Mantle sources and magma evolution in Europe’s largest rare earth element belt: Gardar Province, SW Greenland

Tuesday, 15 December 2020: 08:00
William Hutchison1, Adrian Finch1, Anouk Borst2, Michael A.W. Marks3, Aubrey Zerkle2, Eva E Stüeken1 and Adrian Boyce4, (1)University of St Andrews, School of Earth and Environmental Sciences, St Andrews, United Kingdom, (2)University of St Andrews, St Andrews, United Kingdom, (3)University of Tübingen, Department of Geosciences, Tübingen, Germany, (4)Scottish Universities Environmental Research Center at the University of Glasgow, East Kilbride, United Kingdom
Recorded Presentation
Abstract:
Alkaline igneous complexes are key feature of intracontinental rifts and are the most important natural sources of rare earth elements (REE). Although a wide variety of magmatic processes are known to drastically increase (and decrease) the total REE content of an alkaline magma body, there is great uncertainty about the importance of the mantle source in the genesis of REE-rich magmas. It is often unclear whether neighbouring alkaline complexes reflect similar mantle sources or similar magmatic processes (i.e. degree of partial melting and fractionation). Sulfur isotopes allow us to test these hypotheses because they are unaffected by high-temperature partial melting processes.

Here, we present new S isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of magmatic units from the Mesoproterozoic Gardar Province in SW Greenland (which hosts Europe’s largest REE deposits). We show that virtually all Gardar melts preserve the δ34S signature of their mantle source and that their values (1–5 ‰) represent a subduction-influenced source. Comparing various generations of Gardar magmas we find that δ34S, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb) are highly enriched in the alkaline complexes and clusters of dykes spatially associated with these complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline igneous complexes are sourced from localized mantle domains enriched in δ34S, REE and volatiles (particularly, F). Since volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest that the co-location of volatile species and incompatible metals at high concentrations in the mantle source is a vital first-step in the genesis alkaline igneous systems, and argue that an enriched mantle source is critical to the formation of world-class alkaline REE deposits.

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