A NEW study by Chinese researchers explains why so few carbonatite intrusions yield economically viable rare earth element (REE) deposits, even though carbonatites host over half the world’s REE reserves. Researchers from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, led the study, which pinpoints magma emplacement depth — and thus pressure — as the decisive factor controlling REE enrichment.

Published in Nature Communications, the study used high-temperature, high-pressure experiments to simulate magma crystallization 6-20 km underground. Results reveal a critical boundary near 10 km depth (0.3 GPa) that splits magma evolution into two pathways with starkly different outcomes for REE concentration.

When magma intrudes at shallow depths, apatite crystallizes early. This silicon- and sodium-rich apatite forms a crystal “cage” that traps rare earths, preventing them from migrating and accumulating. Low pressure also drives rapid release of low-salinity hydrothermal fluids, which are poor REE transporters, further hindering ore formation.

In contrast, deep emplacement triggers early olivine crystallization, which removes silicon from the melt and blocks formation of the REE-trapping apatite. Higher pressure allows magma to retain more water, delaying fluid separation and promoting evolution into an alkali- and volatile-rich “salt melt.” REEs are highly soluble in this melt, enabling progressive enrichment and the formation of transitional minerals like huanghoite — paving the way for commercial REE minerals such as bastnaesite.

The study is the first to link emplacement pressure, mineral sequence, melt properties, and REE enrichment into a complete chain. According to USGS data, China holds 44 million tons of REE — 48.4% of world reserves — underscoring the importance of improving exploration models for these critical materials.(SD-Agencies)