Overview

Project Highlights:

  • Understanding low temperature, REE concentration, in the context of addressing modern needs for REE supplies
  • Exploring “slow tectonic” basin processes through the development of monazite crystals
  • Implications for understanding oceanic chemical change in post-Paleozoic times

Large, long-lived, mud-dominated sedimentary basins are the sites of some of the most economically and societally-significant processes in geology, including the generation of oil and gas, and the formation of metallic mineral deposits. However, the processes involved as mud transforms into mudstone and (where mountain-building occurs) into slate, are cryptic and poorly constrained. One source of information that can help illuminate some of these processes are authigenic growths of the mineral monazite (a rare earth element phosphate mineral) that are common in some classic mudrocks, such as in the Welsh Basin or in lagerstätte e.g. Sirius Passet, Greenland. Markedly zoned, both chemically and isotopically, and radiometrically dateable, these low-temperature rare earth element sinks can be used to track fluid-rock interactions, in the concentration of economically important ‘immobile’ elements. The current generation of high-precision and ultra-high resolution analytical equipment offers the potential to use these diagenetic phenomena to track the movement of both fluids and chemical elements underground through both time and space, and illuminate the fundamental geological processes involved. This PhD, jointly between the University of Leicester and the British Geological Survey (NIGL) explores this potential, applying state-of-the-art techniques to these complex, remarkable proxies for underground rock transformation.    

A scanning electron microscope image of a monazite from the Sirius Passet Formation, Greenland.

Methodology

The student will initially examine the literature and unpublished works to build up an understanding of the distribution and isotopic range of  U-Th - Pb, and Sm-Nd, within authigenic monazites. The aim of this will be to optimize and develop the best precision geochronometers for within-crystal dating and also to capture as wide a range of parameters as possible for interpreting basin subsidence history. In the first year, the student will collect and prepare samples from the Welsh Basin, to represent the stratigraphic succession of the Cambrian to Silurian, and proximal to distal portions of the Basin. Fieldwork will involve logging the successions for sampling, characterisation of the samples, and compilation of any available data. Mineral separation will be carried out at NIGL. With the selected samples, the student will (1) date zonations within the samples using micro-analytical, high precision U-Pb and (2) geochemically characterise the samples using LA-(MC)-ICP-MS. The student will receive full training in these techniques and be taught how to run the samples for themselves. The data will be used to assess growth behaviour of the minerals, and build up a picture of the evolution of the Basin from the development of the minerals. In the second year, the work from the Welsh Basin will be contrasted with work from other Basins, e.g. Brabant Basin, Belgium, or other mudrcosk that display authigenic monazites, e.g. Sirius Passet, Greenland, to eventually produce a generic model for the transportation, and concentration of REE in mudrock-dominated systems.

The student will join a well established research group with expertise in geochemistry, applied geology and paleoenvironments.

Training and Skills

In addition to the geochemical techniques described above, and geochemical modelling, there will be opportunities to present within lab groups and seminars at the University of Leicester and to national and international meetings. Training will be provided on science writing for international publications, and for research proposals.

Timeline

Year 1: focus on understanding the distribution and isotope range of  U-Th - Pb, and Sm-Nd  isotope distribution within authigenic monazites, with the aim of optimizing and developing the best precision geochronometer for within-crystal dating and for basin subsidence measurements. Collect and prepare samples from the Welsh Basin, from across the Cambrian to Silurian stratigraphy.

Year 2: apply best-practice methodology to the Welsh basin samples, to map the timing and movement of REE within and through the sedimentary pile. Scope out comparison sites, e.g. Brabant Basin, Belgium or Sirius Passet, Greenland. Present initial results at a national/international conference.

Years 3: extend analytical studies to comparison sites.  From the mineral data, develop a model for REE behaviour during basin diagenesis.  Complete thesis.

Partners and collaboration (including CASE)

A strength of the project is that the student will benefit from working closely with experts at NERC Isotope Geosciences Laboratory, at Keyworth, who have worked and developed microanalytical geochemistry for their careers.

Further Details

Please contact Prof. Jan Zalasiewicz at the University of Leicester (jaz1@le.ac.uk) for further details.