When it comes to the lava they produce, no two volcanoes are the same. The composition of basaltic lava varies greatly from one part of Earth’s surface to another.
For decades, geochemists have argued this variation is because each volcano is drawing from a different rock reservoir in the mantle, the 2,900-kilometer-thick (1,800-mile-thick) portion of the planet that underlies the crust. A new study hints that the mantle may be more uniform than scientists thought.
Oceanic crust being subducted into the mantle at plate boundaries, the traditional argument goes, introduces new chemical elements to create these highly distinct reservoirs in the deep mantle. This idea, however, doesn’t key with geophysical understandings of Earth processes, which say that convection should mix all that mantle material into a uniform soup.
Volcanoes, according to that model of uniform mantle, should produce identical lava. So why don’t they?
Tracing the Journey of Magma
In an attempt to answer this question, Matthijs Smit from the University of British Columbia and Ellen Kooijman from the Swedish Museum of Natural History collated data from more than 200,000 basalt samples, collected from oceanic island volcanoes situated above mantle hot spots all around the world. They determined the chemical composition of these lavas.
Hot spot magmas start out deep in the mantle with lots of nickel in them. As they travel from their source, nickel crystallizes out of the melt, so magmas that make it to the surface and erupt as lava have a lower proportion of nickel than they did when they first formed. Scientists can therefore use the amount of nickel in lava to gauge how far it has traveled. Pairing this with other chemical information, they can chart the evolution of a magma’s chemistry, from melt to eruption.
“We actually found that they all follow a very distinct trend,” Smit said.
“The mantle that is down there makes a very specific, distinct, single kind of juice.”
That, Smit said, wouldn’t be possible if they came from isolated, chemically distinct mantle reservoirs.
“If all the melts started out differently,” Smit said, “we shouldn’t see any clear trends.”
The implication is that all hot spot basalts start from a single source magma derived from partial melting of a homogeneous deep mantle. “The mantle that is down there makes a very specific, distinct, single kind of juice,” Smit said. “It comes out everywhere, and it just starts to live its own life from then on.”
Old slabs of crust, he said, likely lie buried in the mantle, but only in the uppermost part.
As the magma travels up through the mantle and crust, it interacts with these crustal remains, leading to changes in its chemistry. “Depending on where the lava goes, depending on what kind of lithosphere it has to go through, what kind of crust it sees, it evolves in its own unique flavor,” Smit explained.
These different “flavors,” he said, account for the vast range of lavas found in hot spot volcanic regions around the globe. The research was published in Nature Geoscience.
A New Paradigm?
The study, Smit said, represents a completely new way of understanding Earth’s mantle. It “makes everything so much simpler, and basically turns everything upside down. The complexity’s at the top, not at the bottom.”
“It also makes the mantle more simple, which is probably more consistent with nature,” Smit added.
Ananya Mallik, a researcher with the University of Arizona who was not involved in the study, said she found aspects of the research “compelling.”
“It’s an interesting idea that they put forth,” she said. “I think it may be a starting point for us to investigate this further.”
She pointed out, however, that earlier research has found that oceanic crust reaching the deep mantle is needed to explain the high nickel content of ocean island basalts. “This study does not cite that paper,” she said. “I think that needs reconciliation.”
“To most mantle geochemists, these would have to be alarming conclusions, running contrary to a scientific consensus crafted over the past four decades.”
The study was “too simplistic,” said Marco Brenna, a petrologist with the University of Otago in Aotearoa New Zealand who was not involved in the study. Its use of a global data set, he said, “hides the local heterogeneities.”
“The problem is that there are more…unknown parameters than we can put our fingers on, and therefore every model, especially global ones that just blur the local particularities, will be misleading at best,” he wrote in an email.
Jon Woodhead, a geochemist at the University of Melbourne who was not involved in the study, wrote in an email that he initially also found the premise of the study “very hard to accept.”
“To most mantle geochemists these would have to be alarming conclusions, running contrary to a scientific consensus crafted over the past four decades,” he wrote. However, he noted that he tested the idea using some geochemical data he had on hand. He wrote that “to my surprise,” he found “very convincing trends” that corroborated the study’s findings.
“It does seem clear,” Woodhead wrote, “that there is potentially a previously unrecognized issue at play here.”
“Further research will likely be required before we can say whether any modification, or even upheaval, of the existing paradigm will result,” he wrote.
—Bill Morris, Science Writer
