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Here’s what you’ll learn from reading this story:
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To understand the mantle—the largest layer of Earth’s rocky body—scientists drill deep cores from the Earth.
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Scientists drilled the deepest core and recovered serpentinized peridotite that forms when salt water interacts with mantle rock.
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Although this is the deepest in the mantle that scientists have ever drilled, the mission did not reveal a virgin mantle that is beyond the boundaries of the Mohorovičić discontinuity, or Moho.
If you want to understand the geology of our home planet, studying the mantle is a great place to start. Separating the planet’s rocky crust and molten outer core, the mantle makes up 70 percent of Earth’s mass and 84 percent of its volume. But despite its great influence on the planet’s geological processes, scientists have never directly sampled rocks from this hugely important geological layer.
And that’s understandable, especially when you consider that the crust is roughly 9 to 12 miles thick on average. Fortunately, that average contains outliers—areas of the earth where the crust is actually incredibly thin and the fault exposes the mantle through cracks. One such area is the Mid-Atlantic Ridge, specifically near an underwater mountain called the Atlantis Massif.
To the south of this massif is an area known as the Lost City—a hydrothermal field whose vent fluids are highly alkaline and rich in hydrogen, methane, and other carbon compounds. This makes the zone a particularly compelling candidate to explain how early life evolved on Earth. In addition, it contains mantle rock that interacts with seawater in a process known as “serpentinization,” which changes the rock’s structure and gives it a green, marble-like appearance.
It was here, 800 meters south of this area, in May 2023 that members of the International Ocean Discovery Program (IODP)—aboard the JOIDES Resolution, a 470-foot-long research vessel hired by the US National Science Foundation—extracted a 1,268-meter core containing abyssal peridotites, which are the primary rocks that make up the Earth’s upper mantle. The results of the study have been published in the journal Science.
Although this makes this particular drill core the deepest sample of the mantle so far, going that deep into the rock was not the goal of this record-breaking expedition.
“We had only planned to drill to 200 meters, because that was the deepest we have ever managed to drill into the mantle rock,” said Johan Lissenberg, a petrologist at Cardiff University and co-author of the study. Nature. He said that the digging was so easy that they advanced three times faster than usual. The team eventually dug a staggering 1,268 meters, stopping only because of the mission’s limited operations window.
Andrew McCaig—study co-author and University of Leeds scientist—said in an article by The Conversation that, according to a preliminary analysis of the rock, the composition of the core contains a variety of peridotite called harzburgite that forms through partial melting of the mantle rock. It also contains rocks known as gabbros, which are coarse-grained igneous rocks. These two rocks then reacted chemically with the seawater, changing their composition.
While this core represents an incredible opportunity to learn more about the Earth’s mantle, as well as provide an in-depth look at the geological substrate on which the Lost City rests, the mission did not complete the “grand challenge” of crossing the Mohorovičić discontinuity. Otherwise known as the Moho, the Mohorovičić discontinuity is recognized as the true boundary between the pristine crust and mantle.
Future missions may continue to explore this site near the Atlantis Massif, but unfortunately, those missions will not include Resolution JOIDES—NSF declined to fund further core drilling through 2024. As scientists are finally knocking on the door of the most ubiquitous geological layer on Earth, the future of these types of drilling missions is now uncertain.
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