Overview

Project Highlights:

  • Fieldwork in the European Alps
  • Training in multi-method geochronology in state-of-the-art laboratories
  • Develop a new chemical framework for linking rock age to mineralogical process

Geochronology fundamentally underpins our knowledge of how the continental crust forms and evolves by providing the rates and timescales of burial, metamorphism and deformation. High spatial resolution in-situ analyses (via laser ablation) allow for the precise and accurate measurement of isotope ratios from individual geochronometer minerals within thin sections. These isotope ratios provide tightly constrained ages that can be linked to petrographic observations and mineral chemical analyses, all of which underpin the modern field of ‘petrochronology’ [1]. There is a still considerable debate about the importance and role of changing metamorphic conditions, bulk rock chemistry, deformation and fluid infiltration in determining when the geological clock starts ticking in deformed and metamorphosed rocks that have experienced a lengthy and protracted geological history.

In-situ U-Th-Pb geochronology datasets from metamorphosed and deformed rocks commonly yield a range of dates that spans more time than the analytical uncertainty of a single “age” would suggest. This span of ages therefore suggests either that: (1) protracted crystallization took place over a range of pressure, temperature and deformation (P-T-d) conditions, (2) there was incomplete isotopic resetting during cooling and exhumation, or (3) there has been analytical mixing of mineral domains of different age. Recent studies have demonstrated that individual samples that have undergone similar P-T-d conditions, i.e. from the same outcrop, can yield strikingly varied mineral dates [2], indicating that the rock’s bulk chemical composition exhibits a strong control on the reactions that allow the geochronometer minerals to crystallise or dissolve [3]. It is also well known that different geochronometer minerals within the same rock respond differently to pressure, temperature and deformation [4,5].

The major aim of this project is to develop new U-Th-Pb petrochronological tools and workflows to help constrain how and when time is recorded in deformed rocks during burial and exhumation of the continental crust. This will be achieved by: (1) analysing different samples that are closely spatially associated (e.g. on the sub-metre scale) but which have different bulk chemical compositions, and (2) analysing rocks of similar bulk composition in less strained versus more strained localities. A suite of analytical datasets using the petrochronology approach will be applied to each rock unit, encompassing imaging techniques, petrography, microstructural analysis, in-situ U-Th-Pb geochronology, and modelling of metamorphic conditions. Integration of these data will inform how different geochronometers respond during the deformation and metamorphism of a rock unit.

Multi-layer SEM element map of a typical deformed metapelite that shows the location of geochronometer minerals apatite, monazite, muscovite and biotite. By comparing chemical compositions and locations of the same geochronometer minerals in neighbouring rocks of different bulk composition or different magnitude of deformation we will be able to determine geochronometer-forming reactions and link “age” to “process” more tightly. Image created by J. Lissenburg, Cardiff University.

Methodology

Two sample suites will be investigated during this project. The COSC drill core (Collisional Orogen in the Scandinavian Caledonides ICDP drilling project [6]) will be used to investigate the effect of deformation on the geochronometer record in the same bulk composition. These samples are housed in Berlin. Samples will be also collected from the Lepontine Dome in the Alps [3] in either years 1 or 2. Samples will be processed at the Open University (OU). The project will use a wide variety of imaging and analytical techniques. Imaging and petrography will be carried out by optical petrography, back-scattered electron (BSE) and cathodoluminescence (CL) imaging using a Scanning Electron Microscope (SEM), and elemental mapping using both electron microprobe (EMPS) and laser ablation ICP-MS. Mineral chemical analyses will be carried out using electron microprobe and LA-ICP-MS. In-situ geochronology will be conducted using laser ablation methods for U-Th-Pb at the BGS. Additional techniques that the student may use, and help develop, include Rb-Sr on micas at the OU, and Lu-Hf on garnet at the BGS. The student will also carry out metamorphic modelling using phase equilibria modelling.

Training and Skills

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 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/). The OU is a centre of excellence for public engagement with research.

Timeline

Year 1: Initial induction, literature review and familiarisation with the OU and BGS sample collections. Sampling trip to the COSC drill core in Berlin and fieldwork in the European Alps. Sample preparation, initial data collection.

Year 2: Interpretation of first data set. Second field season where required. Presentation at national conference. CENTA 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 the British Geological Survey in Keyworth, Notts, where the student will spend at least a couple of months during their PhD.

Further Details

Students should have a strong background in hard rock geology and enthusiasm for fieldwork and detailed labwork. Experience of fieldwork in mountainous areas and a love of igneous or metamorphic petrology is 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 the British Geological Survey (https://www.bgs.ac.uk/sciencefacilities/laboratories/geochemistry/gtf/home.html).

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

Applications should include: