Investigating mantle-to-crust transfer processes of critical metals and volatiles at Kerguelen

Platinum-group elements (PGE) and rare-earth element (REE) are essential resources for energy transition technologies and sustainable development. They are classified as critical and strategic metals by the European Union. Economically-viable deposits of PGE and REE in the Earth’s crust are rare. More than 90% of the extracted PGE comes from a single mining site in South Africa (Bushveld complex), while 60% of REE production is controlled by three deposits: Bayan Obo in China (40%), Mountain Pass in the United States (10%) and Mount Weld in Australia (10%). PGE and REE resources are therefore very limited, leading to supply tensions with the increase in demand. Understanding transport and enrichment processes of these metals in geological materials is essential to establish innovative strategies for the rational and sustainable exploitation of secondary resources and recycling.

Enrichment processes of PGE and REE in the crust are linked to the intrusion and evolution of magmas enriched in volatile elements, mainly sulfur (S) and carbon (C). These volatiles are crucial for the transfer of metals via magmas, from their formation in the Earth’s mantle to the crust. The Kerguelen archipelago is ideal for studying these mantle-to-crust transfer processes. Magmatic activity ranging from (mid-oceanic) ridge- to mantle plume-type, took place there over more than 130 millions years, involving the formation of various magmas enriched in volatiles. Furthermore, mantle rocks are also accessible on Kerguelen island under the form of enclaves in magmas. Preliminary studies show that some magmas from the Kerguelen plateau display PGE abundances more than 10 times higher than in the oceanic and continental crust, as well as high S and C concentrations exceeding predictions for the intra-oceanic domain. However, the origin of these unique enrichments is unknown because the mantle-to-crust transfer processes of magmas remain poorly understood.

KERGMET project aims to sample magmatic and mantle rocks from Kerguelen island (Gallieni, Jeanne d’Arc and Ronarc’h peninsulas) to understand mantle-to-crust transfer processes of metals and volatiles and their evolution through time. The targeted sites will make it possible to compare metal and volatile contents between magmas of different types and mantle rocks variably modified by the transfer of magmas to the crust. Geochemical analyses of the rocks will be carried out in several laboratories within an international research group, including the Laboratoire Magmas et Volcans (LMV; Université Clermont Auvergne, Clermont-Ferrand, France), Curtin and Monash University (Australia), the Institut de Physique du Globe de Paris and Université Paris-Saclay (UPS). Geochronological analyses to obtain absolute ages of sample cristallization will be performed at LMV, Curtin and UPS. High-pressure and high-temperature experiments at LMV and Curtin will make it possible to determine the physicochemical conditions, in particular the role of S and C, favoring the transfer of metals in magmas. The results will make it possible to identify which processes lead to the formation of metal deposits in the crust. This work will provide new insights into the global cycles of S, C and metals in the Earth’s mantle, and open new perspectives for the rational and sustainable use of critical materials.