Overview

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

Our understanding of the history of the Earth is fundamentally underpinned by geological time: when events happened, how quickly events happened and how long events lasted. Geochronologists determine not only “when” key geological events occurred, but also “how fast”? and “for how long?”. To answer those questions, we need to determine how rocks record processes such as deformation, metamorphism, magmatism and fluid flow. These processes may be long- or short-lived or may be continuous or pulsed. Distinguishing between these options has important implications for tectonic process and for the development of mineralisation in major crustal shear zones.

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, they incorporate trace elements that allow processes such as deformation, fluid infiltration and/or changes in pressure and temperature to be ‘fingerprinted’. Furthermore, following recent analytical advances, micas can be dated by a variety of radioactive decay schemes, including 40Ar/39Ar, Rb-Sr and K-Ca [2]. The challenge at the moment is that it is difficult to link the “process” that affects mica crystallisation to the recorded age, as the processes and factors listed above can affect the age record in micas differently depending on the different geological clock that is being used to “tell the time”.

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 new methods for dating micas in-situ, including via Rb-Sr and K-Ca using LA-ICP-MS techniques.

This project will make use of an outstanding natural laboratory for shear zone evolution, the North and South Armorican Shear Zones in Brittany, France [3,4], Figure 1. These shear zones separate two major terranes in the Armorican Massif and were active during the Variscan orogeny, about 300-400 Myrs ago. The structures are well exposed, and cut through a number of different lithologies, enabling us to investigate the role and magnitude of a number of potential factors such as bulk composition, metamorphic evolution, deformation and fluid infiltration that could affect how minerals in the shear zone record 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.

a) Geological map of 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].

Methodology

Fieldwork to collect carefully-characterised samples will be carried out in Brittany, France, where both the North and South Armorican Shear Zones will be investigated. We will concentrate on a number of different locations where shearing has affected different lithologies and where there are a variety of levels of mineralisation.

Samples will initially be processed at the Open University (OU), where oriented thin sections and polished thick sections will be made for further analysis. Careful petrographic observation and major element geochemical analyses on the OU electron microprobe will help to identify different generations and compositions of mica. Electron backscatter diffraction (EBSD) analysis to help characterise deformation histories will be obtained at Portsmouth, and detailed trace element geochemical compositions of the micas will be determined at the Portsmouth LA-ICP-MS facility.

The student will join a strong team of geochemists working together to develop in-situ laser ablation Rb-Sr and K-Ca mica geochronology at the OU [e.g. 6] to complement and supplement our current expertise in 40Ar/39Ar dating [e.g. 7]. Ages of different mica generations and chemical zones will be compared using the different techniques with the ultimate aim of developing a new framework for age interpretation.

Training and Skills

The student 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.

Specific scientific training will include safe fieldwork planning, fieldwork first aid, rock preparation (crushing, mineral separation/picking, thin-section making), data collection using a variety of cutting-edge geochemical instruments, and interpretation using a variety of chemical and statistical plotting methods. The student will join the University of Portsmouth’s 10-day field trip to Brittany to provide regional context.

The School of Environment, Earth and Ecosystem Sciences has a thriving postgraduate community. Online teaching opportunities including teaching on OU undergraduate modules and Massive Open Online Courses (MOOCs) are available via the OU Virtual Learning Environment.

Additionally, our students can gain excellent skills in science outreach by contributing travel experiences to platforms such as TravelingGeologist (http://www.travelinggeologist.com/), and Fieldwork Diaries (http://www.fieldworkdiaries.com/), or contributing to the CENTA student research blog (https://centaresearch.wordpress.com/).

Timeline

Year 1: Initial induction, literature review and familiarisation with the Portsmouth sample collections. 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 and initial data collection.

Year 2: Interpretation of first data set. Second field season (1 week). Presentation at national conference. Work placement (2 weeks).

Year 3: Interpretation of second data set. Preparation of papers. Presentation at an international conference. Writing and submission of thesis

Partners and collaboration (including CASE)

This project will involve training, supervision and lab work at Portsmouth University (https://www.port.ac.uk/research/research-areas/areas-of-expertise/crustal-evolution).

Further Details

Students should have a strong background in hard rock geology, experience in labwork and enthusiasm for fieldwork. Experience of making detailed observations during fieldwork, 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 (http://www.open.ac.uk/science/environment-earth-ecosystems/research/dynamic_earth) and Crustal Evolution at Portsmouth University (https://www.port.ac.uk/research/research-areas/areas-of-expertise/crustal-evolution).

Please contact Clare Warren, clare.warren@open.ac.uk for further information.

Applications should include: