The influence of lithospheric structure and composition on the distribution of CO2-rich intraplate volcanism and REE mineralisation
Lead Research Organisation:
University of Cambridge
Department Name: Earth Sciences
Abstract
Most of the Earth's volcanism occurs at the diverging and converging boundaries of tectonic plates. While igneous activity is less common in intraplate continental settings, these magmas hold essential clues on the evolution of Earth's outer rigid shell (lithosphere) and the generation and distribution of commercially significant natural resources, including rare earth elements (REE) and critical metals (e.g. Nb & Ti), essential for renewable energy and other infrastructure.
The type of continental intraplate magmatism is governed by the lithospheric thickness and composition and temperature in the lithosphere and underlying mantle. Intraplate basalts are produced by partial melting in the asthenosphere beneath thin (<60-70 km) lithosphere, whereas carbonatites and associated alkaline igneous rocks, which host most of the world's REE deposits, are emplaced through thicker (~70-120 km) lithosphere. This contrasts with diamondiferous kimberlites and lamproites that occur in Precambrian cratons, with lithosphere over 150 km thick.
Carbonatite complexes are found in continental rift zones, Large Igneous Provinces and syn- to post-collisional zones. Previous studies of their origins have largely focused on their relationship with surface geology but here we innovate by examining their distribution with deep lithospheric structure. Our work is timely because:
1. It follows the rapid recent growth in the number of seismic stations around the world, which now permits seismic tomography to map lateral variations in lithospheric structure at higher resolution beneath most continental areas than previously possible. High-resolution geophysical models of lithospheric structure together with access to samples collected from global carbonatite complexes of different ages enables novel, unique insights into their distribution in space and time.
2. Keeping up with increasing demand to supply REEs, essential for products in emerging clean, green technologies, electronic devices and petroleum-refining catalysts, presents a serious global challenge. Over 95% of the world's usable REEs are produced from the Bayan Obo mine (China), raising concerns over the security of supply for Europe (e.g. EURARE project http://www.eurare.eu/about.html) and highlighting the need to identify sources of potential future deposits. This exploration is essential for long-term planning and growth, but Europe currently has no mines supplying REEs, although there are a number of areas with geology often associated with REE mineralisation, including alkaline igneous rocks found in south-west Greenland, Finland and Scandinavia.
The main goal of our research is to quantify the relationship between lithospheric thickness and the global distribution of carbonatite complexes with their associated REE, Ti, Nb and Ta deposits, and assess how this has varied during Earth's evolution. This relates to large-scale, complex interactions within the Earth system. We have conducted pilot studies at global and regional scales, focused on E Africa and western S America, which revealed: (i) a correlation between the distribution of continental magma type and lithospheric thickness; (ii) a strong relationship between carbonatite complexes and the margins of ancient cratons; (iii) Phanerozoic carbonatite magmatism is often located on thinner lithosphere than its Precambrian counterpart. We seek to build on the pilot study by improving the resolution of the geophysical data, expanding the analysis to all the continents, and compiling comprehensive geochemical datasets for carbonatite complexes and associated alkaline igneous rocks of different ages together with their REE deposits. Using in-depth statistical analysis of the geophysical and geochemical data, we will quantify the correlations, establish how they varied over geological time, build a probabilistic predictive model for the location of the rocks and deposits, and advance our knowledge of lithospheric evolution
The type of continental intraplate magmatism is governed by the lithospheric thickness and composition and temperature in the lithosphere and underlying mantle. Intraplate basalts are produced by partial melting in the asthenosphere beneath thin (<60-70 km) lithosphere, whereas carbonatites and associated alkaline igneous rocks, which host most of the world's REE deposits, are emplaced through thicker (~70-120 km) lithosphere. This contrasts with diamondiferous kimberlites and lamproites that occur in Precambrian cratons, with lithosphere over 150 km thick.
Carbonatite complexes are found in continental rift zones, Large Igneous Provinces and syn- to post-collisional zones. Previous studies of their origins have largely focused on their relationship with surface geology but here we innovate by examining their distribution with deep lithospheric structure. Our work is timely because:
1. It follows the rapid recent growth in the number of seismic stations around the world, which now permits seismic tomography to map lateral variations in lithospheric structure at higher resolution beneath most continental areas than previously possible. High-resolution geophysical models of lithospheric structure together with access to samples collected from global carbonatite complexes of different ages enables novel, unique insights into their distribution in space and time.
2. Keeping up with increasing demand to supply REEs, essential for products in emerging clean, green technologies, electronic devices and petroleum-refining catalysts, presents a serious global challenge. Over 95% of the world's usable REEs are produced from the Bayan Obo mine (China), raising concerns over the security of supply for Europe (e.g. EURARE project http://www.eurare.eu/about.html) and highlighting the need to identify sources of potential future deposits. This exploration is essential for long-term planning and growth, but Europe currently has no mines supplying REEs, although there are a number of areas with geology often associated with REE mineralisation, including alkaline igneous rocks found in south-west Greenland, Finland and Scandinavia.
The main goal of our research is to quantify the relationship between lithospheric thickness and the global distribution of carbonatite complexes with their associated REE, Ti, Nb and Ta deposits, and assess how this has varied during Earth's evolution. This relates to large-scale, complex interactions within the Earth system. We have conducted pilot studies at global and regional scales, focused on E Africa and western S America, which revealed: (i) a correlation between the distribution of continental magma type and lithospheric thickness; (ii) a strong relationship between carbonatite complexes and the margins of ancient cratons; (iii) Phanerozoic carbonatite magmatism is often located on thinner lithosphere than its Precambrian counterpart. We seek to build on the pilot study by improving the resolution of the geophysical data, expanding the analysis to all the continents, and compiling comprehensive geochemical datasets for carbonatite complexes and associated alkaline igneous rocks of different ages together with their REE deposits. Using in-depth statistical analysis of the geophysical and geochemical data, we will quantify the correlations, establish how they varied over geological time, build a probabilistic predictive model for the location of the rocks and deposits, and advance our knowledge of lithospheric evolution
