They Drilled Deeper Than Ever Before, But What They Found Wasn't What They Expected
In May 2023, a team of researchers aboard the JOIDES Resolution achieved a groundbreaking feat, drilling 1,268 meters into the Earth's crust and reaching mantle-altered rock closer than ever before. Working near the Lost City hydrothermal field in the Atlantic Ocean, the scientists extracted a core of abyssal peridotite, marking the deepest successful mantle sampling mission to date. While they didn't cross the official boundary between the crust and mantle, the mission yielded vital insights that could revolutionize our understanding of Earth's largest layer.
The mantle, comprising 70% of Earth's mass and 84% of its volume, remains largely a mystery. No previous mission has successfully retrieved pristine mantle rock from below the Mohorovičić discontinuity, known as the Moho, which marks the true threshold between the crust and mantle. The Earth's crust, typically 9 to 12 miles thick, has made direct access to the mantle nearly impossible.
However, certain locations offer better opportunities. The Mid-Atlantic Ridge, particularly the Atlantis Massif, features unusually thin crust and faulting that has cracked the rocky layers open, making it a prime spot for exploration. Nearby, the Lost City vent field is renowned for its highly alkaline hydrothermal activity and concentration of hydrogen, methane, and carbon compounds, even being considered a plausible environment for the origins of life.
Record-Breaking Drill at the Atlantis Massif
Funded by the U.S. National Science Foundation, researchers aboard the 470-foot-long JOIDES Resolution exceeded expectations during their drilling at a site 800 meters south of the Lost City. Initially planning to drill only 200 meters, the deepest previous record in mantle rock, the team unexpectedly encountered easy drilling conditions, allowing them to go over six times deeper.
Johan Lissenberg, a petrologist at Cardiff University, explained, "We had only planned to drill 200 meters, because that was the deepest people had ever managed to drill in mantle rock. The drilling was so easy that they progressed three times faster than usual." The drill finally stopped at 1,268 meters, not due to equipment failure or rock hardness, but simply because the mission's operations window had ended.
New Insights from Serpentinized Mantle Rock
The core recovered during the mission contained abyssal peridotites, the dominant rock in the upper mantle. Preliminary analysis revealed the presence of harzburgite, a variety of peridotite formed through partial melting of mantle rock, and gabbros, coarse-grained igneous rocks. Both rock types had undergone chemical changes due to prolonged exposure to seawater, a process known as serpentinization, which alters the structure and composition of the mantle rock, giving it a green, marble-like texture.
According to The Conversation, study co-author Andrew McCaig from the University of Leeds emphasized the scientific value of these samples, not only for their geologic content but for their relevance to understanding the Lost City's geologic foundation. However, despite the success of this drill, scientists still fell short of breaking through the Moho, leaving the goal of sampling pristine mantle unmet.
Momentum Stalled Just as Breakthroughs Emerge
The NSF's decision to decline further core drilling with JOIDES Resolution beyond 2024 puts the future of deep mantle research at risk. Just as researchers are finally within reach of Earth's most elusive layer, the tools may be taken off the table, raising questions about the future of deep-sea drilling efforts and the potential breakthroughs that could be lost.
