrolled in the 2024 BGS University Funding Initiative (BUFI) cohort, where a wide range of BGS- and externally-led training is available.

Partners and collaboration

The student will receive both training and external supervision at Portsmouth University and the BGS. This will include regular visits to both places as well as virtual meetings with supervisors. Furthermore, the student will be able to incorporate additional laboratory analyses at the BGS’ isotope laboratories, where they will be able to interact and work with internationally renowned scientists and have access to specialist instrumentation and analytical expertise. The BGS has recently installed the latest mass spectrometry technology, for which this project will be able to assist with method and application development.

Further details

Further details on how to contact the supervisor for this project and how to apply for this project can be found here:

For any enquiries related to this project please contact Dr Barbara Kunz (OU) [email protected].

To apply to this project:

You must include a CENTA studentship application form, downloadable from: CENTA Studentship Application Form 2024.
You must include a CV with the names of at least two referees (preferably three) who can comment on your academic abilities.
Your application materials, including the CENTA application form 2024, your CV and the Open University application form must be emailed to [email protected]. Instructions on how to apply to the Open University are to be found on https://www5.open.ac.uk/stem/environment-earth-ecosystem-sciences/research/phd-students/current-opportunities-and-how-apply, please ensure you read this webpage before applying as you will need to obtain the relevant OU application form from here. Please quote CENTA 2024-OU5  when completing the application form.

Applications must be submitted by 23:59 GMT on Wednesday 10th January 2024.

The student will join a strong team of geochemists at the OU and UoP working together to develop in-situ laser ablation Rb-Sr petrochronology techniques.

Students should have a strong background in hard rock geology, with experience in petrographical and geochemical techniques and enthusiasm for fieldwork. Experience of making detailed observations during fieldwork and of thin sections, and a strong interest in (micro)structural and/or metamorphic petrology and geochemistry are highly desirable.

The successful student will join a well-established team researching Dynamic Earth processes at the Open University (https://www.open.ac.uk/stem/environment-earth-ecosystem-sciences/research/dynamic-earth) and Crustal Evolution at Portsmouth University (https://www.port.ac.uk/research/research-areas/areas-of-expertise/crustal-evolution).

Both the OU (https://www.open.ac.uk/stem/environment-earth-ecosystem-sciences/research/analytical-facilities/earth-science-laboratories) and Portsmouth (https://www.port.ac.uk/about-us/our-facilities/lab-and-testing-facilities/mass-spectrometry-and-laser-ablation-laboratory) have the necessary cutting-edge lab facilities to conduct this research.

Possible timeline

Year 1

Initial induction, literature review. Regional context field trip to Brittany in April with the University of Portsmouth (10 days) and further sampling fieldwork in Brittany, France (5 days). Sample preparation, method development at the OU and initial data collection at the OU & Portsmouth.

Year 2

Initial interpretation of data set. Second field season (1 week). Continued method development and data collection; Presentation at national conference. Work placement (2 weeks).

Year 3

Final interpretation of data set. Preparation of papers. Presentation at an international conference. Writing and submission of thesis.