Submitted by Dr C.M. Martin-Jones on Wed, 14/05/2025 - 09:46
Geoscientists from Cambridge have uncovered a new explanation for the diversity of rocks in an alkaline-subalkaline igneous complex that hosts Canada’s only rare earth element mine, offering fresh insights into the origins of similar, often mineral-rich, complexes worldwide.
According to the research, the distinct rock types—including subalkaline granitic rocks alongside more unusual alkaline rocks enriched in sodium and potassium—formed as magma cooled at a depth of around 15 kilometres in the crust, hovering near a pressure threshold that allowed both rock types to form.
“Using our new detailed model simulation, we can now realistically replicate how alkaline-subalkaline complexes like this form,” said Carrie Soderman, lead author of the study from Cambridge Earth Sciences. “In this igneous complex, pressure conditions are key,” she said. Their results overturn decades of geological thinking, which placed this critical pressure threshold much deeper in the Earth, within the upper mantle.
The work is part of a project, led by Cambridge Earth Science’s Owen Weller, that aims to shed light on the geological conditions that lead to rare earth enrichment and help pinpoint future viable deposits.
Alkaline-subalkaline complexes are an important natural host of rare earth elements and other critical metals essential for high-tech devices and clean energy technologies, such as wind turbines and electric vehicles. As demand for these raw materials continues to rise, concern over the international security of future supplies is growing.
Weller and team have developed a new thermodynamic model that predicts how rock types evolve as magma cools and different minerals crystallize. This model is specifically calibrated for both alkaline and subalkaline rocks and accounts for ten different elements, allowing for a realistic investigation of magma evolution.
They want to understand why some alkaline-subalkaline igneous complexes host rare earth deposits, whereas others are barren. “First we had to go back to basics and ask how these rocks form in the first place,” said Soderman, because, “it turns out there was a big gap in our understanding.”
Geologists have been puzzled as to why alkaline and subalkaline rocks are frequently found in close association for decades. “There’s been this understanding that their evolution is distinct and yet these rock types co-occur in many locations globally,” said Soderman.
Magma cools and solidifies into different rock types depending on pressure conditions, because of changes in the amount and type of minerals that form. At lower pressures, granites and other subalkaline rocks form whereas, at higher pressures, alkaline rocks form. Previously, scientists thought the transition between alkaline and subalkaline magmas occurred at around 10 kilobars—roughly 35 kilometres deep, in Earth’s upper mantle, and potentially too deep to be a viable explanation for the co-occurrence of alkaline and subalkaline rocks in many complexes. “That’s the textbook thinking,” said Weller. “But we now know that the threshold is much shallower, around 4 kilobars, or 15 kilometres.”
Their investigations of the Blatchford Lake Igneous Complex in Canada revealed that the 4 kbar 'tipping point' represented a fork in the road, where magma just above or below this pressure diverged onto a path toward either subalkaline or alkaline rock. “This newly identified lower-pressure tipping point provides a compelling explanation for the widespread occurrence of alkaline-subalkaline complexes worldwide because magma chambers are commonly located at these depths in the Earth,” said Weller.
Left to right: Owen Weller, Carrie Soderman and Charlie Beard (now Utrecht University) on fieldwork in Greenland, where similar alkaline igneous complexes occur.
The results also hint at a sweet spot for rare earth element enrichment, said Soderman. “It seems like this magma was well primed to become a rare earth element deposit.”
She added that the rocks at this site are also enriched in the particularly valuable ‘heavy’ rare earth elements (e.g. neodymium), that are sought-after for manufacture into magnets. “Our modelling tells us that this crystallization pressure potentially maximized the rare earth potential.”
Now they have a clearer picture of how alkaline-subalkaline complexes form, the team plan to explore the controls of rare earth enrichment by applying their model to other mixed igneous complexes in Greenland, Scotland, Namibia and the Côte d'Ivoire. “The aim is to have a workflow that can both explain enrichment in an observed location, as well as be predictive, indicating where maximum enrichment might occur in igneous systems.”
Reference: Soderman, C. R., Weller, O. M., Beard, C. D., Riel, N., Green, E. C., & Holland, T. J. (2025). A mid-crustal tipping point between silica-undersaturated and silica-oversaturated magmas. Nature Geoscience, 1-8. https://doi.org/10.1038/s41561-025-01695-3
Feature image: Eudialyte, amphibole and feldspar pegmatite layers at Ilimmaasaq in Greenland. Credit: Charlie Beard.
Read more about the group’s research on our blog.