About the Project
Lead Supervisor
Prof Jens C. Andersen, Camborne School of Mines, College of Engineering, Mathematics and Physical Sciences, University of Exeter
Additional Supervisors
Prof Wolfgang Maier, School of Earth and Ocean Sciences, Cardiff University
Prof Marian Holness, Department of Earth Sciences, University of Cambridge
Location: Penryn Campus, University of Exeter, Penryn, Cornwall, TR10 9FE
This project is one of a number that are in competition for funding from the NERC GW4+ Doctoral Training Partnership (GW4+ DTP). The partnership aims to provide a broad training in the Earth, Environmental and Life sciences, designed to train tomorrow’s leaders in scientific research, business, technology and policy-making. For further details about the programme please see http://nercgw4plus.ac.uk/
For eligible successful applicants, the studentships comprises:
- An stipend for 3.5 years (currently £15,009 p.a. for 2019/20) in line with UK Research and Innovation rates
- Payment of university tuition fees;
- A research budget of £11,000 for an international conference, lab, field and research expenses;
- A training budget of £3,250 for specialist training courses and expenses.
- Travel and accommodation is covered for all compulsory DTP cohort events.
- No course fees for courses run by the DTP
We are currently advertising projects for a total of 10 studentships at the University of Exeter
Project Background
Layered mafic-ultramafic intrusions host important resources of energy-critical raw materials. These include V, Ga, Cr, Ni, Co and the PGE. Chromite is the source of Cr, while Ni, Co and the platinum-group elements typically associates with sulphide minerals. Economic resources of these typically formed in response to magma replenishment and mixing. In contrast, there is no consistent model for the occurrence of V and Ga in iron-titanium oxides.
The Skaergaard intrusion (Greenland) is the prime example of closed system crystallisation of tholeiitic magma. However, the crystallisation process remains controversial. A particular issue relates to whether the magma followed silica or iron enrichment during fractionation, and therefore, what conditions might govern the crystallisation of iron-titanium oxides. The Bushveld Complex (South Africa), in contrast, involved multiple magma injections and host economic oxides in the evolved parts of the intrusion (the Upper Zone).
Indeed, the formation of iron-titanium oxide rich layers is enigmatic. How is it possible to create layers of substantial thickness that carry 80-90% oxides from the magmas that (at the same time) would crystallize more than 90% silicates? If they formed by equilibrium crystallisation, the sorting during the accumulation would have to be extremely efficient. However, unlike chromitite layers that require disequilibrium crystallisation and are commonly explained by magma mixing, the oxide rich layers display no evidence for magma recharge. So how did they form?
Project Aims and Methods
During the project, you will explore the evidence for silicate-silicate liquid immiscibility during the formation of iron-titanium oxide rich successions in mafic-ultramafic intrusions. You will be working on an extensive collection of legacy samples from the Skaergaard intrusion and the Bushveld complex, but the project team will pursue opportunities for field work and sampling during the project. The principal methods will be detailed examinations using reflected and transmitted light optical microscopy, QEMSCAN, EPMA and LA-ICP-MS.
The crystallisation of cumulate rocks involves the following stages: 1. the crystallisation of minerals in a magma; 2. selective fractionation of these crystals into a semi-solid cumulate; and 3. migration and crystallisation of magma that became trapped between the crystals. This progressive crystallisation may be interrupted when new magmas replenish the magma chamber leading to reversals within the cumulate succession. Such events are critical for the deposition of chromite, sulphides and the PGE.
Recent research suggests that the evolution of many mafic-ultramafic magmas leads to exsolution of conjugate Fe-rich (with up to 40 wt% FeO) and Si-rich liquids. Because of specific gravity differences, the Fe-rich liquids accumulate at the base of the magma chamber, while the Si-rich liquid rises into the roof zone. The physical evidence for this process is retained in interstitial symplectites and reaction rims within the cumulate, as well as trapped inclusions of conjugate melt fractions in cumulus crystals.
We wish to learn how widespread this process may be, and how the ore constituents partition during immiscibility. We also wish to explore if these liquids may offer a linkage between the processes in mafic-ultramafic intrusions and formation of the enigmatic Nelsonite ores.
Funding Notes
NERC GW4+ funded studentship available for September 2020 entry. For eligible students, the studentship will provide funding of fees and a stipend which is currently £15,009 per annum for 2019-20. The GW4+ DTP consists of the GW4 Alliance of research-intensive universities: the University of Bath, University of Bristol, Cardiff University and the University of Exeter plus five unique and prestigious Research Organisation partners: British Antarctic Survey, British Geological Survey, Centre for Ecology & Hydrology, the Natural History Museum and Plymouth Marine Laboratory.
References
Background reading list
Charlier B et al. (2011) Large-scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the Daly gap. Geology 39(10), 907-910.
Holness MB et al. (2007) A textural record of solidification and cooling in the Skaergaard intrusion, East Greenland. J. Petrol. 48, 2359– 2377.
Holness MB et al. (2017) The Skaergaard intrusion of East Greenland: paradigms, problems and new perspectives. Elements. 13, 391–396.
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