Standard tectonic models face a challenge after researchers discovered that a metamorphic rock descended over 90 kilometers into Earth’s mantle approximately four million years ago, recording an unexpected thermal transition from hot to cold conditions.
Researchers from Göttingen University and an international team analyzed a rare high-pressure rock known as eclogite, which traveled more than 90 kilometers—about 56 miles—underground roughly four million years ago deep beneath the surface of Papua New Guinea. Inside this distinctive red-and-green material, scientists found microscopic crystals of coesite, a mineral that forms only under extreme subterranean pressures.
The research team reconstructed the rock’s pressure-temperature history by examining these microscopic inclusions using advanced analytical techniques. Published in Nature Geoscience, the findings reveal a thermal path that contradicts current geodynamic simulations.
Rock Descends Through Unexpected Thermal Transition
Subduction zones occur when converging tectonic plates force one slab beneath another, pulling surface materials down into the mantle. Standard thermal models assume that descending rocks experience a steady, predictable increase in temperature as they sink deeper into the planet.
The Papua New Guinea sample shattered that assumption. At a depth of approximately 45 kilometers—about 28 miles—the rock had already reached roughly 700°C. By the time it descended to its maximum depth past 90 kilometers, temperatures reached approximately 800°C. The rock was only about 100 degrees Celsius cooler at half its descent than it was at its ultimate depth, meaning it experienced intense heating near the surface followed by a much slower rate of warming than models predict.
Finding this change from hot to cold subduction was unexpected as this contradicts our current thermal models of subduction zones.
Dr. Jan Schönig, Geoscience Center at Göttingen University
Two Decades of Fieldwork Lead to Discovery
Suzanne Baldwin, Professor Emerita of Earth and Environmental Sciences at Syracuse University, was part of a team on an NSF-funded expedition to remote islands off the eastern coast of Papua New Guinea in 2003—the same year the university celebrated its men’s basketball team winning its first-ever National Championship. That fieldwork identified what researchers term the world’s youngest ultrahigh-pressure terrane.
This study was based upon a sample that Syracuse University scientists collected on an NSF-funded expedition to remote islands of eastern Papua New Guinea over two decades ago.
Suzanne Baldwin, Professor Emerita of Earth and Environmental Sciences at Syracuse University
State-of-the-art analytical techniques developed long after the initial collection allowed researchers to unlock secrets trapped inside the minerals. The project united scientists from Syracuse, Göttingen University in Germany, and other institutions globally.
The entire pressure-temperature-time path that rocks take from the surface to extreme depths and back to the surface needs to be documented and incorporated into models of subduction zone evolution.
Suzanne Baldwin, Professor Emerita of Earth and Environmental Sciences at Syracuse University
Researchers Test Two Heat Hypotheses
Researchers have proposed two primary hypotheses to explain why the rock experienced such unusual thermal conditions. First, intense shearing forces along tectonic plate boundaries could generate substantial friction and additional heating at relatively shallow depths. Second, the subduction zone itself may have been young enough that it had not yet developed the cooler temperatures expected by conventional long-term models. Funding for the research was provided by the German Research Foundation.
Better-calibrated subduction models allow geologists to evaluate conditions associated with earthquakes and volcanic activity. Refined models also improve tracking for geochemical cycles, including the deep carbon cycle that governs planetary climate stability. Both mechanisms may have operated simultaneously.