IAP-25-030

Formation of rare metal (Li-Ta-Sn) melts on the Precambrian Earth

The secular evolution of Earth’s continental crust is marked by a distinct geochemical change between 3.0 and 2.5 Ga (Brown et al. 2020; Nebel et al. 2018). This change reflects global shifts in geological and geodynamic processes, and is marked by the onset of widespread crustal reworking, and the first appearance of rare-metal enriched melts. These melts particularly manifest in the geological record as Archean-aged lithium–caesium–tantalum (LCT) pegmatites – highly differentiated small-volume granitic intrusions that now dominate global lithium production.

Highly evolved granitic melts form via reworking of existing crust, and there is increasing agreement that mineralised pegmatites result from low-degree anatexis (Webber et al 2019). However, only a subset of pegmatites are actually mineralised in lithium (Li), and this has led to intense speculation about the processes responsible for the high degree of Li enrichment necessary (up to 0.5 wt.%) to form Li ore minerals (e.g. Gardiner 2024). The lack of suitable metasedimentary source rocks in some of the most prospective Li pegmatite Archean terranes (e.g. Zimbabwe, Australia) has led to further questions regarding the nature of the source to pegmatites. Models include incorporation of Li-enriched sedimentary material; multi-cycle recycling of igneous crust; or even a deeper mantle origin (Koopmans et al. 2024, Smithies et al. 2025).

In this project, we will conduct a petrological study of Archean-aged pegmatites in several terranes (e.g. Western Australia, Zimbabwe and North America), to better constrain the source and processes involved in the formation of mineralised pegmatites, and place their genesis within the broader Archean crustal evolution.

We will deploy novel geochronological and isotopic tracers to unpick the nature of the crustal source to mineralised Li pegmatites, and compare this to unmineralized examples. Pegmatites are rich in feldspar and micas, and we will use tools including U-Pb in cassiterite, U-Pb-Sr in apatite, Rb-Sr in muscovite and Pb-Pb in feldspar to determine ages, likely crustal material input into pegmatite sources, and to compare with surrounding host rocks. We will deploy petrological modelling tools to model the partial melting of putative source rocks and how this process might generate Li-rich melt. These tools together will help to build a genetic petrological 4D model.

This research will inform both on our thinking of how Li pegmatites form, and also tackle the broader questions regarding late Archean continent formation, stabilisation and evolution, helping to answer the question of why lithium pegmatites appear at this point in Earth history, and are so rich in specialty metals.

The project involves fieldwork, geochemical lab work, and melt modelling to derive a petrological and geological framework.

Fieldwork will take place in one or more Archean terranes, and we will sample examples of both mineralised and unmineralized pegmatites, including surrounding Archean host rocks (e.g. TTG, potassic granites and greenstone belts).

Petrological and geochemical analysis includes whole-rock data, and appropriate mineral data via EPMA and LA-ICPMS. Hosting granitoids will be dated and characterised via U-Pb and Hf isotopes in zircon. However, such methods are difficult to deploy to pegmatites as zircon tends to be absent or highly metamict, and we will explore alternative mineral-based tools to date the pegmatites, to link them to potential source rocks. We will deploy a range of non-traditional isotopic techniques – which may include apatite, feldspar and mica-based analyses – using the facilities both at the St Andrews geochronology laboratory (StAGE) and the Isotope and Tracing Facility, British Geological Survey. These include use of laser ablation LA-ICPMS on both multicollector and triple quadruple instruments.

Empirical analysis can be underpinned by quantitative petrological modelling, and we will use tools such as MAGEMin to constrain the P-T-X of crustal melting and to help link these processes into the broader geodynamics.

Year 1

Field season to conduct detailed mapping and sample collection in key regions of interest.
Literature review.
Bulk-rock, and in-situ trace-element and isotope geochemistry
U-Pb Geochronology
MDSG conference

Year 2

Possible 2nd field season. Further geochemical analysis, including isotope methods

Year 3

Petrological modelling, and international conference

Year 3.5

Write up papers, submit thesis

Training will be provided for all aspects of the scientific research (i.e., fieldwork, lab work, modelling). The PhD will gain experience in geochemical analysis including LA-ICPMS, thermodynamic melt modelling, data management, fieldwork, and in multi-disciplinary research. They will work with leading experts in the field, present at several international conferences, and benefit from training and support offered by UStA’s Academic Skills Project led by the Centre for Educational Enhancement and Development team (alongside training provided by the Iapetus DTP). They will become part of a vibrant research culture in the School of Earth and Environmental Sciences, with MSc, PhD and postdocs working on a range of research projects in the PALS (Pegmatites and Laser) and PaStA (Petrology at St Andrews) research groups. It is anticipated a number of publications will arise from this study, and training in scientific manuscript preparation and presentation will be given.