Recent research by Chinese scientists explains how deposits of rare earth elements (REE) are formed, proving that the emplacement depth (pressure) of carbonatitic magma is a key factor controlling the abnormal concentration of REE, which are indispensable raw materials in fields such as new energy and high-tech industries.

While more than half the world's rare earth reserves are derived from a type of igneous rock known as carbonatite, less than 10 percent of carbonatite bodies form economically viable rare earth deposits, which has puzzled scientists for a long time.

Associate researcher Xue Shuo and researcher Yang Wubin from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, along with their collaborative team, have discovered that magmatic emplacement depth determines REE mineralization potential.

The discovery explains the distribution pattern of global carbonatite-type rare earth deposits. For example, China's Bayan Obo and Maoniuping deposits were all formed from magmatic intrusions at depths greater than 10 km, according to Yang. The GIG team's report on the formation of giant carbonatite rare earth deposits controlled by deep-seated magma chambers was published in the international academic journal Nature Communications recently.

Using high-temperature and high-pressure experiments, the team simulated the cooling and crystallization process of carbonatitic magma in the upper-middle crust, approximately 6-20 km underground. They found that, with a boundary at around 10 km depth (corresponding to a pressure of about 0.3 GPa), the magma's evolution follows two distinct evolutionary pathways.

"When carbonatitic magma is emplaced at shallow depths, apatite crystallizes earlier," said Xue. The apatite formed under such conditions is rich in silicon and sodium, and its crystal structure acts like a specialized "cage", firmly trapping rare earth elements within the lattice. This leads to the early sequestration of rare earth elements, making it difficult for them to migrate and accumulate further, he added.

"Meanwhile, the low-pressure environment promotes the release of large volumes of low-salinity hydrothermal fluids from the magma," Xue said, adding that such fluids have a limited capacity to transport rare earth elements and are unable to effectively concentrate the residual rare earths, thus hindering the formation of economically viable ore deposits at later stages.

When carbonatitic magma is emplaced at greater depths, olivine crystallizes first, consuming a significant amount of silicon from the magma, said Xue. This prevents subsequently crystallized apatite from forming the cage-like structure, making it difficult to accommodate and lock in rare earth elements.

Meanwhile, the high-pressure environment allows the magma to dissolve more water, delaying the separation of hydrothermal fluids and promoting evolution of the system into an alkali — and volatile-rich "salt melt".

REEs exhibit high solubility in such salt melts, enabling them to continuously enrich in the residual melt. The process leads to the crystallization of substantial transitional minerals such as huanghoite, laying a solid foundation for the large-scale precipitation of economically valuable rare earth minerals like bastnaesite in later-stage evolution.

The GIG study is the first to establish a complete causal chain linking pressure, mineral crystallization sequence, melt properties and rare earth enrichment. Yang said it deepens understanding of the mechanisms behind rare earth elements enrichment and offers new insights for the exploration of carbonatite-type rare earth deposits.

According to data from the United States Geological Survey, China's REE reserves are 44 million tons, accounting to 48.4 percent of the world's total.

Among the rare earth-rich lands in China, Bayan Obo, located in Baotou of the Inner Mongolia autonomous region, holds unique significance. With its vast reserves accounting for approximately 90 percent of the country's total rare earth resources and about 40 percent of the world's proven total, Bayan Obo has earned the well-deserved title of the "Hometown of Rare Earths".

"In contrast, many shallowly emplaced carbonatite bodies, such as Alno in Sweden and Ol Doinyo Lengai in Tanzania, although they may contain rare earth elements, often exhibit dispersed and non-concentrated mineralization, lacking economic viability for mining," Yang said.

"Unraveling the origin of the Bayan Obo deposit is not only driven by human curiosity about Earth's evolution and natural laws, but also serves as an essential step toward advancing theoretical insights that can guide future ore exploration and sustainable, green extraction practices."

zhengcaixiong@chinadaily.com.cn